FOUS30 KWBC 101916
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 PM EDT Mon Jul 10 2023

Day 1
Valid 16Z Mon Jul 10 2023 - 12Z Tue Jul 11 2023

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR THE CHAMPLAIN
VALLEY AND MUCH OF VERMONT...
...WIDESPREAD SIGNIFICANT FLASH FLOODING LIKELY THROUGH TONIGHT...

...16Z Update...

...Northeast...

Several changes to note as we progress through this very busy
Monday. The High Risk area was extended southward to cover the
rest of southern Vermont with the Special Update. No big changes
were made to the High Risk area since that special update.
Remarkably, the latest 12Z HREF guidance has increased 100-year
ARI exceedance probabilities to over 80% now near the northwest
corner of VT, near St. Albans, with over 70% chances from
Burlington north to near the Canadian border along I-89 to near
the spine of the Green Mtns.

The surrounding Moderate Risk area was trimmed from the southwest
to eliminate northwestern CT and the southwest corner of MA.
Unfortunately the amount of trimming couldn't continue to follow
the radar signature because the surface low driving this continues
to strengthen and will pivot more Atlantic moisture into New
England through tonight, so the back edge of the rain for western
MA, VT, and NH is unlikely to continue to progress as daytime
heating allows more widely scattered convection to begin
redeveloping behind the plume of moisture. The heavy rain
advancing to the west of Boston, including Worcester is likely to
produce flash flooding, so the Moderate Risk area was expanded
eastward for those storms moving through over the next couple
hours. In similar fashion, for eastern CT (and eventually RI once
the main band of storms moves through) while the rain has stopped
here for now, additional convection is expected to form through
the afternoon with daytime heating, and with very low FFGs, still
could result in additional flash flooding, though nowhere to the
extent to what has already occurred. By around sunset with
diminishing solar heating so too will the flash flood threat from
any afternoon storms.

No changes were made to the Slight Risk area from MA north, with
trims on the southwestern edge out of southern NY and western CT
made. With a nearly stationary band of heavy rain set up in the
eastern Lake Ontario region down through Syracuse due to the
potent upper low pivoting southwest of that region, the Slight
Risk area was expanded westward until that convection dissipates.
Recent radar trends have been showing the band strengthening, so
this may take several hours.

For similar reasons, the possibility of widely scattered, but
potentially strong afternoon storms this afternoon for southern NY
and eastern Long Islands has prompted keeping the Marginal Risk
largely the same.

...Southeast...

Trimmed the Slight Risk out of central SC in coordination with
CAE/Columbia, SC forecast office. The storms have been strong,
locally producing rainfall rates to 1.5 inches per hour, but they
are moving rapidly eastward. This rapid movement will greatly
diminish the flash flooding potential, and the Slight along the SC
coast is largely for the storms moving over potentially sensitive
areas over the next 1-3 hours, with any following scattered cells
behind the main line of convection. This is considered a low-end
Slight for today. Along the Gulf Coast, largely as expected, most
of the thunderstorm activity is off the coast, and those that are
over north FL are moving quickly, so the Marginal was maintained
with no changes.

...Southern Plains...

Training showers and storms have developed across northeast TX and
into northern LA, but fortunately they're moving quickly enough
and are widely spaced apart from one another enough over areas
that have high enough FFGs that the flash flooding threat remains
low. The Slight for southern OK and far northern TX remains in
place, and is for expected storms that will develop late this
afternoon through the overnight, developing into an MCS. The MCS
will be fast moving, but there are sensitive areas of west central
OK and far northern TX that may get enough rain, especially with
any cell mergers that occur ahead of the main line of storms that
the flash flooding threat is largely elevated. The MCS has yet to
form as of this update. Supporting the threat for flash flooding
is the highly anomalous amounts of atmospheric moisture that will
be advected into the region ahead of the storms, with PWATs likely
exceeding 2 inches. This will allow the storms to be very
efficient rain-makers compared to the typical hot and dry weather
that is normal of mid-July. Thus, the sheer strength and abundance
of moisture available to the storms may overcome their fast motion
to result in more widely scattered flash flooding across the
Slight Risk area.

...Washington State...

A progressing but vigorous shortwave trough over southern WA will
lift into the northeastern part of the state by late today. The
showers and storms over central WA will develop into more of a
comma-shaped appearance over the next several hours, with the
Slight Risk area highlighting where the comma-head region is
expected to develop. Thunderstorms are expected to develop within
this region this afternoon, which could produce heavy rainfall
rates. The flash flooding risk is heightened in those areas where
the storms form or move over existing burn scars, canyons to the
east of the Cascades, and any other flood-sensitive and more
urbanized areas of north-central WA. In coordination with
OTX/Spokane, WA forecast office, a Slight Risk targeting the
afternoon and every evening hours today was introduced.

Elsewhere into ID and MT few changes from inherited were made.
These areas are ahead of the aforementioned shortwave, so while
there is some potential for backbuilding, any individual shower
and thunderstorm cells will be moving quickly, which will keep the
flash flooding threat isolated.

Wegman

...Previous Discussion...

...New England and Upstate New York...

A 500mb shortwave rotating within a large and broad longwave
trough across the eastern CONUS will sharpen into a closed low
over Upstate NY this morning and then pivot very slowly eastward
through the day. This will drive intensifying downstream ascent
through height falls and PVA, overlapped efficiently with the
diffluent LFQ of a poleward arcing jet streak strengthening to
90-100 kts downstream of the primary trough axis. The increasing
negative tilt to this feature will drive the 850mb low across
central New England, with flow returning off the Atlantic and Gulf
of Maine northwest back into New England at 20-25 kts, with the
surface low tracking slowly northeast just inland of I-95.

During this evolution, the large scale ascent will intensify
across New England, resulting in an expansive area of heavy
rainfall today. The 850mb inflow will drive impressive WAA
northwestward, which will manifest as an impressive theta-e ridge
lifting cyclonically into a TROWAL over New England. This theta-e
ridge will then converge into a sharp mid-level deformation axis
oriented N-S to produce locally even more intense ascent and
provide a focus for heavy rainfall. This then produces a
concerning setup for training and repeated rounds of heavy
rainfall as mean 0-6km winds remain S to N through the day to
impinge along this axis while inflow remains out of the E/SE. With
PWs progged to be 1.75-2", around +2 sigma according to the NAEFS
ensemble tables and above the 90th percentile from the SPC
sounding climatology, modest 250-750 J/kg of MUCAPE through a
deeply saturated column, and warm cloud depths above 11,000 ft,
warm/efficient rainfall rates will likely peak above 2"/hr, and
the HRRR suggest 15-min rainfall may exceed 1" at times (short
term 4+"/hr rates).

These rates in themselves could be enough to overwhelm soils to
result in runoff and flash flooding, but what makes this event
more alarming is that they will be occurring atop pre-saturated
grounds. 14-day rainfall from AHPS has been 200-300% of normal
across much of central and northern New England, leading to USGS
streamflow anomalies that are almost uniformly above the 90th
percentile, and in some places are well above all-time record
flows. This has also resulted in FFG that is as low as 0.75"/1hr
and 1-1.5"/3hrs, further indicating the sensitivity of the local
soils. This suggests that even moderate rainfall will become
runoff as the soils are fully hydrophobic. With rain rates of
2+"/hr likely, and training of echoes repeating through the day,
many places will likely receive more than 3" of rain as reflected
by all the ensemble probabilities, with local maxima of 5-8"
probable noted by HREF probabilities reaching 60% for 5" and even
20-25% for 8". While there remains some uncertainty as to the
exact placement of the heaviest rainfall axis, the HREF EAS
probabilities for 3" are above 40% in western VT which is a
notable value for this threshold. This indicates that guidance is
coming into better agreement on the axis of heaviest rainfall
which will focus beneath the deformation axis aligned near Lake
Champlain.

The inherited high risk and surrounding EROs still seem warranted
for what could be a considerable flash flood event for the region.
While rainfall on Sunday across parts of VT/NY was not quite as
high as predicted, soils are still extremely vulnerable, and the
high risk overlaps well the highest EAS probabilities and
24-hr/100yr RI exceedance probabilities which reach 40-50%.
Repeated rounds of convection will likely plague much of Upstate
NY and New England through the day, with WAA/TROWAL heavy rainfall transitioning by evening to a deformation band of additional
moderate to heavy rain, and the high risk overlaps where the
longest duration is expected. Minor cosmetic changes were made to
the ERO, with the most significant adjustment actually spreading
the MDT risk into the White Mountains of NH where impressive moist
upslope flow could focus a secondary maxima in rainfall atop
already saturated soils.


...Southeast...
A broad but amplified longwave trough will continue to extend down
into the Southeast today, with the base of the trough lingering
along the Gulf Coast. This will result in more flattened, almost
zonal, mid-level flow across the Gulf Coast/Southeast, within
which weak vorticity impulses embedded within shortwaves will
traverse eastward through the day. At the surface, a stationary
front will ripple from Texas to South Carolina, interacting with
the weak impulses aloft to produce waves of low pressure along it. Additionally, a zonally oriented jet streak across the lower TN
VLY will pivot eastward and begin to arc to the north during the
aftn, placing increasingly favorable diffluence within the RRQ
aloft, to provide additional ascent. The result of this should be
plentiful deep layer ascent through D1 to produce waves of
convection.

South of this front, 850mb winds will emerge out of the Gulf of
Mexico at 20-30 kts to provide impressive inflow, which will then
impinge into and along the front. This will surge PWs to as high
as 2.25 inches, more than 2 standard deviations above the climo
mean according to NAEFS, and produce IVT above 500 kg/m/s, highest
in GA/SC. This moisture plume will overlap with MUCAPE that will
approach 2000 J/kg to produce favorable thermodynamics for intense
rain rates within convection, especially during peak heating.
Rainfall rates as progged by the HREF will be 1-2"/hr, and with
mean 0-6km winds aligned to the front and at least obliquely to
the Corfidi vectors, it is likely these rates will train west to
east through the day. Additionally, with higher instability
lingering over the Gulf of Mexico, Corfidi vectors that are
progged to collapse to just 5-10 kts indicates the potential for
some backbuilding to prolong rainfall near the Gulf Coast.

Anywhere training occurs will experience more than 3 inches of
rain today, but there may be two local maxima as reflected by HREF probabilities of around 20% for 5". The first is along the Gulf
Coast from near Plaquemines Parish, LA through the Big Bend area
of the Florida Panhandle, where backbuilding into the Gulf of
Mexico will likely train to the northeast to enhance the duration
of heavy rainfall. The second area is along the coast of South
Carolina where westerly flow along the front will allow for
potentially significant training while also pinning the sea breeze
near the coast to also prolong heavy rainfall potential.
Consideration was made for two slight risk areas here, but recent
rainfall, especially along the Gulf Coast has been light so souls
are likely still hydrophillic there noted by FFG that is generally
3"/3hrs. However, after coordination with WFO CHS/ILM, a narrow
SLGT risk was added for the coastal plain of SC where HREF
exceedance probabilities are greater, and recent rainfall has been
as high as 5" in isolated areas.


...Southern Plains...
An anomalous mid-level trough encompassing much of the eastern
third of the CONUS, and an amplifying ridge centered over the
Desert Southwest will leave pronounced NW flow across the Southern
Plains today. Within this flow regime, weak shortwave impulses and
associated vorticity maxima will rotate down across the region to
drive bouts of locally enhanced ascent. As the ridge to the west
intensifies this evening, and a potent vorticity max shifts out of
Colorado, low-level flow will back in response, driving a warm
front northeastward and producing impressive WAA into an elevated
boundary late. This WAA will surge PWs to around 1.75 inches,
coincident with a ribbon of MUCAPE reaching 2000-3000 J/kg,
providing robust thermodynamics for heavy rain producing
convection, with thunderstorms likely organizing into an MCS
through effective bulk shear of 40-50 kts.

The high-res guidance is all in agreement that an MCS will
develop, but as is typical with these subtle shortwaves in summer,
the placement varies widely. This is noted in high-res simulated
reflectivity, as well as HREF EAS probabilities that are minimal
for 1 inch. However, the HREF neighborhood probabilities for 1"/hr
and 2"/hr rates peak along and north of the Red River Valley of
the South into Oklahoma, which is also an area that has received
heavy rainfall the past 7-day noted by AHPS departures that are
300-600% of normal. Storm motions may generally be progressive
within this MCS, but some increasingly right-angled Corfidi
vectors to the mean flow suggest at least short-duration training
is possible, and the HREF 3-hr maximum probability for 3" exceeds
30% tonight. Locally, this event could produce 3-5" of rainfall,
which if it falls atop pre-conditioned soils from recent MCS
events could lead to instances of flash flooding. Confidence in
the position of the SLGT risk is admittedly modest due to
uncertainty in the MCS track, but it was modified cosmetically to
account for the newest guidance and most vulnerable antecedent
soil conditions.


...Upper Midwest and Great Lakes...
A cold front will dig southward from Canada today, but gradually
become more elongated west to east across the northern tier of the
CONUS. This evolution will be driven by a potent vorticity lobe
rotating around the anomalous closed low near the Hudson Bay, with
a secondary lobe swinging southward into MN late in the day help
to stretch the front. By the end of D1 this front should finally
eject southeast towards the Ohio Valley.

(cont'd)
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 112012
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 PM EDT Tue Jul 11 2023

Day 1
Valid 16Z Tue Jul 11 2023 - 12Z Wed Jul 12 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-MISSOURI BASIN AND LOWER MISSISSIPPI VALLEY...


16Z Update...
Overall, made minor changes to the previous outlook based on the
12Z HREF guidance and current trends. A small Marginal Risk was
added to portions of southeastern Arizona and southwestern New
Mexico to reflect increasing PWs and weak steering flow which may
support slow-moving storms capable of producing locally heavy
amounts during the afternoon into the evening hours.

Pereira

Previous Discussion...
...Northern High Plains through the Mid-Missouri Valley and
Southern Great Lakes...
Nearly zonal flow this morning will gradually transition more to
NW flow later today as a shortwave digs into the mean longwave
trough across the eastern CONUS. This evolution will help push a
wavering surface front to the south while maintaining a NW to SE
axis parallel to the increasing flow aloft. A 300mb jet streak
coming out of the Pacific Northwest will flatten at the same time,
leaving favorable LFQ diffluence atop the front and into the
Northern Plains. The overlap of this diffluence with subtle height
falls ahead of shortwave impulses and their associated PVA, will
impinge into the weak baroclinic boundary to drive deep layer
ascent from eastern MT through the Mid-Missouri Valley. This
ascent will occur into favorable thermodynamics characterized by
PWs surging on return flow ahead of the front to 1.25 to 1.5
inches, approaching +2 standard deviations from the climo mean
according to NAEFS, and MUCAPE surging northward within a corridor
ahead of the front reaching 2000 J/kg. This impressive overlap of
ascent and moisture/instability will result in scattered to
widespread showers and thunderstorms with rain rates exceeding
1"/hr likely according to HREF neighborhood probabilities. 0-6km
mean winds of 15-25 kts suggest progressive storms, but with the
wind aligned to the front, some short term training is possible
which could result in isolated instances of flash flooding
necessitating a continuation of the MRGL risk with only subtle
cosmetic adjustments.

Although the overall flash flood risk is modest today, there is an
area of higher probabilities from eastern SD/NE into IA and far
northern MO. Here, both the baroclinic and instability gradient
will be most intense, supporting an axis of greater risk for an
MCS (or possibly two) to drop southeast into the higher moisture
and along the 500mb thickness gradient tonight. The guidance is in
good agreement that an MCS will form as a shortwave digs along
this gradient and interacts with 0-6km effective bulk shear of
40-50 kts. However, timing and placement is still uncertain which
is reflected by modest HREF EAS probabilities for 1" of rainfall.
While parts of NE/IA have been wetter than normal the past
14-days, soils appear generally hydrophilic as reflected by low
streamflow anomalies, so only the longest duration of heaviest
rainfall would likely result in scattered instances of flash
flooding. The greatest risk area for this according to the HREF
and GEFS is across western IA, so the SLGT risk was tailored to
best match these probabilities with some extension to account for
lower FFG/more sensitive soils.

Farther to the east towards Chicago, IL and Detroit, MI, some
aftn/evening showers and thunderstorms may develop along the front
as it continues to sag southeast into the area. There is a narrow
corridor both temporally and spatially for an overlap of
convergence for ascent along the front interacting with a rich
theta-e plume surging northeastward. Confidence is generally low
in evolution as any slowing of the front could reduce the
available moisture considerably. However, HREF probabilities have
increased for 1" of rainfall due to convection, and the mean flow
appears aligned to the boundary during this time. This could
produce some short duration training/backbuilding into the higher
theta-e plume to support an isolated flash flood risk.


...Southern Plains through the Gulf Coast...
A stationary front aligned west to east from the Southern Plains
through South Carolina will begin to lift north as a warm front
and weaken today. This evolution will be driven by an expanding
high pressure ridge centered over the Desert Southwest which will
elongate to the east to drive bulging heights along the western
and central Gulf Coast. The longwave trough that has been plaguing
the region the past few days will be displaced eastward during
this evolution, helping to additionally weaken the surface
boundary. The slow height rises combined with the weakening trough
and associated displacement of the upper jet streak will result in
weakened ascent today for the Gulf Coast region. Still, a corridor
of enhanced PWs of 2-2.25" overlapping MUCAPE of around 2000 J/kg
should allow for at least scattered thunderstorms today, with
storms moving slowly within the general weakly forced environment.
HREF probabilities suggest a good chance for 1-2"/hr rates in
thunderstorms that develop today, which through slow motions could
produce locally more than 3 inches of rainfall in some areas.

Although in general this area will see only an isolated risk for
flash flooding, there appears to be a subtle maximum in flash
flood risk due to higher convective coverage across northern
Louisiana. Here, some better backbuilding is likely as the weak
warm front lifts northward allowing storms to regenerate into
higher instability, which will enhance the duration of heavy rain
rates. Additionally, these aftn/eve thunderstorms could precede an
MCS (or possibly two) that is progged by the high-res to sweep
southeast along the increasing CAPE gradient tonight. In areas
that receive both leading thunderstorms and these MCSs, locally
more than 5 inches of rain is possible as reflected by the newest
HREF exceedance probabilities. While much of LA has been dry
recently noted by 14-day rainfall generally 50-75% of normal,
there is an axis in northern LA and far southern AR that has
received more than 150% of rainfall during this same temporal
window. This is also the area that has the greatest risk for the
heaviest rain today, and although HREF 3-hr FFG probabilities are
still only 20-25%, they have increased since the last run. After
coordination with SHV/JAN/LZK, a small SLGT risk area was added.


...Northern New England...
Lingering light to potentially moderate rainfall rates will
persist through the morning, primarily across far northern VT, NH,
and ME. This rain will be driven by a pivoting deformation axis in
the vicinity of a negatively tilted shortwave lifting into Maine,
and an occluded front connecting two surface low pressures also
shifting northeast through the morning. This shortwave and
accompanying occluded boundary will likely eject into Canada by
the late aftn, allowing for much drier air advecting in from the
west, noted by a rapid decrease in mid-level RH, to finally bring
an end to the area rainfall. PWs in the morning will still be
generally 1.25 - 1.5 inches, about +1 standard deviation above the
climo mean according to NAEFS, overlapping a corridor of 500-750
J/kg MUCAPE ahead of the front. These thermodynamics will support
at least brief rainfall rates of 0.5-1"/hr, but with generally
fast storm motions to the north. While the total coverage of
rainfall this morning appears to be limited, and total rainfall
will also likely be modest noted by HREF probabilities for 1"
peaking around 20-50%, this will be falling atop extremely
saturated soils and likely still ongoing flash flooding from
Monday's rain event. This suggests the inherited MRGL risk is
still needed as any additional rainfall could quickly become
runoff, but the overall threat will be isolated and ending by the
evening.


Weiss



Day 2
Valid 12Z Wed Jul 12 2023 - 12Z Thu Jul 13 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MID
MISSISSIPPI/LOWER MISSOURI VALLEYS, OZARKS, AND SOUTHERN GREAT
LAKES...

...Mid Mississippi Valley through the Great Lakes...
Consensus of the CAMs shows ongoing convection early in the period
across southern Wisconsin/northern Illinois. A mid-level
shortwave embedded within the base of a broad upper trough, along
with an upper jet couplet, are expected to help support storms
continuing from late in the Day 1 period through the early hours
of Day 2, with storms translating west-east into southern Michigan
by the afternoon hours, and possibly continuing into the
evening/overnight. Deepening moisture ahead of the wave (PWs
around 1.75 inches) along with the potential for training storms
are expected to contribute to a heavy rainfall/flash flooding
threat. A Slight Risk was maintained from southern
Wisconsin/northern Illinois through southern Michigan. Models
have shown a fairly consistent signal across this region, but have
shifted a bit farther to the north, which is reflected in the
Slight Risk. HREF neighborhood probabilities indicate that
accumulations of 2-3 inches can be expected within the Slight Risk
area, especially across southeastern Michigan.

Models have been less consistent farther to the southwest across
the mid Mississippi/lower Missouri Valley and Ozark regions.
There remains a signal for at least locally heavy rainfall to
develop, but the general consensus is farther southwest than
previous runs. A majority of the CAMs show the heaviest amounts
developing during the evening/overnight hours as convection
develops along the trailing front and propagates southeast across
the region. PWs are forecast to increase to 1.75-2 inches, with training/backbuilding storms contributing to the heavy rainfall
threat. While confidence in the occurence of heavy rainfall
remains fairly high, the run-to-run model variability continues to
contribute to lower confidence in placement. A Slight Risk was
maintained for portions of the region, but was restricted to areas
where the 12Z HREF showed higher probabilities for accumulations
exceeding 2 inches.

...Gulf Coast States...
Slow-moving thunderstorms are again expected near the Gulf Coast
on Wednesday, with two areas of focus possible. The first will be
across far southern Arkansas/northern Louisiana where a lingering
shortwave from Day 1 may persist across this region in the first
half of Day 2. Uncertainty is high as to how intense this will
still be as the LLJ veers Wednesday morning and instability gets
overturned, but lingering heavy rain atop soils likely saturated
from Day 1 rainfall necessitated an introduction of a Marginal
risk for this area. If the Day 1 amounts come to fruition and
the signal remains strong for additional heavy amounts on Day 2,
an upgrade to a Slight Risk may be required for portions of
southern Arkansas and northern Louisiana. Farther to the south
and east, from eastern LA through the northern FL peninsula, a
signal exists for some very slow-moving thunderstorms as Corfidi
vectors collapse to just 5 kts and align into the instability
offshore into the Gulf of Mexico to suggest some training.
Low-level inflow from the Gulf will keep PWs above 2 inches, with
some local backing possible as the remnant shortwave/MCV from
northern LA shifts southward. This could produce scattered
slow-moving thunderstorms with 2+ in/hr rain rates, leading to
locally more than 3 inches of rain on Wednesday.

...Southwest...
A Marginal Risk was added to portions of southeastern Arizona and
southwestern New Mexico. Anomalous moisture and weak steering
flow will contribute to slow-moving afternoon/evening storms that
may produce locally heavy amounts, with isolated flash flooding a
concern, especially across burn scar and urbanized areas.

Pereira/Weiss
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 120829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Wed Jul 12 2023

Day 1
Valid 12Z Wed Jul 12 2023 - 12Z Thu Jul 13 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE LOWER
MISSISSIPPI VALLEY...


...Lower Mississippi Valley and Gulf Coast States...
An impressive vorticity maxima associated with a potent shortwave
will be drifting SE from the Arklatex this morning. This feature
is progged to drift slowly southeast while weakening through the
day, but maintain a modest trough along the central Gulf Coast
with weak accompanying height falls. This shortwave will be the
primary mechanism for ascent today, but a dissipating surface
trough and potentially a lingering outflow boundary from other
convection could also serve as a focus for low-level convergence
and ascent. The environment will be extremely favorable for heavy
rainfall, however, with PWs 2-2.25 inches, +1 to +2 standard
deviations above the climo mean according to the NAEFS ensemble
tables, with overlapped MUCAPE of 2000 J/kg. These thermodynamics
acted upon by the mesoscale features will likely yield scattered
to widespread heavy rain producing convection today.

The guidance has become more aggressive with thunderstorm
potential today, and that led to the (then D2) slight risk upgrade
for LA/AR. The area of greatest risk for heavy rainfall will be
beneath the aforementioned vorticity max as it will likely
manifest as an impressive MCS with slow moving thunderstorms
possibly backbuilding along outflow boundaries to prolong
rainfall. There is still uncertainty as to how this will evolve
through the morning, but it is likely that as the LLJ of 20-25 kts
veers it will less efficiently upglide atop outflows and lead to a
reduction in coverage and intensity of convection. The timing of
this is uncertain, but it is likely there will be several hours of
moderate to heavy rainfall with rates 1"/hr or more before waning
late morning into early aftn. The remnant MCV that is likely to
develop will then serve as an additional focus for convection as
it shifts southeast through the day, providing additional rounds
of heavy rain as far as the Florida Panhandle. The inherited SLGT
was expanded to the Gulf Coast as Corfidi vectors of just 5-10 kts
aligned against the mean flow and into the higher instability
suggests a good potential for backbuilding storms, and with rain
rates of 2"/hr more, this could produce 3-5" of rain in some areas
as reflected by HREF neighborhood probabilities. This could result
in scattered flash flood instances despite relatively dry
antecedent soils, with locally significant flash flooding possible
across far southern AR/far northwest LA where event total rainfall
(from pre-12Z through the morning) could reach 10 inches.


...Mid Mississippi Valley through the Great Lakes and into the
Northeast...
A broad but expansive trough centered near the Hudson Bay will
maintain lowered heights and cyclonic flow into the eastern 2/3 of
the CONUS today and tonight. Within this cyclonic flow, multiple
shortwave impulses and accompanying vorticity maxima will rotate
west to east in conjunction with a 90-100 kt zonally oriented
upper jet streak to produce increasing deep layer ascent. This
synoptic lift will impinge into a stationary front that will be
draped west to east across the area and waver through the day.
This will not only result in locally enhanced convergence for
ascent, but may manifest as waves of low pressure in response to
the vorticity maxima rotating overhead. While the exact timing of
these shortwaves and any low pressure waves are a challenge to
resolve even on this time range, it is likely scattered to
widespread thunderstorms will be the result.

Thermodynamics will be quite impressive across a large portion of
this area, suggesting the widespread thunderstorms could all pose
a heavy rain threat. PWs south of the stationary front will reach
1.75-2 inches as low-level SW flow reaches 30 kts and impinges
into the front. This will supply the high PWs and collocated
MUCAPE of 1000-3000 J/kg northward to enhance the environment,
while also lifting isentropically atop the front and any residual
outflow boundaries. Within the widespread convection, there is a
signal for two areas of heaviest rainfall noted by the best
convergence of moisture transport vectors along the greatest
instability/PW gradients.

The first is draped west to east along the stationary front from
eastern IA through the thumb of lower Michigan. Here, convergence
of the LLJ into the front will help turn the 0-6km mean winds to
be parallel to the front, leading to an increased likelihood for
training of cells which the HREF indicates will have rainfall
rates of 1-2"/hr. This will likely result in waves of convection
repeating across the area. The high-res guidance is actually in
pretty good agreement and alignment for today, noted by HREF EAS
probabilities for 1" and 2" that are quite high from Chicago, IL
eastward to Detroit, MI and just north. Other than Chicago, this
area has generally been dry the last 2 weeks, which is reflected
by high FFG. However, training of echoes to result in locally more
than 3" of rain and some exceedance of the FFG, which could result
in scattered instances of flash flooding.

The other area of concern is around Missouri where the most
intense CAPE gradient and best moisture confluence will likely
help develop convection in the evening which will then grow
upscale into an MCS tonight. Some of the lead convection will
likely develop along a residual outflow from prior thunderstorms,
so uncertainty in the exact placement remains high. However, once
convection begins it should expand rapidly and organize through
30-40kts of effective bulk shear. The guidance again is insistent
that an impressive MCS will develop, and as Corfidi vectors
collapse to around 5 kts this suggests some training from NW to SE
with backbuilding also likely into the higher instability. Much of
Missouri has been dry recently which has maintained high FFG, and
the HREF exceedance probabilities are modest. However, training of
1-2"/hr rain rates could still produce 3-4" of rain in some areas
as noted by the HREF probabilities, and the SLGT risk was adjusted
just cosmetically for updated guidance.


...Southwest...
The inherited MRGL risk for southeast AZ and far southwest NM was
maintained, although the signal for excessive rainfall is modest.
The difference today from the prior few days is that the area has
received rain recently, so there are pockets of 7-day AHPS
rainfall that are 100-150% of normal. FFG is still quite high, but
does feature some pockets of 0.75-1"/1hr, and with PWs nearing +1
standard deviation according to NAEFS, the HREF suggests at least
a 15-20% for 1"/hr rates. With storm motions progged to be just
around 5 kts, and redevelopment probable on subsequent outflow
boundaries in the pulse environment, isolated flash flooding is
possible, especially in any urban areas.


Weiss


Day 2
Valid 12Z Thu Jul 13 2023 - 12Z Fri Jul 14 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OHIO VALLEY/CENTRAL APPALACHIANS AND THE INTERIOR NORTHEAST...


...Mid-Mississippi Valley through the Central Appalachians and
Northeast...
Broad mid-level NW flow will continue across Plains and
Mississippi Valley, lifting into more SW flow across the
Appalachians and Northeast as the broad and expansive mid-level
trough centered near the Hudson Bay persists. This will provide an
environment waves of convection moving through the flow. Most of
this development on Thursday will occur along a slow moving cold
front, and leading pre-frontal trough, that will become elongated
as it approaches the base of the flattening but still broad and
expansive trough, with locally enhanced ascent occurring
downstream of weak impulses through PVA and height falls. Overall
deep layer ascent appears modest on Thursday, so convection should
be generally confined to the front and any mid-level shortwaves,
suggesting a generally focused enhanced rainfall risk. Despite
modest overall ascent, the thermodynamic environment will be quite
favorable, so convection that does develop should be efficient
rain makers. PWs will rise to 1.75 to 2 inches on W/SW 850mb
inflow of 20 kts, with MUCAPE surging during peak heating south of
the front to as high as 3000 J/kg, with the highest ribbon
positioned from Arkansas through Ohio.

While there is some uncertainty into exactly where the strongest
convection will develop due to relying on residual outflows from
D1 and timing of shortwaves aloft, the trend has been for activity
to push a little south from previous across the MS VLY, and is now
weaker/more progressive, prompting a removal of the SLGT from
Missouri.

However, convective trends among the global models has become a
bit more organized and intense from the Ohio Valley into New
England. Although much of the activity Thursday, which should be
widespread, will be generally progressive, effective bulk shear of
30-40 kts will allow for storms to organize into multicells and
then train to the northeast, with additional enhanced ascent
occurring where flow can favorably upslope into terrain. The
favorable thermodynamics across this region will support rain
rates of 1-1.5"/hr, which through training could produce 2-3" of
rain or locally higher amounts in some areas. This is reflected by
at least modest 3"/24hr probabilities in the global ensembles, and
some higher focused 3"/12hr probs from the HREF. While confidence
in the exact evolution and development is modest, this region is
still quite saturated from recent rainfall that is around 150% of
normal in the eastern OH VLY, and 300-600% of normal in New
England/Upstate NY, leading to well above normal streamflow and
low FFG. The SLGT risk areas were drawn to best reflect the
highest QPF probabilities overlapping some of the most sensitive
soils.


...Central Plains...
Broad NW flow on the upwind side of an expansive mid-level trough
will drive multiple impulses through the flow and down across the
region on Thursday. These impulses will combine with modest upper
diffluence within the tail of a departing jet streak to the east,
and convergence along a weakening front/surface trough moving
across the area. A departing MCS in the morning could also produce
some priming rainfall, but the greater risk for excessive rain
will occur in the evening/at night. During that time, ESE 850mb
inflow will surge to 20-30 kts as a wave of low pressure develops
across the High Plains along a stationary front over Colorado.
This will draw PWs above 1.5 inches northward, combined with
resupply of MUCAPE approaching 2000 J/kg. This moist ascent will
drive a theta-e ridge northward, supporting expanding convection
along a surface trough as another impulse moves overhead. Timing
of the impulse and its affect on the impinging surface boundary is
in question, and if they do not overlap the coverage of storms
could be less. However, the ensembles suggest a high probability
of at least 1" of rain from near the Raton Mesa northward through
northern Iowa, with training of intense rates possible. This rain
occurring atop compromised FFG from recent heavy rainfall could
produce a few instances of flash flooding.

Weiss

$$
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FOUS30 KWBC 131944
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
343 PM EDT Thu Jul 13 2023

Day 1
Valid 16Z Thu Jul 13 2023 - 12Z Fri Jul 14 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR
PORTIONS OF CENTRAL MISSISSIPPI/WEST_CENTRAL ALABAMA AND SOUTHERN
VERMONT...

...Northeast...
Only minor changes were made across the Northeast and New England
for this update as guidance continues to depict a strong line of
thunderstorms capable of containing 1-2"/hr rainfall rates. For
the most part this line will be progressive across northern New
York and Vermont, but sufficient moisture and instability are
likely to allow for quick 1-3" totals that exceed low FFG. For the
Moderate Risk area in and near southern southern VT, there is
potential for rounds of heavy rain to overlap very saturated
terrain leading to greater chances for flooding impacts. This
potential is particularly seen in 12z HREF neighborhood probs for
1-hr FFG exceedance centered over southwest VT by 00z tonight.

The overall setup includes mid-level northwest flow will continue
across the Great Plains and Mississippi Valley, lifting into more
southwest flow across the Appalachians and Northeast as the broad
and expansive mid-level trough centered near the Hudson Bay
persists. This will provide an environment allowing waves of
convection moving through the flow. Most of this development on
Thursday will occur along a slow-moving cold front and leading
pre-frontal trough. PWs will rise to 1.5-1.75" on W/SW 850mb
inflow of 20 kts, with SB CAPE surging during peak heating south
of the front to as high as 2500 J/kg. The Moderate Risk area has
the potential for overlapping of sufficient moisture, instability,
and mean southwest flow parallel to the associated frontal
boundary promoting possible brief cell training. The training
potential has diminished in recent guidance, but there still is
potential for the QLCS to slow along the southwest flank from
southern VT, western MA, and northern CT by tonight. Much of
Vermont and New England remains vulnerable due to recent extreme
rainfall, where areas of 300-600% of the seven day average
rainfall has fallen. The above ingredients support 2"/hr totals,
with locally higher amounts possible if training occurs. Within
the Moderate risk area, expect several instances of flash flooding
and impacts due to rapid runoff where terrain cannot adequately
absorb heavy rain.


...Mid-South to Central Gulf Coast...
A Moderate Risk remains over central MS and was expanded into
parts of west-central AL due to recent radar and CAM trends. MRMS
estimates 8-12" of rain already occurred this morning along the
AL/MS border to the east of Meridian, AL. Very intense rainfall
rates of 2-3.5" can be found within the training bands of
convection, which can easily add up to over 5" total rainfall
within an hour or two. Areas underneath these extreme rainfall
rates can expected rapid inundation and significant flash
flooding, even in areas where flooding is uncommon. See MPD 714
for more information.

A previous MCV that was initially the catalyst for the efficient
rainfall producing thunderstorms over MS and AL remains over the
area as a separate shortwave approaches from the NW and is acting
to further invigorate approaching updrafts. The low level inflow
per JAN's 12z sounding is roughly double the mean 850-400 hPa
wind, which is very efficient from a heavy rain perspective and
generally a feature of warm core lows, with a freezing level up to
almost 16k feet. Additionally, this part of the country typically
has plenty of atmospheric moisture content, but PWs of 2-2.5" are
into the 90th climatological percentile. It is expected for
ongoing activity to continue over the current area of central
AL/MS for at least a few more hours until the approaching
shortwave from the NW shifts the activity further south this
afternoon. As the shortwave continues progressing from the
Mid-Mississippi Valley and LA, the threat for excessive rainfall
during the afternoon and evening hours on Thursday is expected to
shift towards the north-central and northeast Gulf Coasts. Here,
the 12z HREF probabilities of at least 3"+ and 5"+ in 24 hours
continue to support a Slight Risk, which was somewhat reconfigured
based on the latest guidance. Some guidance hints at very heavy
rain occurring over the western FL Panhandle tonight in sort of a
similar fashion to the ongoing AL/MS activity, but there still
remains far too much uncertainty regarding specifics in this
region.


...Mid-Mississippi Valley through the Central Appalachians...
Convective trends among the global models and CAMs has become a
bit more organized and intense from the Ohio Valley into the
central Appalachians. Although much of the activity Thursday,
which should be widespread, will be generally progressive,
effective bulk shear of 30-40 kts will allow for storms to
organize into multicells and then train to the northeast, with
additional enhanced ascent occurring where flow can favorably
upslope into terrain. The favorable thermodynamics across this
region will support hourly rain totals to 2", which through brief
training could produce 2-4" of rain or locally higher amounts in
some areas, which would be particularly problematic in the coal
fields of WV and KY. Confidence in the exact evolution and
development remains modest. Spotty areas have seen above normal
rainfall over the past week in this region. The Slight Risk was
maintained from the previous outlook. By the overnight hours,
developing activity progressing over the Appalachians may make it
into central VA (similar to recent HRRR runs). The airmass in
place will support heavy rain as PWs increase to around 1.75", but
too much uncertainty remains to warrant a risk increase.


...Central to Southern High Plains...
Broad northwest flow on the upwind side of an expansive mid-level
trough will drive multiple shortwaves down across the region on
Thursday. These impulses will combine with modest upper diffluence
within the tail of a departing jet streak to the east and
convergence along a weakening front/surface trough moving across
the area. A departing MCS in the morning could also produce some
priming rainfall, but the greater risk for excessive rain will
occur in the evening/at night. During that time, east-southeast
low-level inflow will surge to 20-30 kts as a wave of low pressure
develops across the High Plains along a stationary front over
Colorado. This will draw PWs above 1.5 inches northward, with
values as high as 1-1.25" in the High Plains of CO, combined with
resupply of MUCAPE approaching 2000 J/kg. This moist ascent will
drive a theta-e ridge northward, supporting expanding convection
along a surface trough as another impulse moves overhead. Timing
of the impulse and its affect on the impinging surface boundary is
in question, and if they do not overlap the coverage of storms
could be less. The Marginal Risk area captures the heavy rain
signals seen in the 12z guidance, which remain scattered with not
great agreement in placement. Hourly rain totals to 2.5" and
local amounts to 4" are possible wherever cells can train/merge or
mesocyclones can form. Some areas in the Marginal Risk have seen
greater than 600% of their average seven day rainfall, so soils
should be rather saturated. Slight Risk impacts cannot be ruled
out.


...Near the southern AZ/NM border...
Precipitable water values southern AZ remain 1.5"+ this period,
with rather weak flow, implying slow cell motions for any
thunderstorms that might develop. ML CAPE should rise to 1000+
J/kg. Temperatures at 700 hPa are near 15C around the time of
convective initiation, implying modest mid-level capping. This
should allow for isolated to widely scattered thunderstorms with
locally heavy rainfall, like has been seen over the past few days.

Snell/Roth


Day 2
Valid 12Z Fri Jul 14 2023 - 12Z Sat Jul 15 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN NEW ENGLAND, THE MID-ATLANTIC, AND FROM THE MID-SOUTH TO
THE GULF COAST...

...Northeast and Mid-Atlantic...
The Slight Risk was mostly maintained, but expanded across
southeast VA and into central NC with the 20z update. A weakening
frontal boundary and pooling anomalous moisture creates conditions
ripe for scattered instances of heavy rain and flash flooding
potential. Continued troughing across the eastern Lower 48 due to
a deep parent upper low over south-central Canada will be
reinforced by shortwaves embedded within the cyclonic flow which
pivot northward on Friday. One of these shortwaves is progged to
lift across New England early in the period, helping to drive a
weakening cold front eastward along the Mid-Atlantic and
Northeast. Ahead of this cold front, a warm front is progged to
surge north into Canada, leaving impressive thermodynamics within
the warm sector across the Northeast. SW flow within this warm
sector is progged to reach 20-25 kts, advecting anomalous PWs of
1.75" and MUCAPE of 1000-2000 J/kg northeastward. As a surface
trough sharpens during the afternoon, coinciding with peak
instability, widespread showers and thunderstorms should develop
and then lift northeast along this trough with rain totals up to
2"/hr. Aligned mean winds indicate localized training is likely,
lengthening the duration of rain rates which will otherwise be
limited by 20-25 kts of storm motions, though erosion of
instability should allow convection to shift somewhat with time.
This training could result in an axis of 2" of rainfall, with
local amounts of 4". The antecedent soils are saturated across New
England. These vulnerable soils indicate that any heavy rainfall
will quickly become runoff and may lead to renewed flash flooding,
as well as an elevated potential near more urban areas. Although
uncertainty remains with how convective evolution will occur
Friday, the Slight Risk was adjusted to match the best 12z model
consensus for heavy rainfall. Better certainty across New England
will exist after early morning activity comes to an end, as any
lingering mesoscale boundaries could focus heavy rain along the
urbanized region between CT and central MA. It is this region
where the 12z HREF highlights the best chances (40-60%) for at
least 3" of rain and the potential for scattered flash floods.

Farther south along the Mid-Atlantic coast and central NC. A pre
frontal trough and lingering boundary from possible Thursday night
activity will help spark slow-moving afternoon thunderstorms
within a moisture rich environment. PWs of 2-2.4" and SB CAPE

2000 J/kg will allow for efficient rainfall. These storms will

also occur within 20 kts of southwesterly low-level flow, possibly
limiting their southeast progression enough to produce 3-5"
rainfall totals. 12z HREF supports this idea with scattered 15-30%
neighborhood probabilities for at least 5" in 6 hours, with the
threat diminishing after 03z.


...Central Plains, Midwest, Mid-South and Central/Eastern Gulf
Coast...
The previous Slight Risk areas extending from eastern KS to the
Gulf Coast were adjusted to highlight where confidence is highest
for scattered instances of flash flooding and intense rainfall
rates associated with slow-moving storms. The area extending from
KS/OK to northern AR may have activity entering the area Friday
morning that could warrant a targeted upgrade, but most flash
flood risk throughout the Plains, Midwest, and Mississippi Valley
is too uncertain or localized to maintain the Slight Risk at the
moment. However, confidence is higher that intense rainfall may
fall over saturated soils from parts of the Mid-South to the
central/eastern Gulf Coast.

For the Slight Risk area, a residual MCV that impacted parts of
AL/MS on Thursday with significant rainfall is expected to drop
southeastward overnight and center near the FL Panhandle on
Friday. Even though this feature is expected to weaken overall,
very high PWs and 2000-3000 J/kg MUCAPE when combined with weak
steering flow will aid in localized heavy rain totals. The forcing
for the eventual storms is most in question, whether it be the MCV
itself or aided by lingering outflow boundaries or a sea breeze,
but the overall environment and CAM guidance supports the SLGT
designation. The ingredients suggest that hourly totals to 3",
with local amounts to 6", would be expected. Some guidance shows
the potential for local 8" amounts in or near the region, which
cannot be ruled out given the history of this MCV as it has pushed
from the Lower Mississippi Valley on Wednesday. With very
saturated soils along the MS/AL border from 2-10" rainfall totals
on Thursday, the SLGT was also extended in this direction to
account for overlap with elevated 12z HREF probs for at least 3"
of rain.

Through the central U.S. and Midwest, thunderstorms will be driven
by shortwaves and associated vorticity maxima rotating eastward in
northwest to west mid-level flow, impinging upon an elongated cold
front draped across the Mid-Mississippi Valley. The shortwave
interacting with this front will drive impressive ascent into
favorable thermodynamics characterized by PWs rising to ~2"
co-located with a corridor of 2000-3000 J/kg MUCAPE. The 850mb
inflow out of the SSW at 20-30 kts ahead of the approaching cold
front from the NW will robustly resupply these thermodynamics into
the area, and as effective bulk shear climbs to 50 kts Friday
night, organized convection, possibly a potent MCS, will move
across the area. Due to the inherent difficulty in forecasting the
placement of these MCSs and the potential for fast forward
prorogation, opted to go with a Marginal Risk rather than a broad
SLGT and highlight the potential for localized flash flooding. If
hires guidance becomes more aligned with an area that could see
overlapping storms or training, a targeted SLGT is possible.


...In and near southern NM...
Precipitable water values rise toward 1.75" in southern AZ and the
low-level flow picks up out of the west-southwest as 700 hPa
troughing develops across NM. However, temperatures at 700 hPa
are on the rise, with the atmosphere modestly capped, which
despite the available moisture should keep thunderstorm coverage
isolated to widely scattered similar to previous days. A Marginal
Risk of excessive rainfall was maintained to cover the potential
for hourly totals to 2" where cells manage to merge and/or train.

Snell/Roth
$$
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FOUS30 KWBC 141612
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1211 PM EDT Fri Jul 14 2023

Day 1
Valid 16Z Fri Jul 14 2023 - 12Z Sat Jul 15 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER HUDSON VALLEY THROUGH WESTERN AND CENTRAL
MASSACHUSETTS...

...16Z Outlook Update...
The ongoing forecast is generally on track, although several
changes have been made based on morning convective trends and 12Z
model guidance.

...Northeast...

A very small northwestward adjustment to the Moderate Risk area
was made, and a westward expansion of the Slight was made across
northeastern Pennsylvania. Through the remainder of the day,
models focus most convective development from northeastern PA
through southern NY, with lower chances of storms farther
southeast across Connecticut in the wake of morning convection in
that area. The categorical risk adjustments are also consistent
with model trends. Areas of 1-2 inch/hr rain rates and
backbuilding/training could affect especially the Moderate Risk
area and spread eastward from northeastern PA into the NYC Metro
later this afternoon and early evening.

...Mid-Atlantic...
Robust convective development across central/eastern North
Carolina this morning has left behind an outflow boundary across
that area, leaving some doubt regarding convective coverage in
eastern Virginia this afternoon. Surface dewpoints are still in
the 70-76F range in eastern Virginia though, and any sunshine in
the wake of morning rainfall could destabilize the area and cause
a few storms to develop. With the greater focus for convective
development now evident across central/eastern NC per 12Z guidance
and surface observations, the Slight Risk has been adjusted to
account for better flash flood potential in NC today. It is worth
noting that HREF QPF totals of 1-3 inches across southern/central
North Carolina could materialize across areas that have received
copious amounts of rainfall this morning. This area will be
monitored for the possibility of a targeted Moderate Risk upgrade
should a corridor of training convection materialize for a few
hours this afternoon.

...Mid-Mississippi Valley to the Florida Panhandle...
A persistent MCS continues to result in localized flash flood
potential across coastal areas of the Florida Panhandle this
morning. Latest CAMs/observations suggest that the ongoing MCS
could backbuild westward toward southern Alabama, while additional
deep convection develops to the north toward the MS/AL border
area. Convective mergers and training associated with these
features could result in locally heavy rainfall totals - perhaps
exceeding 5 inches in a few spots. This area will also be
monitored for the possibility of a targeted Moderate risk upgrade
especially if one or two axes of extreme (2.5+ inch/hr) rain rates
can persist for multiple hours. It is worth noting that models
redevelop storms near the central Florida Panhandle between 06-12Z
that could also produce locally heavy rainfall and rain rates
exceeding 2.5 inches/hr, though that rain should occur across
areas of fairly high FFG thresholds (3+ inch/hr), suggesting that
multiple hours of heavier rates would be needed for any widespread
flash flood potential.

Another area of concern exists from the MS/OH River confluence
eastward into western Kentucky. Recent CAMs suggest that
localized training potential could materialize there as upstream
MCS activity reaches the region after 06Z tonight. The timing and
extent of convection is a bit uncertain, although the potential
was high enough to expand the Marginal area eastward and include a
Slight Risk for this update.

Cook

...Previous Discussion...

...Northeast...

Confidence continues to increase in the potential for even more
flash flooding in the hard-hit areas of the lower Hudson Valley
through the Berkshires and into central Massachusetts. Additional
heavy rainfall occurred through this exact region overnight, and
flash flooding is ongoing across portions of the lower Hudson
Valley and southern Catskills as training and backbuilding storms
develop and move across that area. These storms are expected to
largely dissipate over the next few hours, but given their
longevity so far, that remains to be seen. In fact, recent radar
trends have been showing additional shower and thunderstorm
formation across the Catskills and into the Capital Region. Around
noon today, the second round of storms, for which the Moderate
Risk is in effect, is expected to begin from northeastern PA
through southeastern NY and northern NJ. Each individual storm
cluster is likely to be moving at a reasonably fast clip. However,
the combination of very favorable antecedent conditions (flash
flooding is ongoing now in many of these areas!), and then this
second round, which is actually likely to consist of several
rounds of storms generally forming around the Catskills/Poconos
then moving off to the north and east into New England.

A cold front moving across New York and Pennsylvania will push
east today as a warm front races north across New England, leaving
much of New England in the warm sector, consisting of PWATs to
1.75 inches. An upper level shortwave across Ohio and the
Virginias will rapidly lift northeastward today which will add to
the atmospheric lift. With daytime heating, most unstable CAPE
will rapidly increase locally to as high as 3,000 J/kg, and more
broadly to 2,000 J/kg across the lower Hudson Valley and southern
New England, which may allow for storms to begin even as early as
late morning. Regardless, this will be plenty of fuel for storms
to produce rainfall rates as high as 2 inches per hour. With
training and backbuilding likely due to largely parallel mid-level
flow to the cold front, local amounts could approach 6 inches in
the hardest hit areas, which again are very likely to be the same
areas that have much lower FFG values from rainfall both in the
recent past and ongoing.

The Moderate Risk was coordinated with all of the impacted
forecast offices: BGM/Binghamton, NY, ALY/ Albany, NY, OKX/Upton,
NY, BOX/Norton, MA, and GYX/Gray, ME.

...Mid-Atlantic...

Multiple rounds of training showers and thunderstorms are ongoing
across southern VA and northern NC. The training storms moving
east along an east-west oriented boundary resulted in numerous
flash flood warnings from far southern WV through southern VA
overnight. A moisture plume consisting of PWATs approaching 2.25
inches will be drawn northward ahead of a cold front also moving
across the Northeast. CAMs guidance suggests central NC will be
the primary area that will see strong storms capable of 2+
inch/hour rainfall rates this afternoon. However, given the
sensitivity of the soils across southwest VA, this area was
included in the newly expanded Slight risk, despite somewhat lower
signal for heavy rains later today. Storms will form in this
highly favorable environment around midday today across central
and western NC, likely including somewhat lower coverage into
south central VA. As the storms move east into the evening, storm
mergers are likely as the storms that form along the mountains in
western NC catch up to the slower moving storms across central NC,
resulting in a greater likelihood of flash flooding across NC,
where fortunately FFGs are at least somewhat higher. Regardless,
given the impressive amount of moisture these storms will have at
their disposal, the heavy rainfall rates are likely to locally
exceed FFGs.


...Mid-Mississippi Valley to the FL Panhandle...

Yet another pair of MCSs are ongoing across the far northern TX
Panhandle and northeastern OK this morning. The storms towards the
Panhandle are expected to gradually weaken as they move into
western OK, while the storms over northeastern OK are expected to
grow upscale into a larger MCS across AR by midday, tracking into
MS and TN during the afternoon. As second round of storms starting
the day in NE will race southeastward into AR overnight and
eventually weaken across northern MS early Saturday morning. These
repeated rounds of storms, some of which are expected to move over
hard-hit areas of eastern MS and far western AL, are expected to
cause widely scattered flash flooding across eastern OK, AR and
into MS. Thus, the Slight Risk area inherited was expanded well to
the west to include central OK. Depending on how much agreement
can occur between the CAMs regarding tonight's convection, further
westward expansions into the Panhandles may be needed with a new
round of showers and thunderstorms that may develop there late
tonight.

The Slight risk area extends down to the FL Panhandle for storms
that are developing now and will persist through the morning.
Close proximity to the very warm Gulf will supply the storms with
an abundance of moisture, and the slow-moving storms could back
build or have a second round move through from the inland MCS's
weakening as they move towards the area. FFGs in this area are
rather high, so the higher flash flooding threat is likely further
inland near the MS/AL border due to yesterday's very heavy rains,
but flash flooding is possible, particularly in urbanized areas
around Pensacola.

Wegman


Day 2
Valid 12Z Sat Jul 15 2023 - 12Z Sun Jul 16 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR IN AND NEAR
NORTHEAST NEW MEXICO & THE OKLAHOMA/TEXAS PANHANDLES...

...Northeast NM/TX and OK Panhandles...

Model agreement has deteriorated somewhat regarding the likelihood
of an MCS forming near or over the Panhandles late tonight. Thus,
the Slight Risk is a lower-confidence one. However, given storms
that are ongoing now and the likelihood of another MCS forming at
the end of Day 1, should a stronger one form Saturday night, as
some of the CAMs suggest, then the potential it could cause flash
flooding is certainly there and would fall in the Slight category
as regards the coverage of flooding it could produce. Due to the
uncertainty as to whether or not it will form, the Slight risk was
maintained for this forecast cycle with few changes with
continuity being the primary driver of the forecast.

...US from the Mississippi East...

A weak and highly diffuse signal for general showers and
thunderstorms continues across almost the entirety of the eastern
half of the country east of the Mississippi River. Based on the
CAMs that extend through the first half of the period for the day
on Saturday, it's likely that one or two lines of storms will
cross the country from west to east, but will be moving at a rapid
pace with little thunderstorm activity between the lines of
storms. This weather pattern favors a very large area of mostly
light daily rainfall totals. However, given the abundance of
moisture that will continue to be advected northward ahead of
these storms, they could still easily grow strong enough to
produce locally heavy rainfall totals to 2 inches. The areas most
likely to see flash flooding are those with the most favorable
antecedent conditions and low FFGs. Since there is little to no
signal for repeating or training storms, it could not be
pinpointed where any higher rainfall threat will be located on
Saturday, and thus no upgraded areas were introduced with this
forecast update. Should a better signal arise in portions of the
Marginal Risk area, a localized Slight is quite possible given the
abundant moisture streaming northward ahead of any lines of storms.

Wegman

$$
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FOUS30 KWBC 150826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Sat Jul 15 2023

Day 1
Valid 12Z Sat Jul 15 2023 - 12Z Sun Jul 16 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHEAST NEW MEXICO, THE TEXAS PANHANDLE, CENTRAL APPALACHIANS,
THE UPPER SUSQUEHANNA RIVER BASIN, AND NORTHERN NEW JERSEY...

...Eastern Half of the Country...

A digging upper level low starting out the day today over northern
Manitoba will dig southward, reaching the US/Canadian border in MN
late tonight. An upper level wave rotating around that low and its
associated cold front will sweep eastward across the Midwest
today. The convection ongoing from MI southwest to IL is occurring
partially because of that upper level forcing. It is expected to
sweep eastward along with its forcing across the Midwest and into
the Mid-Atlantic tonight. PWATs ahead of this convection will
advect northward out of the Southeast, rising to between 1.5 and
1.75 inches broadly across the Northeast. This will provide plenty
of moisture and instability for the storms as they cross the
Midwest. Fortunately, as has been the case in recent days, the
guidance remains consistent that the storms will be quite
progressive, so no one area is expected to see excessive amounts
of rain. Thus, the large Marginal Risk area across almost the
entire half of the country east of the Mississippi remains in
place. After sunset this evening, the storms will be into the
Mid-Atlantic, but weakening due to the loss of solar heating.
However, the upper level flow will favor more northward movement
of the storms with a slowing eastward progression. This will
result in a larger area of light to moderate rainfall to develop
into PA and NY, resulting in a longer-duration but lighter
intensity of rain. This too will favor generally low chances of
flash flooding. The two Slight Risks generally over the Virginias
and again over PA/NY are largely in place owing to the much lower
FFGs in those areas, as the areal rainfall totals will be similar
to areas outside the Slight Risk areas.

Across the Southeast, the MCS starting out the day over MS will
devolve into a broken line of widely scattered storms, which with
plenty of available atmospheric moisture will support continued
convective development both ahead of and behind that line of
storms, as the real air mass change largely stays north from the
TN Valley north. As there's little indication that the storms will
be able to organize today, largely due to lack of forcing, the
Marginal Risk area remains across the Southeast. Somewhat heavier
rains are possible across the FL Panhandle today, where sea breeze
convection may merge with advancing storms from the north,
resulting in locally heavier rainfall totals. As FFGs are also
quite high here, especially over the eastern Panhandle, which
guidance is in good agreement will see the bulk of the heaviest
rainfall today, a high-end Marginal remains in place, which may
need to be upgraded to a localized Slight with future forecast
updates.

...Southern High Plains...

Storms ongoing as of the time of this writing over much of the
Slight Risk area from northeastern NM into northwest TX are
helping prime the soils and lower FFGs to increase the flash
flooding risk when an MCS develops this afternoon into this
evening across this region. A complex of storms will develop over
northeast NM, then turn eastward as it moves over the southern TX
Panhandle, following the upper Red River basin on the TX side of
the river overnight tonight. The guidance has shifted south and
west with the axis of heaviest rain associated with this MCS,
which is in keeping with typical MCS behavior where the south and
western sides of the MCS are more likely to get held up and move
more slowly than the rest of the complex of storms. The Slight
Risk area was shifted south and west, removing all of OK and much
of the northeastern TX Panhandle from the Slight with this update.

Wegman


Day 2
Valid 12Z Sun Jul 16 2023 - 12Z Mon Jul 17 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF NEW
ENGLAND, EASTERN NEW YORK, NORTHERN NEW JERSEY, AND NORTHEASTERN PENNSYLVANIA...

...Northeast...

A potent cold front that starts out the day near the 80 W meridian
will push east through the Mid-Atlantic and into western New
England by early Monday morning. The front will be supported by a
potent upper level low over the Great Lakes, which will swing a
potent upper level disturbance eastward into the Northeast.
Meanwhile, a highly anomalous moisture plume with PWATs exceeding
2 inches, or up to 3 sigma above normal, will advect northward
into New England ahead of this front. Ample moisture and forcing
will set the stage for widespread moderate rain, with embedded
heavier convective elements likely. CAMs guidance is in okay
agreement that steady rain well ahead of the front will be ongoing
right at the start of the period Sunday morning across western
portions of the Moderate risk area in PA/NY/NJ. As the morning
goes on, convection across southern areas of the Moderate Risk
area will increase in coverage with daytime heating, though
convective elements are likely to persist into northern New
England with steadier rain in between the convective elements that
far north. This will be due to the influx of additional Atlantic
moisture into New England. Nearly all of the Moderate Risk area is characterized by very sensitive soils and low FFGs due to numerous
recent previous days with heavy rainfall and flooding. The
addition of multiple hours of steady rain, which will be heavy at
times, is likely to initiate new flash flooding and worsen ongoing
river flooding. During the afternoon, the first round of rain will
move into eastern New England, but a secondary line of heavy
showers and thunderstorms is expected to develop over NY and NJ
and follow the first line eastward behind it. While this second
round should be progressive enough to keep the duration of heavy
rain rather short, the brief time frame in between rounds of rain
may cause additional flooding to result from this second line.
After sunset, the rain will continue slowly pushing eastward to
coastal New England and Maine, while the second line largely
dissipates with the loss of solar heating. Much of the event
should be over by around midnight except across eastern ME.

The Moderate Risk area was expanded in several regions. First, it
was expanded south and west into far northeastern PA and much of
northern NJ, owing to the flood sensitivity resulting from the
training thunderstorms over the area a few hours ago. While
there's added uncertainty as to how far south and west more
widespread storms capable of heavy rainfall rates will get, the
very wet soils in the area may make up for that uncertainty
resulting in scattered flash flooding into the Catskills and
eastern Poconos. The Moderate was also expanded northward to
include more of northern VT given the increased signal in the
guidance, along with storms that caused flash flooding yesterday
evening as well. Finally, the Moderate was also expanded to
include all of coastal CT and much of RI due to increased signal
in the guidance there as well. There remains significant
uncertainty as to how much rain will fall over the NYC area and
Long Island, as southerly onshore flow off the Atlantic may hinder
instability and prevent stronger storms from developing until
they're further inland. Thus, NYC and Long Island remain in a
high-end Slight Risk for now. The Moderate was shrunk a row of
counties westward away from the MA, NH and ME coastline for
similar reasons where proximity to the ocean may hinder stronger
storms and thus, higher rainfall totals from being realized. The
surrounding Slight Risk area remains largely the same.

Elsewhere the Marginal Risk area was shrunk from the west to
remove western NY, PA, MD, and VA with better agreement that the
rainfall will be largely east of those regions by the start of the
period.

...Gulf Coast...

No significant changes expected along the Gulf Coast as the
trailing end of the stalled front helps focus additional
convection over southern MS, AL, GA, the FL Panhandle and eastern
LA. Isolated flash flooding is possible where any storms may
locally organize, though there is little confidence on exactly
where that will happen.

...Lower Missouri River Valley...

No significant changes on the potential for an MCS to drop locally
heavy rains from KS through western IN Sunday afternoon. With a
few urban centers in the area, there is local potential for more
than isolated flash flooding, particularly near Kansas City, but
confidence is too low at this time for any localized upgrades,
though they remain possible.

Wegman

Day 3
Valid 12Z Mon Jul 17 2023 - 12Z Tue Jul 18 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE
DAKOTAS THROUGH THE MID-MISSISSIPPI RIVER VALLEY, OHIO VALLEY, AND
NORTHERN MID-ATLANTIC, AS WELL AS MUCH OF THE FLORIDA PANHANDLE,
CENTRAL GULF COAST, AND SOUTHEAST ARIZONA...

An advancing warm front driven by a developing LLJ Monday night
will cause heavy rain to break out across the Plains into MO as a
moisture-rich air mass runs into a stalled out front. Further east
into OH and into PA/NY, a slow-moving MCS over the lower OH Valley
will track up the valley and into the Mid-Atlantic along that same
front, though the precipitation will be on the eastern/cold
frontal side. A Slight risk upgrade may eventually be needed,
particularly from NE through MO but confidence and weak signal has
precluded that for now.

The southern end of a stalled out front and abundant moisture will
increase convective coverage over much of the FL Peninsula,
particularly the eastern half. The storms will have potential to train/backbuild as they'll be very slow-moving, as is typical with
a stalled out front. PWATs will remain as high as 2.25 inches so
the storms will have plenty of moisture available for high
rainfall rates of up to 3 inches per hour.

A small Marginal risk area was added for the central Gulf Coast,
particularly southeast LA for occasional storms tracking eastward
from LA out into the Gulf. There's reasonable agreement among the
guidance that storms may be more concentrated in and around New
Orleans.

Finally, monsoonal flow into southeast AZ may cause isolated flash
flooding in sensitive and urbanized areas from isolated
thunderstorms.

Wegman
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 160834
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Sun Jul 16 2023

Day 1
Valid 12Z Sun Jul 16 2023 - 12Z Mon Jul 17 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF NEW
ENGLAND, DOWNSTATE NEW YORK, NEW JERSEY, THE PHILADELPHIA AREA OF
PENNSYLVANIA, AND NORTHERN DELAWARE...

...Northeast...

A potent cold front and attendant negatively-tilted upper level
shortwave has caused a large area of rainfall to develop over the
Mid-Atlantic from the Delmarva through New York State. The showers
and thunderstorms are moving over a large flood-sensitive area
from northeastern PA through much of New England, which is causing
renewed flash flooding over western portions of the Moderate Risk
area. As the individual thunderstorm cells track northeastward,
new thunderstorms are expected to develop around midday from the
Eastern Shore and the Delmarva northeast across much of the
Moderate Risk area. This will further worsen any ongoing flash
flooding, as well as cause additional flash flooding to develop.
Besides the highly favorable antecedent conditions, a plume of
anomalously moisture-rich air is being advected northward ahead of
the cold front. PWATs may exceed 2.25 inches along the immediate
Jersey coast northeastward through coastal New England, which in
some areas is over 3 sigma above normal for mid-July. Since July
is the warmest month of the year climatologically, and warm air
can hold the most moisture, an anomaly 3 sigma above normal is
highly unusual. The showers and thunderstorms that have both
already formed and will continue to develop across the Northeast
today into tonight will have access to that level of moisture,
making it highly likely that the strongest storms will have the
potential to produce prodigious rainfall rates. Already this
morning the storms that formed over northeastern PA are locally
producing rainfall rates to 2 inches per hour. Add in diurnal
instability late this morning through this afternoon and the
storms will have even greater rainfall producing potential. Thus,
the Moderate Risk is looking on track, and is a higher-end
Moderate Risk from NYC and northern NJ northeastward through
central NH. The I-84 corridor from CT through MA is particularly
vulnerable, as numerous showers and storms have developed in this
region out ahead of the main line of rain now crossing into NJ,
with additional storms likely to develop later this afternoon.
While these areas mentioned have a somewhat higher risk of flash
flooding today, the signal was not consistent that the storms
producing the heaviest rain will be widespread enough anywhere in
the Moderate Risk area that High Risk-level flash flooding is
anticipated. Rather, much of the guidance has "streaks" of higher
rainfall totals in the 4-6 inch range corresponding with
semi-stationary lines of training thunderstorms, which will be
interspersed with areas with much less rain in between storms.
This characterizes better potential for scattered instances of
flash flooding with localized significant/considerable flash
flooding, and not widespread flash flooding with numerous
instances of significant flash flooding that would characterize a
High Risk flash flooding day. In coordination with many of the
forecast offices, the Moderate Risk was maintained with this
update.

A few notable changes were made to the area of the Moderate Risk,
however. The biggest change was to expand the Moderate southward
quite a bit to now include the Philadelphia area, much of NJ, and
the northern tip of Delaware, including Wilmington. Both guidance
and present radar trends suggest the flash flooding potential in
this area has increased markedly with heavy rain likely to move
over the aforementioned urbanized areas more prone to flash
flooding both early this morning and redeveloping late this
morning through this afternoon in this area. This change was
coordinated with the PHI/Mt. Holly, NJ forecast office.

The Moderate Risk area was maintained across NYC and the
surrounding metro, and expanded a bit to include more of the
eastern suburbs on Long Island, including much of Nassau County,
as urbanization supports a higher flash flooding potential. The
eastward shift in the axis of heaviest rainfall now puts NYC
squarely in the cross-hairs of the potential for training
thunderstorms.

As mentioned the highest threat continues from northeastern CT
through central MA and into NH, as highly favorable antecedent
conditions combine with likely training thunderstorms embedded
within a broad area of moderate rainfall for much of the day
today. Should the rainfall over-perform the forecast, this is the
most likely area to potentially need a High Risk upgrade later
today, but this will be highly contingent on more widespread high
rainfall totals, as opposed to individual storms delivering quick
hits of heavy rain over the area. The Moderate Risk was also
expanded a little up into southwestern ME as this area is also
sensitive from previous rainfall events and the storms should have
no problem maintaining themselves as they push north.

For a bit of good news, the slight eastward shift allowed for a
small trimming of the western peripheries of the Slight and
Marginal risk areas over central PA and upstate NY. Much of the
rain these areas are expected to see is occurring presently and
should be over for the day around sunrise and the start of this
Day 1 period. However, the caveat is that additional afternoon
storms are likely to develop over eastern NY and far eastern PA,
which may form a bit further west.

...Gulf Coast and Southeast...

The aforementioned eastward shift in the axis of heaviest rainfall
has effectively pushed most of the forecasted rain to the
immediate coast or off the coast of the Carolinas. Thus, the
previous Marginal Risk in this area was dropped with no flash
flooding anticipated. The trailing end of the front will extend
into FL and west along the immediate Gulf Coast. A few of these
areas had localized heavy rain in previous days, which is likely
to be more widespread today as compared to previous days, however,
given the high FFGs along the immediate Gulf coast, think any
flash flooding will be very localized. Some return flow will allow
for a few showers and storms to develop along the Red River
forming the TX/OK border, but they are not expected to be
organized enough to pose any more than an isolated flash flooding
threat, so the Marginal Risk for this region was maintained with
minimal changes.

...Central Plains through the Chicagoland Area...

Widely scattered showers and thunderstorms will form this evening
through the overnight hours from KS/NE east through northwestern
IN. The storms should be fast-moving and actually have a somewhat
limited supply of moisture, so heavy rainfall totals are not
expected. The inherited Marginal Risk area was largely left the
same.

Wegman


Day 2
Valid 12Z Mon Jul 17 2023 - 12Z Tue Jul 18 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR THE
MID-MISSOURI RIVER VALLEY, OHIO VALLEY AND NORTHERN MID-ATLANTIC,
AS WELL AS MUCH OF THE FLORIDA PENINSULA, CENTRAL GULF COAST, AND
SOUTHEAST ARIZONA...

No major changes were made to the various Marginal Risk areas
across the country on Monday. A secondary front pushing south and
east across the Midwest will provide the focus for widely
scattered showers and storms over the Ohio Valley today. Since the
main plume of moisture responsible for the heavy rain Sunday in
the Northeast will be well off the coast by Monday, this secondary
front will have much less access to atmospheric moisture. This
will dramatically cut down on the storm coverage and intensity,
which in turn will limit the flash flooding threat as well.
Nonetheless, isolated training/backbuilding storms can't be ruled
out, with the strongest signal from southeast OH into western PA.
The Marginal Risk was maintained across this area.

The southern end of the same front will be between forcings, and
therefore the signal for heavy rain has diminished, thus the
biggest change from inherited was to drop much of MO and IL from
the Marginal Risk area. Meanwhile further northwest across the
Mid-Missouri River Valley, a developing LLJ will begin pushing the
front east as a warm front Monday night. This interface will be
the focal point for scattered storms, primarily over SD and
northeastern NE. Isolated flash flooding can't be ruled out. This
same LLJ will be much more influential and impactful further south
and east on Day 3/Tuesday.

The FL Peninsula will remain on the southern end of the impressive
moisture plume now entirely off the rest of the East Coast. Thus,
scattered but largely disorganized convection may still produce
flash flooding since any storms will have ample moisture to draw
from. The greatest threat will be along the Treasure Coast.

A weaker signal persists over southeastern LA, but with possible
flash flooding from Day 1/Sunday, opted to leave the Marginal as a
low-end threat for Monday. This is the lowest confidence of the
various Marginal Risk areas.

Finally, weak monsoonal flow may produce a renewed round of
afternoon showers and thunderstorms over southeastern AZ. This
area was hit by convection the past couple days, so an isolated
flash flooding threat can't be ruled out.

Wegman

Day 3
Valid 12Z Tue Jul 18 2023 - 12Z Wed Jul 19 2023

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE LOWER
MISSOURI VALLEY TO THE LOWER OHIO VALLEY AS WELL AS FOR PORTIONS
OF THE NORTHEAST...

...Lower Missouri Valley to the Lower Ohio Valley...

An MCS at the nose of strengthening LLJ is expected to develop
over eastern MO and IL Tuesday evening, which will intensify as it
slowly moves east across IL and IN Tuesday night. The LLJ will
strengthen to 50 kts in the mid-levels Tuesday night as it advects
plentiful Gulf moisture northeastward into a warm front draped
across the area. This front will be the primary source of lift for
strong and slow-moving thunderstorms capable of over 2 inch per
hour rainfall rates in the strongest storms. While the area has
been about average for rainfall over the past 2 weeks based on
AHPS data, the potential for very heavy rain would likely overcome
these antecedent conditions should the heavy rain persist for a
long enough period of time. Since the front will be slow moving,
this will support training thunderstorms which will move
southeastward with time. The most persistent areas of rain may see
rainfall totals exceed 5 inches for storm total rainfall from this
event.

There is considerable uncertainty as to where the MCS is likely to
develop. The areas mentioned above are most likely to see the
greatest rainfall, but the spread covers from central MO east to
southwestern OH. Assuming the signal remains high for localized
heavy rain, and with better consistency among the guidance with
future forecast cycles, a Moderate Risk upgrade is possible. For
now the high-end Slight Risk encompasses from St. Louis east
across southern IL and southwestern IN.

...Northeast...

The front pushing east across the Mid-Atlantic will have
considerable upper level support from a deepening negatively
tilted longwave trough. However, moisture will be limited as the
primary moisture plume will be well off the coast. Thus, while
storms will have the potential to grow strong with the strong
forcing, the relative lack of moisture will limit strength to some
extent, but moreso coverage. Thus, the Slight Risk is primarily
driven by highly favorable antecedent conditions that will be
ongoing after today's rainfall event. Tuesday's storms will be
less strong and with less coverage, but since FFGs are likely to
be low, widely scattered flooding is still possible. The Slight
Risk was expanded southward to include all of the Hudson Valley
and into northern NJ with this update, following the axis of
heaviest rainfall.

...South Florida...

Typical sea breeze convection will be ongoing with the stationary
tropical air mass persisting. The Marginal Risk was nudged
northward to include more of central FL with this update.

...Southeast AZ...

Monsoonal flow is likely to result in another round of afternoon
showers and thunderstorms over the same area as previous days.
Isolated flash flooding in sensitive areas can't be ruled out.

Wegman
$$
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FOUS30 KWBC 171533
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1132 AM EDT Mon Jul 17 2023

Day 1
Valid 16Z Mon Jul 17 2023 - 12Z Tue Jul 18 2023

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE
EASTERN GREAT LAKES, THROUGH THE OHIO VALLEY AND PORTIONS OF THE
LOWER TO MIDDLE MISSOURI VALLEY, AS WELL AS THE EAST CENTRAL TO
SOUTHEAST FLORIDA COAST, SOUTHERN LOUISIANA, AND ARIZONA...

1600Z Update...

Mainly cosmetic changes were made to the early morning D1 ERO. The
06Z and 12Z HREF guidance suggests a bit of a stronger/heavier QPF
signal to areas of southeast FL later today with some relatively
high probabilities of seeing 3+ inch rainfall totals from
convection in the 18Z to 00Z time frame near the urban corridor.
Thus have expanded the Marginal Risk farther south to include
Miami-Dade County. Otherwise very minor adjustments/tweaks were
made to the Marginal Risk areas across the central Gulf Coast and
also the OH Valley, Middle MS Valley and central Plains. Finally,
with some monsoonal moisture making a somewhat stronger presence
in the Southwest, it was decided to pull the Marginal Risk a
little farther north to include portions of the Mogollon Rim and
this will account for some terrain-induced and slow-moving
convective cells this afternoon and evening.

Orrison

Previous discussion...

...Eastern Lakes, Ohio Valley into southern Missouri, southeast
Kansas and far northeast Oklahoma...
A broad marginal risk area was maintained for the upcoming day 1
period along and ahead of the next frontal boundary pushing
through the eastern Great Lakes and toward the Ohio Valley. The
latest simulated hi res radars depict potential for scattered
convection across these regions where PW values are near seasonal
norms and instability will range from 1000-200 j/kg through the
Ohio Valley and values 3000+ j/kg from eastern Kansas/far
northeast Oklahoma into southern Missouri. The previous marginal
risk was extended westward across southern Missouri into southeast
Kansas/far northeast Oklahoma to capture model heavy qpf potential
in the max instability axis. The marginal risk area coincides
well with the higher HREF neighborhood probabilities for 1"+
amounts, peaking at 50-80% across the marginal risk area. At the
moment, convection should be fairly progressive limiting the
upside potential for heavier totals. This is reflected in much
low HREF probabilities for 2"+ amounts.

...Middle Missouri Valley...
The front dropping south from the Northern Plains into the Central
Plains/Mid Missouri Valley will become nearly stationary Monday
afternoon from the Central Plains, east southeast into the Mid
Mississippi Valley. Strengthening southeasterly to southerly
inflow into this front will support potential for organized
convection to press east southeast along and north of the front
late Monday afternoon into the early hours of Tuesday. There is
general model agreement on heavy rainfall potential as this
convection presses southeastward, but not very good agreement on
placement of max amounts. This is reflected in very low EAS
probabilities for 1"+ amounts, maxing out at 5% over southern SD
into far north central NE, while HREF neighborhood probabilities
for 1"+ amounts are higher 40-50%+ across this area. Risk level
kept at marginal given this spread.

...East central Florida coastal area...
An axis of above average PW values expected to persist day 1
across much of central FL, with anomalies 2 to 3 standard
deviations above the mean. With the low level flow expected to
remain westerly, convection that does fire will likely progress
eastward and maximize somewhere along the east central Florida
coast Monday afternoon. HREF probabilities reflect this with the
highest 2"+ probabilities, 50-80%, along the east central FL coast
from approximately Melbourne to Miami. This is also where the
Ensemble Agreement Scale (EAS) shows the highest probabilities for
1 and 2"+ amounts. For these reasons, the previous broader
marginal risk area across FL was decreased in size and
concentrated along the east central FL coast where the
aforementioned higher probabilities coincide with the more
urbanized regions.

...Southern Louisiana...
Only some slight changes to the previous marginal risk outlook
with the northern edge trimmed south by approximately 50 miles to
fit the latest model qpf output. At the moment, the model max
axis of additional precip is across far southeast LA just to the
south of the urban areas from New Orleans to Baton Rouge and
Lafayette, but can not rule out convection still affecting the
urban areas where there will be the greatest runoff risk day 1
from isolated rainfall amounts of 1-2 inches Monday afternoon.

...Southeast Arizona...
The latest model consensus is for any monsoonal showers to remain
primarily across southeast Arizona and southward into northwestern
Mexico. This is reflected in the latest HREF probabilities that
show any probabilities greater than 25% for 1"+ amounts centered
close to the border over south central Arizona. Activity over the
past 24 hours was mostly south of the southern AZ border with this
trend again likely for day 1. While there may be some scattered
convection north into the Mogollon Rim area, moisture is limited
here and should preclude any runoff issue threat. The previous
marginal risk area was trimmed on the north and northeast side
from southwest New Mexico into central AZ.

Oravec


Day 2
Valid 12Z Tue Jul 18 2023 - 12Z Wed Jul 19 2023

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE LOWER
MISSOURI VALLEY INTO THE MID MISSISSIPPI VALLEY AND LOWER OHIO
VALLEY AS WELL AS FOR PORTIONS OF THE NORTHEAST...


...Lower Missouri Valley, Middle Mississippi Valley into the Lower
Ohio Valley...
The stationary frontal boundary expected to stretch west northwest
to east southeast from the Central Plains into Lower Missouri
Valley, Mid Mississippi Valley and Lower Ohio Valley region should
be the focus for another round of organized convection late
Tuesday afternoon into the early hours of Wednesday. There will
be potential for an MCS/MCC to progress east southeastward along
and north of this front. The initial southerly low level flow
into this front will attain more westerly component after 0000 UTC
Wed across northern to central MO into southern Illinois. This
will support potential for backbuilding and training of cells in a
west northwest to east southeasterly direction. There is a model
signal for a narrow axis of very heavy rainfall amounts from
northern Missouri into southern Illinois and western Kentucky.
The day 2 Excessive Rainfall Outlook continues to depict a slight
risk across these areas, although the threat axis was narrowed
from the previous outlook, trimmed on the northern side, to
reflect the current qpf axis range. Given the training potential
with this system, an upgrade to moderate risk is possible with
later issuances, especially as we get into the day 1 time period
where hi res guidance may offer better agreement with a max axis.
In the areas of training, rainfall totals in excess of 5 inches
are possible.

...Northeast...
The cold front moving through the eastern Lakes day 1 will press
east from eastern NY through New England day 2. A narrow axis of
above average PW values in the 1.5-1.75"+ range expected along and
ahead of this front. These values will be lower than PW values
with the heavy rainfall event on Sunday as that anomalous PW axis
will have moved well off the northeast coast during day 1.
Additional convection likely to stretch along this front on
Tuesday, supporting locally heavy rainfall totals from in the
vicinity of northern NJ/eastern NY State, northeast through
western and northwestern New England. The previous marginal and
slight risk areas were trimmed on the western portions across
eastern NY State to reflect model qpf consensus. The new slight
risk area fits well with the 12 hour HREF neighborhood
probabilities ending 0000 UTC Wed. While precip will continue
after 0000 UTC Wed, the last 12 hours of the current HREF forecast
period captures most of the heavy precip potential with this
event. There will be some overlap with heavy rains over the past
24 hours and the day 2 expected heavy axis from southeast NY State
into northwest CT and western MA and east central NH. Much of the
remainder of the heavy rains over the past 24 hours farther to the
southeast over southeast New England will remain to the southeast
of the day 2 expected qpf axis.

...Southeast AZ...
Scattered convection again possible around the peripheries of the
persistent upper high stretching across the Southwest. A small
marginal risk area was maintained over southeast Arizona where
isolated heavy totals may produce local runoff issues.

Oravec

Day 3
Valid 12Z Wed Jul 19 2023 - 12Z Thu Jul 20 2023

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS THE MID TO
UPPER OHIO VALLEY, UPPER TENNESSEE VALLEY INTO THE CENTRAL
APPALACHIANS...

...Mid to Upper OH Valley, Upper Tennessee Valley into the Central Appalachians...
PW values expected to remain above average along and north of the
front forecast to stretch from the Mid Mississippi Valley into the
Upper Tennessee Valley region. Shortwave energy pushing eastward
along the southern edge of the westerlies and the above mentioned
frontal boundary will support potential for convection along and
to the north of this west to east oriented frontal boundary.
There is a typical amounts of qpf detail spread for a day 3
forecast, but general consensus for the potential for heavy
rainfall amounts in the vicinity of this front. A slight risk was
maintained from the previous forecast issuance, expanded slightly
to the northeast into central to southern West Virginia where
several models show potentially heavy rains moving over the lower
ffg values across this area.

...Central Plains, Lower Missouri Valley, Mid Mississippi Valley...
The marginal risk area was expanded westward from the previous
issuance, from the Lower OH Valley, into the Mid MS Valley, Lower
MO Valley and across the Central Plains of Kansas into southeast
CO and the TX/OK Panhandle region. This was to capture heavy rain
potential in the vicinity of the west to east oriented front
across these areas which also will coincide with the southern edge
of the westerlies where enhanced lift is possible from transitory
shortwaves.

...Upper Mississippi Valley...
The marginal risk from the previous issuance was also expanded
south from the Upper Lakes/northern MN area, through the Upper MS
Valley. This was for convective potential ahead of fairly strong
northern stream height falls forecast to push from the Northern
Plains into the Upper MS Valley during day 3. A narrow axis of
above average PW values and instability along and ahead of the
associated cold front will support potential for scattered
convection Wednesday into early Thursday. At the moment, it
appears convection ahead of this front should be fairly
progressive, keeping the risk level at marginal.

...Southeast Arizona...
With no significant changes to the overall pattern across the
Southwest, maintained a small marginal risk area over southeastern
Arizona similar to the days 1 and 2 time periods. It is possible
the most organized activity remains across northwest Mexico to the
south of the AZ border, with the northern edge of the precip
affecting far southeastern AZ.

Oravec
$$
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FOUS30 KWBC 181934
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
334 PM EDT Tue Jul 18 2023

Day 1
Valid 16Z Tue Jul 18 2023 - 12Z Wed Jul 19 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID MISSISSIPPI VALLEY, LOWER OHIO VALLEY, & LOWER TENNESSEE
VALLEY...

...Lower Missouri Valley, Mid Mississippi Valley into the Lower
Ohio and Lower Tennessee Valleys...
The potential remains for one or two narrow axes of very heavy
rainfall amounts in two rounds, with the potentially more
significant round expected Tuesday night into Wednesday morning
period a hair farther east from the first across portions of the
Lower Missouri Valley/Mid Mississippi Valley/Lower Ohio Valley and
Lower Tennessee Valley region expected near a frontal boundary and
within an area of diffluent 1000-500 hPa heights/thickness which
shifts from MO into western KY with time. The inflow at 850 hPa
should remain persistent out of the west-southwest, growing in
magnitude with time. Precipitable water values should range in
the 1.75-2.25" range. which implies tall, skinny CAPE and
sufficient wet bulb zero height for warm rain processes. The
overall CAPE field is expected to move/shift eastward with time.
An initial round of convection firing late Tuesday morning ahead
of a vort moving east southeast from the Central Plains into
Missouri. Model consensus is that this initial organized area of
convection will be fairly progressive to the southeast across the
risk area during the afternoon and evening hours of Tuesday. This
would then be followed by a second round of greater training
potential convection sometime in the 0300 to 1200 UTC Wednesday
period when the low level inflow increases partially due to the
nocturnal low-level jet, increasing the potential for back
building and downstream training of convection, which due to
increasing instability from west to east could lead to a second
heavy rain streak east of the earlier convective round. While
there is general agreement in this scenario, there are some fairly
differences in where the training episode will set up and whether
or not we'll have one training band or two. Also complicating
matters is the significant variance in rainfall over the past week
across the region, with areas generally west, south, and southeast
of St. Louis the wettest while the remainder of the region has
been fairly dry. The changes in the Moderate Risk area fit well
with the 12z HREF neighborhood probabilities for 5"+ and 8"+
amounts. These totals seem reasonable given the potential for two
rounds of convection which may overlap and the potential for the
training episode in the early hours of Wednesday when more
significant dynamics are at play. Hourly rain totals to 2.5" are
anticipated, which would be more than enough for issues in any
urban areas within the various Risk areas. Cell training and
embedded mesocyclones appear to be the bigger players in heavy
rain production in this pattern. Changes in the Slight and
Moderate risk areas were coordinated with NWS forecast offices in
that region. The expansion made to the Marginal Risk across the
Ohio Valley and WV was made based on the 12z HREF guidance.


...Northeast...
Only slight changes were made to the marginal and slight risk
areas from the previous outlook across the Northeast. The slow
moving front currently over the eastern Great Lakes region will
begin to push more quickly eastward day 1 across the Northeast. A
narrow axis of above average PW values in the 1.5-1.75"+ range are
expected along and ahead of this front. These values will be
lower than PW values were with the heavy rainfall event on Sunday
as that anomalous PW axis has moved well off the northeast coast,
but still would support hourly rain totals to 2". Additional
convection is likely to stretch along this front on Tuesday,
perhaps very soon after this discussion's issuance, supporting
locally heavy rainfall totals from in the vicinity of northern
NJ/eastern NY State, northeast through western and northwestern
New England. The slight risk area fits well with the HREF
neighborhood probabilities for heavy rainfall amounts. There will
be some overlap with heavy rains on Sunday and the expected heavy
axis on Tuesday from southeast NY State into northwest CT and
western MA and east central NH. As the 12z HREF has some signal
for 5" amounts, this should be near the high bar for local amounts
in this region.


...Southwest into the Great Basin...
Widespread monsoonal convection possible across portions of the
Southwest and Great Basin as daytime heating interacts with PWs of
0.75-1", which would foster heavy rainfall particularly in areas
of terrain. An axis of more anomalous PW values are expected to
rotate eastward on the north side of the persistent southwest
upper ridge across portions of NV into western CO, supporting
scattered slow moving convection across these areas. HREF
probabilities do show 20-30% probabilities of .50"+ amounts in an
hour. This could produce some isolated runoff issues, especially
across any burn scar regions, arroyos/dry washes, or slot canyons.


...Florida...
Heavy rainfall over the Space Coast is forecast during daytime
heating, which could have some organization as effective bulk
shear approaches 25 kts, within an area of PWs of 2-2.25". Hourly
rain totals up to 3" are possible, and there is a small signal for
5"+. Enough evidence exists for a new Marginal Risk area in that
location. Down south, other than the usual disorganized but
efficient pulse convection in an anomalously moist airmass during
daytime heating today, an easterly wave approaches the southern
portion of the peninsula and the Keys very late in the period
which could lead to an early morning uptick on Wednesday. Enough
ingredients were available -- particularly moisture and
instability -- to extend the new Marginal Risk southward through
the Keys.

Roth/Oravec


Day 2
Valid 12Z Wed Jul 19 2023 - 12Z Thu Jul 20 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR
TENNESSEE & SOUTHEAST VIRGINIA...

...Tennessee Valley into the Southern Appalachians and southern
Mid-Atlantic states...
Heavy rains should continue into the morning hours of Wednesday
across portions of TN and KY. The previous slight risk area was
reconfigured based on the latest guidance, with a new Slight Risk
area added to southeast VA. An MCV from morning activity could be
the catalyst for the heavy rainfall signal showing up near
Southeast VA. While initial heavy rainfall should be fading with
time on Wednesday morning, additional scattered convection
possible in the vicinity of the frontal boundary stretching from
Southeast VA through the the Tennessee Valley across the Mid
Mississippi Valley and into the Southern Plains. PW values in the
vicinity of this front expected to remain above average, 1.75-2"+.
Additional shortwave energy moving along the southern edge of the
westerlies will enhance upward vertical motion near the front and
above average PW axis. This will support locally heavy rains
across these areas. Model consensus is not cohesive on where the
heavy amounts will be, but enough of a signal exists within the
two Slight Risk areas. Subsequently, just a marginal risk
depicted in the vicinity of the front to the west of the TN Valley
heavy early day 2 rains.


...Southwest into the Great Basin...
An axis of above average PW values -- generally 0.75-1" -- will
continue on the north side of the Southwest upper ridge,
supporting additional scattered convection across northern NV into
central to northern UT and CO, with emphasis likely in area
terrain. Scattered monsoonal convection also possible again into
southeast AZ and NM. Isolated runoff issues, especially across
any burn scar regions, dry washes/arroyos, or slot canyons are
possible from isolated .50-1" rainfall totals.


...Upper Mississippi Valley...
The marginal risk from the previous issuance was slightly
reconfigured but remains more or less the same. This area
describes convective potential ahead of fairly strong northern
stream trough forecast to push from the Northern Plains into the
Upper MS Valley. A narrow axis of above average PW values and
instability along and ahead of the associated cold front will
support potential for scattered convection Wednesday into early
Thursday. At the moment, it appears convection ahead of this
front should be fairly progressive, keeping the risk level at
marginal.


Roth/Oravec

Day 3
Valid 12Z Thu Jul 20 2023 - 12Z Fri Jul 21 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE
CENTRAL/SOUTHERN HIGH PLAINS AND IN AND NEAR THE SOUTHERN &
CENTRAL APPALACHIANS...


...Lee of the Central Rockies into the Central to Southern High
Plains...
Additional shortwave energy expected to rotate east southeastward
on the northeast side of the Southwest upper high, pushing east
southeast from the lee of the Central Rockies into the Central to
Southern Plains. A surface front dropping southward from Central
Plains into the Southern Plains during Thursday with induce a
period of upslope flow, which brings a significant amount of
moisture, with PWs exceeding 1.5" in easternmost CO, which is on
the high end of climatology for any month. Enhanced upper
divergence associated with mid- to upper-level shortwaves and CAPE
values of 2000-3000 J/kg will support increasing convection
Thursday afternoon into Thursday night/early Friday along and
north of this front. There is fairly good model agreement on an
axis of moderate to heavy rains across southeast WY, southwest NE,
eastern CO, western to central KS and northern OK. Stream flows
are still mostly above average across these areas, although much
of the area has been dry over the past week. Hourly rain totals
to 2.5" with local amounts of 4-6" are possible based on the above
ingredients and the available guidance. There remains spread in
the guidance on the exact location, which left the risk level
Slight. A Moderate Risk cannot be ruled out should the guidance
become more agreeable or a little wetter.


...Southern to Central Appalachians into the Northern
Mid-Atlantic...
A strong northern stream upper level trough will be pushing east
through the Great Lakes, OH Valley, Southern to Central
Appalachians region day 3, becoming increasingly negatively tilted
with time. Strong upper difluence and defined frontal convergence
in an axis PW values 1.5-1.75" and Cape values 1000-2500 j/kg will
support widespread scattered convection along and ahead of the
associated cold front. Expanded the slight risk to cover where
potentially heavy rains coincide with the relatively low FFG
values stretching from the Southern to Central Appalachians into
the Northern Mid Atlantic.

Roth/Oravec
$$
--- SBBSecho 3.14-Linux
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FOUS30 KWBC 191929
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
328 PM EDT Wed Jul 19 2023

Day 1
Valid 16Z Wed Jul 19 2023 - 12Z Thu Jul 20 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR
WESTERN KENTUCKY & WESTERN TENNESSEE...


...Tennessee Valley...
Heavy rains continue across portions of MO, western KY, and TN
within an area of persistent 1000-500 hPa height/thickness
diffluence which should perhaps drift a little east today. While
the 12z HREF is too far northeast with ongoing activity, it could
persist for a few more hours. The excessive rain areas were
shifted westward to match radar trends. Otherwise, there is a
signal in the mesoscale guidance for the initial convection to
shift into eastern TN and becoming potentially problematic this
afternoon, so re-extended the Slight Risk area eastward to account
for the low but existing probabilities of 5"+. With the western
portion of the Slight Risk and the Moderate Risk area, there's the
potential for afternoon convection along a relatively stationary
boundary being pinned by competing forces: current convective
outflow trying to shift the boundary south and west, and some
ridge expansion from west of the area which brings up 700 hPa
towards 12z. Also, convection from the central Plains is expected
to move into the western portion of the Slight and possibly the
Moderate Risk area overnight into early Thursday. Therefore,
there has been no downgrade to the risk, and a High Risk was
considered. For the time being, the area that has seen 6-11" of
rainfall is fairly small and considered too small for a High Risk
upgrade. Should the situation this afternoon be worse than
expected, an upgrade to a High Risk remains possible.


...Southern Mid-Atlantic/North Carolina...
Locally heavy rains also possible Wednesday ahead of a surface
front pushing east along the Mid-Atlantic coast. Kept the small
slight risk area across far southeast Virginia in the urbanized
Hampton Roads region. Farther to the north, the marginal risk was
extended north into far southeast PA and northern NJ to cover
heavy rainfall potential over areas that have relatively low ffg
values.


...Lower Missouri Valley toward the Lower Ohio Valley...
Along the east to west oriented front, locally heavy rains are
possible across much of Kansas into southeast Colorado given the
favorable upper difluence and high PW axis along this front and
sufficient instability. These areas, however, have been
relatively drier than areas farther to the east along the front
through the Lower MO Valley into the TN Valley. At the moment,
the threat level was kept at marginal across these areas.


...Upper Mississippi Valley...
Few changes were made to the marginal risk from the previous
extending northeastward through the Upper MS Valley into the Upper
Lakes region. Models remain consistent in showing a narrow axis
of above average PW values and instability along and ahead of the
associated cold front ahead of strong height falls pushing
eastward from the Northern Plains into the Upper MS Valley. Model
consensus continues to be for fairly progressive convection across
this area. With precip mostly below average over the past few
weeks across the Upper MS Valley and lower than average stream
flows, believe the threat for excessive rainfall will remain
marginal.


...Northern Sierra, Great Basin into portions of the Southwest...
Only some slight changes made to the previous marginal risk areas
the Great Basin and Southwest. PW values expected to remain above
average, 1 to 2+ standard deviations above the mean, from the
northern Sierra, eastward across portions of the Great Basin into
the Central Rockies. Scattered diurnal convection likely in this
above average PW axis, supporting isolated runoff issues. A small
marginal risk area also maintained over southeast AZ into
southwest NM where isolated slow moving cells again possible
Wednesday afternoon.

Roth/Oravec


Day 2
Valid 12Z Thu Jul 20 2023 - 12Z Fri Jul 21 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL/SOUTHERN HIGH PLAINS & THE EASTERN UNITED STATES...


...Lee of the Central Rockies into the Central to Southern High
Plains...
Additional shortwave energy expected to rotate east southeastward
on the northeast side of the Southwest upper high, pushing east
southeast from the lee of the Central Rockies into the Central to
Southern Plains. The shortwave will be progressive to the north
of a surface front dropping southward from Central Plains into the
Southern Plains during Thursday. Post frontal upslope flow and
enhanced upper divergence will help initiate convection in the
vicinity of southeast WY/western NE/northeast CO Thursday
afternoon, but it is forecast to shift southward with time. This
activity will likely enhance into organized convection pushing
southeastward southeast CO/western to central KS/far northeast NM
and the TX/OK Panhandle region Thursday night into the early hours
of Friday in a region of above average PW values and CAPE values
1500 -3000 j/kg. Stream flows are still mostly above average
across these areas, although much of the area has been dry over
the past week. There may be some overlap to locally heavy rains
amounts expected Wednesday afternoon and evening over portions of
southeast CO into western KS, which could leave soils more
saturated. No significant changes to the previous outlook with
only a slight southeast expansion of the slight risk into far
northeast NM and far northwest TX. Widespread 1-2"+ amounts
possible with localized values of 3-5"+. There remains some
spread in the axis of heaviest amounts. Thus far, the mesoscale
guidance shows a progressive convective system, so have kept the
risk level as Slight. This could change should more rain than
expected fall before now and then, or if the expected convective
evolution changes to some degree.


...In and near KY and TN...
An area of 1000-500 hPa thickness and height diffluence across the
region implies the possibility of organized convection across the
area. Recent rains across portions of KY and TN have left soils
increasingly saturated. There is some signal, particularly
amongst the 12z NAM and 12z Canadian Regional, which is seen
somewhat in the 12z HREF probabilities of 5"+, of another
potential convective episode in this region near and ahead of an
invading cold front. Inflow at 850 hPa is convergence and high
enough in magnitude to worry about organized convection should
could train in bands. With ML CAPE rising to 3000+ J/kg
interacting with PWs of 1.75-2", hourly rain totals to 2.5" are
within the realm of possibility. The new Slight Risk area has
greater than average potential for excessive rainfall/flash
flooding considering recent rains.


...Carolinas into the Northern Mid-Atlantic...
The strong upper level trough will move from Upper MS Valley
through the Great Lakes, OH Valley and Southern to Central
Appalachians Thursday and Thursday night. Increasingly difluent
upper flow, defined frontal convergence and southwest upslope into
the Appalachians will support increasing convective coverage ahead
of this front. The guidance is attempting to center in more on
portions of PA and northern WV, so constrained the Slight Risk to
those areas. Hourly rain totals to 2" with local 3" amounts are
possible. Some of the region, particularly central PA, has seen a
very wet week. Coordination with the CTP/State College PA
forecast office helped lead to this reconfiguration.


...Eastern L.P. of Michigan...
A Slight Risk remains across the eastern portions of the L.P. of
Michigan where flash flood guidance values are relatively low in
the wake of a wet week and an organized convective squall line
along and ahead of the above mentioned cold front which will
likely push across Thursday afternoon with hourly rainfall amounts
up to 1.5", and local amounts of 3" where cells merge, manage to
train for a couple hours, or a mesocyclones manage to form.

Roth/Oravec


Day 3
Valid 12Z Fri Jul 21 2023 - 12Z Sat Jul 22 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR THE
MID-SOUTH & NORTHEAST NEW MEXICO...

...Mid-South/Southeast...
An axis of above average PW values will stretch west to east from
the Southern Plains into the Lower MS Valley/TN Valley and
Southeast in the vicinity of the west to east oriented front
forecast to be moving slowly south and southeast day 3.
Shortwaves moving west to east along the southern edge of the
westerlies and this frontal zone will enhance lift in the above
average PW axis and support potential for widespread scattered
convection along and ahead of the boundary. Most of the guidance
has shifted south in this region, so the Slight Risk follows suit
better matching the intermediate 12z ECMWF and 12z Canadian
Regional solutions, as the atmosphere ahead of the front is
uncapped. The 12z NAM in particular shows a boundary moving ahead
of the front which focuses convection farther south. Further
adjustments to the south are possible as we get closer to the
Friday.


...Southern High Plains...
Low-level easterly upslope flow will strengthen Friday and Friday
night as the cold front continues to sink southward through the
Southern High Plains. To the south across Southern NM, an
easterly wave moving south of the US/Mexican border at 700 hPa
aids in the upslope and moisture surge. The small slight risk
area added overnight across portions of central and northeast NM
still generally works, when viewing the 12z guidance consensus of
potentially heavy rains over northeast New Mexico. This potential
precip area coincides with relatively low flash flood guidance
values where rainfall over the past week has been 2 to 3 times
average in places.


...Northeast Mid-Atlantic into New England...
A strong upper level trough moves from the Appalachians through
New England Friday. Precipitable water values ~1.5" and CAPE of
1000-2000 J/kg should promote convection containing heavy
rainfall. Additional locally heavy precip totals possible from
eastern New York State into central to northern New England, with
hourly rain totals up to 1.75" and local amounts of 2-4" possible.
Considering heavy rains over the past week, and the high stream
flow across these areas, added a Slight Risk for northeast NY and
much of VT.

Roth/Oravec
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 211859
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Fri Jul 21 2023

Day 1
Valid 16Z Fri Jul 21 2023 - 12Z Sat Jul 22 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NEW MEXICO, SOUTHEAST/MID-SOUTH, AND NORTHEAST/MID-ATLANTIC...

1600 UTC Update -- Minor changes were made to the Day 1 ERO based
on the latest observational/mesoanalysis trends and incoming 12Z
CAM guidance (including the 12Z HREF exceedance probabilities). We
did pull the back edge of the Slight Risk area farther east across
eastern NY-NJ. Meanwhile, lingering MCS/MCV across AR and southern
MO will continue to get rejuvenation from the more unstable
airmass downstream across the Slight Risk area over the Lower TN
Valley (mixed layer CAPEs currently 1500-2500 J/Kg).

Hurley

Previous discussion below..

...Northeast and Mid-Atlantic...
Convection is ongoing as of 08z across portions of eastern PA/NY
and should continue to push eastward early this morning. It should
gradually weaken with eastward extent, although the southern
portion of the line is maintaining longer than most guidance
indicated. This does raise some question with regards to how
quickly instability will recover this morning behind this line
from DE/NJ north towards NYC. Not expecting much of a flash flood
risk with this initial line, with the threat primarily expected
with the redevelopment later this morning into the
afternoon/evening hours.

A slowing surface front and persistent upper level divergence will
support the potential of multiple convective rounds today across
portions of the Northeast. Mean flow is out of the west or west
southwest at about 20-30 kts...which would support rather swift
eastward movement of cells. However with the front slowing, lower
level convergence will be rather persistent, which will support a
few rounds of storms, and may also support some brief anchoring of
cells along the convergent axis. Both of these would locally
extend rainfall duration and increase the flash flood risk. PWs
are only modestly above average, so not expecting the most
efficient rainfall producers. But we are in mid summer, and so PWs
are still high enough for heavy rainfall rates within any
convection...and the potential brief training should be enough to
at least partially overcome the modest PW values. HREF data
supports decent coverage of 1" in an hour rainfall, with more
isolated areas around 2". Storm total rain should approach 3" in
spots, and can not rule out an isolated 5" amount.

Do expect that we will see some flash flooding today across
portions of NJ northward into eastern NY, VT and western MA/CT. If
antecedent conditions were neutral this would be a solid Slight
risk, but the fact that soil and streamflow conditions are above
average does begin to push this event towards the higher end of
the Slight risk probability range. Gave some consideration to a
MDT risk upgrade, but the coverage and magnitude of impacts will
probably stay just below MDT risk levels. Will consider this a
higher end Slight risk though, and can not rule out localized
higher end impacts if the wrong basin or urban area gets impacted
by a quick 2-3" of rain.

...Mid-South/Southeast...
An MCS will likely be ongoing across portions of OK at 12z this
morning. The complex should remain rather progressive as it moves
across OK and AR, however heavy short duration rainfall rates are
likely, with a quick 1-3" of rain expected. The forward speed of
the complex will probably keep the flash flood risk isolated in
nature for the most part. While some weakening is possible by
later this morning, do expect the complex to maintain to some
extent as it moves into southern TN and northern MS/AL. As daytime
heating increases, we should see increasing instability out ahead
of the feature, along with enhanced moisture convergence along its
track, especially where interaction with the southward dropping
cold front occurs. Thus additional development along and ahead of
the complex appears likely by this afternoon. This should allow
for some cell merging and/or training along the track of the
complex, and supports an isolated to scattered flash flood risk
extending east southeast across southern TN, northern MS/AL and
eventually northwest GA.

Additional development is also likely Friday night as convergence
along the southward dropping front is progged to increase. Some
west to east training is initially possible with this development
across portions of AR/MS. Activity may then grow upscale into
another MCS that would track southeast across portions of MS/AL.
While this would probably be progressive...this may be round two
for some areas after what transpires this morning and afternoon.
Add all this up, and despite the high FFG, think a broad Slight
risk is warranted from OK into western GA...accounting for
convection this morning/afternoon and for what develops later
tonight into early Saturday.

...New Mexico...
Weak shortwave impulses embedded within an amplifying ridge over
the Desert Southwest will produce ascent within an environment of
increasing heat and instability. HREF guidance suggests MUCAPE
will rise to around 2000 J/kg during peak heating...with
strengthening low level upslope flow and convergence along/behind
a southward dropping front helping initiate convective
development. The region will also be clipped by the right entrance
region of an upper jet to the northeast, enhancing divergence
aloft and supporting a maintenance of deep convection. HREF data
supports a high likelihood of 1" an hour rainfall, with
isolated/localized 2" within an hour possible over northeast NM.
Convection will initially be slow moving near the terrain, but
should eventually build upscale enough to propagate southeast
along the instability axis into the western TX Panhandle. Expect
that some flash flooding will occur over northeast NM where
convection initiates and is initially slow moving, and then a
lower but non-zero risk will exist as activity begins pushing off
to the southeast towards the TX border.

...Southeast Florida...
Not much change to the inherited Marginal risk across southeast
FL. Another day of scattered afternoon air-mass type
thunderstorms is likely across the FL peninsula, but some slightly
greater organization and coverage is possible compared to the past
few days. PWs around 2" will combine with MUCAPE reaching 3000
J/kg to provide extreme thermodynamics to support rainfall rates
of 2-3"/hr within convection. Although most of the development
will occur along the typical sea breeze/gulf breeze with
additional storms occurring along outflows, a weak boundary
dropping south across the peninsula may help slightly increase
convective organization. The 00z HREF guidance still supports
localized 3"+ rainfall amounts, which if occur over a sensitive
urban area, would cause some flooding concerns. With generally
weak low level flow, the latest HREF does focus the highest
rainfall over inland areas, with a bit less of a threat along the
urban coastline. So the risk of flash flooding is probably a bit
lower, but still can not rule out some heavy amounts along the
coast and will thus maintain the Marginal.

Chenard


Day 2
Valid 12Z Sat Jul 22 2023 - 12Z Sun Jul 23 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, CENTRAL PLAINS, SOUTHEAST, AND SOUTH FLORIDA...

...21Z Outlook Update...
The ongoing forecast (with discussion below) is generally on
track. Several areas of thunderstorm development are expected
across the Gulf Coast, Southeast, southern Rockies, and central
Plains especially from Saturday afternoon onward. Main changes
were to expand Marginal flash flood potential 1) into central
South Carolina, where newer guidance depicts 1+ inch rainfall
totals with likely deep convection in that area, 2) across western
Kansas where thunderstorm development is expected across areas of
wet soils from prior rainfall, and 3) across central/southern
Arizona where easterly mid-level flow will allow for a few areas
of thunderstorms to propagate westward off higher terrain and
produce spots of heavier rainfall on rocky/susceptible terrain.

A Marginal area was considered across Maine, although current
guidance is generally uncertain and totals in most models are
relatively light. Still, ground conditions are susceptible for
flash flooding given recent rainfall, and any trend toward heavier
or more widespread rainfall will likely necessitate an upgrade in
later outlooks.

Cook

...Previous Discussion...

...Southeast...
A continued amplification of the eastern CONUS longwave trough
will drive subtle height falls southward, pushing a cold front
towards the Gulf Coast. This front will slow or stall in a
generally west to east fashion Saturday as it becomes entrenched
within more zonal flow at the base of this broad cyclonic flow.
Convergence along this boundary aligned with right entrance region
upper jet dynamics will be enough to support and sustain
convection Saturday into Saturday night over the Southeast.
Instability along and south of the front will be substantial,
generally over 2000 J/KG, and PWs should generally be near or over
2"...thus hourly rainfall over 2" is certainly supported.

There remain some questions regarding convective mode and
evolution through the period. Organized convection may be ongoing
at 12z Saturday, with additional development through the day and
overnight probable as well. The flattening of the trough atop the
front will result in nearly unidirectional low and mid-level flow,
becoming more parallel to the front, which could result in more
significant training of convection. On the other hand the gradual
southward progression of the front and convective propagation
could push activity more steadily off to the south, which would
limit the degree of training. Overall do think the setup is
favorable enough that we will see some areas of flash flooding
evolve, but confidence on exactly when and where remains limited.
FFG is very high over this region, and thus do not really think a
large Slight risk would verify. Instead will probably have a
smaller area where things end up coming together, and so will hold
off on any upgrade at this time and continue to monitor. As we get
more into the high res guidance window we should be able to better
pin down potential risk areas.

...Southwest...
The inherited Marginal risk was adjusted only cosmetically to
account for new guidance, but remains generally unchanged for
portions of AZ and NM. The mid-level ridge will gradually expand
and intensify Saturday, but lobes of vorticity rotating around the
periphery will continue to produce waves of enhanced ascent. This
lift will occur within still favorable thermodynamics as PWs are
slightly above normal and MUCAPE reaches 1000+ J/kg. This will
result in scattered, primarily aftn/eve, convection with rainfall
rates possibly exceeding 1"/hr. Although organization and
intensity may be somewhat muted compared to Friday, storm motions
will remain quite weak and variable, so any heavy rain producing
cell could yield more than 1" of rainfall, which could result in
isolated instances of flash flooding, especially if these cells
move across more vulnerable areas.

...South Florida...
Anther day of afternoon thunderstorms is expected Saturday as an
overlap of impressive instability of 2000-3000 J/kg and PWs around
2" support diurnal convection. Forcing for ascent will increase
once again as the longwave trough across the eastern CONUS
amplifies further to produce at least modest height falls into
Florida, while the associated upper jet streak shifts off the
Southeast coast to leave modest upper diffluence over the
peninsula. The overlap of this ascent and favorable thermodynamics
will produce scattered to widespread thunderstorms with rainfall
rates of at least 2"/hr at times. Although at the moment model
guidance is generally less aggressive with convective organization
compared to Friday, slightly stronger westerly low level flow may
favor localized stationary cells along the urban east coast of the
state. Thus see no reason to deviate from the inherited Marginal
risk.

...Central Plains...
A Marginal risk was introduced across portions of KS/NE and
western MO with this update. Subtle shortwave energy riding over
top the impressive western CONUS ridge will interact with an area
lower level convergence to trigger convective development Saturday afternoon/evening over NE/KS. Organized convection will tend to
drop southward, and PWs are only around climatological average
values. So certainly not a high end or widespread flood threat.
However we'll have plenty of instability to work with, and the
persistent convergence may allow for some initial slow cell
movement. This pattern has generally been pretty productive in
supporting some intense convective clusters, so do tend believe
something closer to the wetter 00z GFS solution.

Chenard
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 220819
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Sat Jul 22 2023

Day 1
Valid 12Z Sat Jul 22 2023 - 12Z Sun Jul 23 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, CENTRAL PLAINS, SOUTHEAST, AND SOUTH FLORIDA...

...Southeast...
A continued amplification of the eastern CONUS longwave trough
will drive subtle height falls southward, pushing a cold front
towards the Gulf Coast. This front will slow or stall in a
generally west to east fashion today as it becomes entrenched
within more zonal flow at the base of this broad cyclonic flow.
Convergence along this boundary aligned with right entrance region
upper jet dynamics will be enough to support and sustain
convection Saturday into Saturday night over the Southeast. HREF
mean shows instability along and south of the front will be
substantial, generally 2000-3000 J/KG, and PWs should generally be
near or over 2". Given this overlap of favorable ingredients not
surprising that the HREF shows substantial probabilities of hourly
rainfall over 2" across this area.

Convection is expected to be ongoing at 12z Saturday across
portions of southern AR into MS and AL. This activity should
survive southeastward in some fashion across portions of AL/GA
through the morning hours. The expected forward progression of
this activity suggests only an isolated at best flash flood risk.
By this afternoon we should see additional convective development
closer to the central Gulf Coast along the low level convergence
axis. The flattening of the trough atop the front will result in
nearly unidirectional low and mid-level flow, becoming more
parallel to the front, which could result in some training of this
convection. However low level flow is rather weak, and not much in
the way of deeper layer shear either. So we are most likely
looking at a pulse convective mode, with propagation along
outflows. This scenarios can still produce heavy rainfall, but the
duration should be somewhat limited given the pulse convective
nature and gradual erosion of instability.

The most concentrated area of convection is expected to be from
southeast LA into southern MS/AL and the western FL Panhandle.
HREF neighborhood probabilities of exceeding 3" are 50-70%, but
drop off to 10-25% when looking at the 5" threshold. HREF EAS
probabilities of 1" and 2" do support a clustering of 1-2"+ totals
along the aforementioned corridor. For most areas of the country
this type of rainfall would solidly support a Slight risk. However
FFG is very high over this region, and despite the aforementioned
rainfall totals seen in the HREF, the HREF FFG exceedance
probabilities never really get above 10%. Thus while we do think
there will be some instances of flash flooding today/tonight, tend
to think the coverage will be too low to support a Slight risk.
Thus a Marginal risk was maintained with this update.

...Eastern North Carolina...
A Marginal risk was added across portions of eastern NC with this
update. Not all that confident in any flash flooding occurring,
but there was a notable uptick in HREF probabilities across this
region. A shortwave will traverse the area today, and guidance
indicates a north south boundary over eastern NC, both enhancing
convergence and maintaining higher instability air to its east.
Guidance is not unanimous in how things evolve today, but HREF 3"+
neighborhood probabilities are 40-70%, with 5" probabilities over
15% as well. FFG is high over this area, but if some of the wetter
scenarios unfold some exceedance is possible...thus think a
Marginal risk is warranted for.

...Southwest into the Plains...
A Marginal risk covers portions of AZ/NM into the TX/OK Panhandle
and southwest KS. The impressive mid level ridge will persist with
subtle lobes of vorticity rotating around the periphery continuing
to produce waves of enhanced ascent. This lift will occur within
still favorable thermodynamics as PWs are slightly above normal
and MUCAPE reaching 1000+ J/kg in spots. This will result in
scattered, primarily aftn/eve, convection with rainfall rates
possibly exceeding 1"/hr. Over NM, organization and intensity may
be somewhat muted compared to Friday, but storm motions will
remain quite weak and variable. Over AZ, PWs are a bit higher
compared to Friday, so would expect a bit higher convective
coverage and rainfall rate potential. Over both NM and AZ any
heavy rain producing cell could yield more than 1" of rainfall,
which could result in isolated instances of flash flooding,
especially if these cells move across more vulnerable areas.

Subtle shortwave energy riding over top the impressive western
CONUS ridge, weak upper divergence on the periphery of the
ridging, and an area of lower level convergence will work together
to trigger convective development Saturday afternoon/evening over
the central Plains. Organized convection will tend to drop
southward, and PWs are only around climatological average values.
So certainly not a high end or widespread flood threat. However
we'll have plenty of instability and shear to work with, so some
intense supercells and/or convective clusters are possible. The
low flash flood risk is probably mainly confined to areas that saw
heavy rainfall the other day over southwest KS, so trimmed back
the eastern extent of the Marginal risk to only include this area.
This convection may also eventually get into the TX Panhandle,
with some potential of cells propagating into this region from NM
as well.

...South Florida...
Anther day of afternoon thunderstorms is expected Saturday as an
overlap of impressive instability of 2000-3000 J/kg and PWs around
2" support diurnal convection. Activity on Friday afternoon
generally under performed compared to HREF guidance...and that
could certainly happen again today. However what we have today
that we did not yesterday is slightly stronger westerly low level
flow, which may favor localized stationary cells along the urban
east coast of the state. So do think the threat today is a bit
higher than Friday, so will maintain the Marginal risk, with HREF
neighborhood probabilities showing a 40-70% chance of some 3"+
rainfall totals.

Chenard


Day 2
Valid 12Z Sun Jul 23 2023 - 12Z Mon Jul 24 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST AND SOUTHEAST...

...Southeast into the Mid Atlantic...
Generally a continuation of the pattern from day 1 with a broad
longwave trough, upper jet, and a low level front in place with
high PWs and instability along and south of it. Northern FL will
need to be watched closely, as the ingredients are in place to
potentially support an organized area of training/backbuilding
convection. An uptick in low level flow along the front and a more
favorable position of the upper jet to the north would support
organized convective development. Meanwhile unidirectional flow
parallel to the front could support a training and backbuilding
threat. Although given the high FFG over this area everything does
need to come together just right to really drive any more
organized flood threat. While that may happen, there are some
indications that the front will have just enough of a southerly
push to keep convection propagating south and east with time, and
other indications that the heaviest convection could end up
focused offshore. For these reasons think the risk remains at a
Marginal level, but will continue to monitor.

Over the Carolinas into VA the convective threat appears to be
increasing compared to prior forecasts. Shortwave energy within
the approaching trough axis is trending a bit stronger in most
guidance, which is driving a bit better mid/upper forcing and
lower level convergence. This has resulted in a notable uptick in
recent GFS and ECMWF QPF over this region. The 00z HREF shows
instability upwards of 1000-2000 J/kg and PWs increasing towards
1.5", with generally scattered/disorganized diurnal convective
development. There are some slow/chaotic cell motions evident in
HREF simulated reflectivities, which would support some cell
merging and a localized flash flood threat given the
thermodynamics in place. The aforementioned trends evident in the
GFS/ECMWF actually support a bit higher PWs over the region along
with a more focused corridor of lower level convergence. Thus will
be interesting to see trends in the HREF to see if it also begins
to trend upwards with the potential organization of convective
activity. Can not rule out eventually needing a Slight risk
somewhere over the Carolinas or VA, but for now think this is a
solid Marginal risk.

...Southwest...
Not much change here compared to Saturday, although the better
moisture does migrate westward by this time, which should lower
the risk over NM. But marginally above average PWs over AZ will
persist, with another day of isolated to scattered diurnally
driven convection. Any storms could drop a quick inch of rain,
which could drive a localized flash flood risk within any more
susceptible basins.

Chenard

Day 3
Valid 12Z Mon Jul 24 2023 - 12Z Tue Jul 25 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, MIDWEST, MID ATLANTIC AND NORTH FLORIDA...

...Mid Atlantic...
The broad longwave trough over the east will continue to drive a
scattered convective threat on Monday. Most model guidance has
been trending a bit stronger with the shortwave energy moving east
within the trough axis. This seems to be driving the development
of a low pressure near the coast and slightly quicker eastward
progression of post frontal drying. These factors are resulting in
a lowered convective threat over most of the Southeast, but a
better chance over portions of the Mid Atlantic. The stronger
forcing as a whole should be able to drive a slightly more
organized convective risk by this time, although there remain
differences in the exact evolution. The thermodynamic environment
appears conducive for storms anywhere from eastern NC into central
NY along a convergence axis. The better potential for higher rates
should be over the southern half of this area where instability
and PWs are higher. But even over PA and NY cells should be able
to drop a quick 1" of rain in 30 to 60 min...and lower FFG over
this area suggests isolated FFG exceedance is possible.

There is some risk of a more targeted corridor of heavier rainfall
where low level convergence is maximized near the potential
developing low. Both the 00z ECMWF and UKMET show this across
portions of northern VA into the Delmarva, but a lack of run to
run consistency means low confidence on these details. While not
an overly impressive setup for flash flooding, the shortwave
energy and strong upper jet, combined with a potential low in the
area, does suggest we'll need to keep a close eye on this region
in the coming days.

...IA/MO/IL...
There is decent model consensus for a nocturnal convective complex
over portions of IA/MO/IL Monday evening/night. Shortwave energy
riding over the southwest ridge and increasing low level moisture transport/convergence should be the driving factors behind the
convective threat. Not looking at anomalous PWs in place...but we
should have extreme instability...thus any upscale convective
growth should be intense. Thus heavy rainfall rates would likely transpire...with the magnitude of any flash flood threat then
coming down to the duration of these rates. That's hard to pin
down at this lead time...as although several indicators suggest a
rather progressive convective complex off to the southeast...the
presence of the extreme instability pool upwind of the low level
inflow could support some backbuilding. All in all nothing jumps
out as a high end flash flood threat, but at least a low threat
would exist with any convective complex that develops. Given the
fairly good model agreement on this evolution, think a Marginal
risk remains warranted...with some location adjustments probable
as we get closer to the event.

...Southwest...
Not much change here compared to day 2, with the better moisture
generally staying over AZ into UT. Marginally above average PWs
over AZ/UT and subtle forcing on the periphery of the mid/upper
level ridge will drive another day of isolated to scattered
diurnally driven convection. Any storms could drop a quick inch of
rain, which could drive a localized flash flood risk within any
more susceptible basins.

...North FL...
The Marginal risk over the eastern CONUS was split into two, with
much of the Southeast cleared from the risk given expected post
frontal drying. However not yet comfortable with removing portions
of northern FL as the front should remain the vicinity, with
continued unidirectional flow and upper jet dynamics supporting a
training convective threat. With that said, guidance is trending
down...with slightly less low level flow and marginally weaker
upper level divergence by this time. So convective organization
may be less and/or stay more confined to offshore areas. If this
trend continues we may eventually be able to remove the Marginal
risk.

Chenard
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 230824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Sun Jul 23 2023

Day 1
Valid 12Z Sun Jul 23 2023 - 12Z Mon Jul 24 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, SOUTHEAST AND MID ATLANTIC...

...Southeast into the Mid-Atlantic...
An area of heavy rainfall appears likely this morning and
afternoon across portions of northwest FL, with ingredients in
place to potentially support an organized area of
training/backbuilding convection. An uptick in 850mb moisture
transport and convergence, and a more favorable position of the
upper jet to the north would support organized convective
development. Meanwhile unidirectional flow parallel to the front
could support a training and backbuilding threat. Most of the 00z
HREF members do show organized convection with some training this
morning. HREF neighborhood probabilities of exceeding 3" and over
80%, with 5" probabilities more in the 30-50% range. These numbers
seem reasonable, and would expect a swath of 2-4" of rainfall with
very localized totals of 4-7" possible. Contemplated a targeted
Slight risk upgrade for this area, but opted against for now. Most
of the high res guidance indicate the organized convection will
eventually develop a southward push that should gradually
accelerate with time...and this progression may make it difficult
to exceed the very high FFG that is in place. Even with the
impressive rainfall the HREF is outputting, HREF FFG exceedance
probabilities don't get much above 5-10%. Thus while localized
flash flooding is possible, tend to think the coverage will remain
localized enough to keep the risk at a marginal level.

By this afternoon we should see additional convective development
over portions of the FL Panhandle into southern AL/GA closer to
the actual surface front. Mean flow parallel to the front may
result in some training here, but also expecting this convection
to gradually push off to the southeast. Recent rainfall has
lowered FFG to some extent across portions of this area, so do
think at least a few instances of FFG exceedance are probable.

Another area of locally heavy rainfall is expected across the
Carolinas into VA. Central and eastern SC looks to be one area of
focus near the stationary front draped across the area. This
boundary should aid in the development of convective clusters,
with local mergers potentially resulting in some higher rainfall
totals. Enough shear is present to support some maintenance of
convective cells, and with PWs forecast around 2" heavy rainfall
rates are likely. Did give some consideration to a Slight risk
here as well, but there's some placement differences amongst the
models, and overall tend to think the coverage of flash flooding
will stay rather localized, thus think a Marginal risk should
suffice.

Convection is also likely across NC into VA with some development
focusing along the terrain of the western Carolinas into southwest
VA. As we go into Sunday night a weak wave of low pressure will
move northeast along the stationary front, which may help
nocturnal convection persist across portions of the
central/eastern Carolinas into southeast VA. This activity may
also pose an isolated flash flood risk.

...Southwest...
The strong mid/upper ridging shifts a bit eastward compared to
Saturday, which should allow the better moisture to migrate a bit
westward. This lowering of moisture and increasing heights over NM
should lower the flash flood risk there. Marginally above average
PWs over AZ will persist and nose northward into southern NV, with
another day of isolated to scattered diurnally driven convection.
Any storms could drop a quick inch of rain (00Z HREF probs for 1"
amounts over the day 1 period peak around 25-40%), which could
drive a localized flash flood risk within any more susceptible
basins.

Chenard


Day 2
Valid 12Z Mon Jul 24 2023 - 12Z Tue Jul 25 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, MIDWEST, MID-ATLANTIC AND NORTH FLORIDA...

...Mid-Atlantic...
The broad longwave trough over the east will continue to drive a
scattered convective threat on Monday as shortwave energy moves
east within the trough axis. The approaching mid level trough axis
combined with a ~100 kt jet at 250mb providing right entrance
upper divergence, should allow for a surface low to intensify and
push off the VA/NC coast Monday. Enhanced convergence associated
with this low may drive a more organized convective threat Monday morning/afternoon across northeast NC into southeast VA.
Definitely some potential for 2-4" rainfall amounts with this
activity as PWs approach 1.8" and instability remains over 1000
j/kg. Can not rule out needing a focused Slight risk, but for now
still think the coverage of any flooding will stay rather
localized given forward cell motions. Plus there is still some
uncertainty on exactly how the details of this setup with the low
and associated frontal structure plays out.

Scattered convection is also expected along/near a surface trough
north of the low extending across VA and into southern NY.
Instability is forecast around or above 1000 j/kg along this
trough axis, and the lower level convergence combined with the
approaching mid level trough should be enough to trigger
convective development. Cells should generally pulse up and down
fairly quickly and not expecting much convective
organization...however the persistent low level convergence and
deep layer southerly flow may allow for some brief
persistence/repeat cells in spots...and rainfall rates will be
high enough to drop a quick 1-2" of rain on a localized basis.
HREF neighborhood probabilities through 00z Tuesday of exceeding
2" are over 30%, with some low 3" probabilities as well. Not
expecting widespread or high end flooding, but localized flash
flooding seems plausible anywhere along/near the low level
convergence axis from northern VA into southern NY.

...IA/MO/IL...
Still seeing decent model consensus for a nocturnal convective
complex over portions of IA/MO/IL Monday evening/night. Shortwave
energy riding over the southwest ridge and increasing low level
moisture transport/convergence should be the driving factors
behind the convective threat. These features are pretty subtle
though, so there is some inherent uncertainty with both the
placement and magnitude of any convective threat. Not looking at
anomalous PWs in place (though they are relatively high this time
of year anyway)...but we should have extreme instability...thus
any upscale convective growth should be intense and heavy rainfall
rates would likely transpire. The magnitude of any flash flood
threat depends on the duration of these rates which is hard to pin
down at this lead time. Although several indicators suggest a
rather progressive convective complex off to the southeast...the
presence of the extreme instability pool upwind of the low level
inflow could support some backbuilding. All in all nothing jumps
out as a high end flash flood threat, but at least a low threat
would exist with any convective complex that develops. Given the
fairly good model agreement on this evolution, think a Marginal
risk remains warranted...with some location adjustments probable
as we get closer to the event. For now, preferred something close
to the consensus seen with the 00z ECMWF/UKMET/Gem Reg.

...Southwest...
On Monday the mid level ridge continues to build over the
southwest, with height anomalies maxing out over CO and NM. This
should keep the better moisture and forcing on the periphery of
the ridge to the west of these areas...generally across AZ,
southeast NV and into UT.

PW anomalies are a bit higher compared to day 1 over AZ/NV/UT,
which combined with subtle forcing on the periphery of the
mid/upper level ridge, will drive another day of isolated to
scattered diurnally driven convection. Any storms could drop a
quick inch of rain, which could drive a localized flash flood risk
within any more susceptible basins.

...North FL...
The Marginal risk over northern FL was maintained as the front
should remain in the vicinity, with continued unidirectional flow
and upper jet dynamics supporting a training convective threat
similar to what should occur Sunday. Global model QPF is inching
up, and the HREF is also indicating some higher QPF potential.
Just like on Sunday, the convection may tend to propagate
southward with time enough to limit the magnitude of the flash
flood risk. However antecedent conditions may be a bit more
sensitive by this time as some of these same areas may see heavy
rainfall Sunday. Combine this potential factor with the overall
persistence of favorable ingredients, and think maintaining the
Marginal risk is the way to go for now.

Chenard

Day 3
Valid 12Z Tue Jul 25 2023 - 12Z Wed Jul 26 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST AND NORTHEAST...

...Northeast...
The mid level trough axis and surface front will work together to
trigger scattered convection across the Northeast on Tuesday. PWs
looks to be marginally above average, and instability is forecast
~2000 j/kg. So seems to be a thermodynamic environment favorable
for heavy rainfall rates, and would expect the potential to be
there for cells to drop a quick 1-2" in spots. Not looking for
convection that is all that organized given the modest mid/upper
support and generally weak shear, so the coverage of any heavier
rainfall rates should be rather isolated. In a typical year this
type of setup would probably not result in a flash flood
risk...but given the continued above average soil saturation and
streamflows over much of this region, do think a localized flash
flood risk is a possibility.

...Southwest...
The core of the mid level ridge remains centered over CO and NM,
with enough subtle forcing on the periphery of the ridge to drive
another day of isolated to scattered diurnally driven convection.
Any storms could drop a quick inch of rain, which could drive a
localized flash flood risk within any more susceptible basins. The
Marginal risk is quite large and pretty much covers most areas
where there is at least some convective threat Tuesday. It seems
unlikely that this area will technically meet the 5%+ coverage of
a Marginal risk, but given the nature of convection in this region
it's hard to pin down exactly where a localized flash flood could
occur. Thus continue to take the safer option of covering a lot of
the region in a low end Marginal risk to account for the
flashiness of some of the basins in the area.

...Elsewhere across the CONUS...
The ECMWF has been rather consistent in bringing a shortwave over
the ridge and into the Dakotas Tuesday, which would result in an
organized convective threat. However other models are not nearly
as aggressive with this feature, and the 00z ECMWF mean is quite a
bit lower, likely indicative of spread within the EC ensembles. So
while there is a conditional localized flash flood risk, opted not
to introduce any Marginal risk area at this time given the low
predictability.

There are also some subtle signs of a heavy rainfall potential
across portions of IN/KY/TN and vicinity. There does appear to be
an axis of enhanced instability in most of the guidance across
this corridor, with some semblance of lower level moisture transport/convergence. Pretty weak QPF signal amongst most of the
deterministic and ensemble guidance suggests the threat is pretty
low. However the fact that there is at least some light QPF
present in most solutions within such a highly unstable airmass
with some forcing in the vicinity...does suggest that there is
some potential for an over performing convective area. The 00z GEM
reg is the lone aggressive QPF output, and while it seems like an
outlier, something similar can not be completely ruled out. Not
nearly enough confidence to introduce any Marginal risk at this
time, but will continue to keep an eye on the area.

Chenard
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 241956
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
356 PM EDT Mon Jul 24 2023

Day 1
Valid 16Z Mon Jul 24 2023 - 12Z Tue Jul 25 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHEAST/MID-ATLANTIC, MID-MISSISSIPPI VALLEY AND OHIO
VALLEY, FLORIDA, AND THE SOUTHWEST/GREAT BASIN...

...Northeast/Mid-Atlantic...
Anomalous trough centered over the Hudson Bay will maintain
lowered heights across the eastern CONUS, and drive a cold front
slowly eastward across the Great Lakes and towards Upstate NY
today. A modest shortwave is progged to rotate through the base of
the trough this aftn/eve causing subtle height falls and PVA into
the Mid-Atlantic and Northeast, combining with at least modest
upper diffluence in the tail of a jet streak to drive deep layer
ascent across the region. This lift will act upon an environment
favorable for heavy rain noted by PWs of 1.25-1.5 inches, around
the 75th percentile according to the SPC sounding climatology, and
MUCAPE of 1000-1500 J/kg to produce convection which the HREF and
HRRR both indicate will have rain rates of 1-2"/hr due to weak
mid-level lapse rates in deep warm cloud depths.

The simulated reflectivity from the high-res today generally
features scattered convection, but the presence of 30-40 kts of
effective bull shear may help drive multi-cell organization in
some areas, especially Upstate NY into western New England. In
general these storms will move progressively to the E/NE on 0-6km
mean winds of 20-25 kts, but some short term training is possible
as Corfidi vectors align to this flow downstream of the primary
trough axis. The HREF probabilities for 1" and 3" of rain are
relatively modest, so despite compromised FFG due to 14-day
rainfall that is 150-300% of normal, the FFG exceedance
probabilities are low. For this reason the MRGL risk was just
adjusted cosmetically, but should more organized convection
develop and train, especially from eastern PA northward through
the Hudson Valley and into western New England, which has at least
a subtle signal in the HRRR and experimental RRFS, some scattered
instances of flash flooding could be possible.


...MS VLY/Ohio Valley...
An MCS moving through Missouri this morning is being driven by a
weak shortwave dropping through NW flow on the upwind side of a
large trough across the eastern CONUS. This MCS should weaken by
the aftn in response to the veering and waning LLJ, but the
accompanying shortwave impinging the weak low-level baroclinic
zone along a weak warm front/stationary front will drive
additional convection to the east this aftn/eve. Subtle height
falls within the mid-level trough will additional aid ascent,
which when acting upon an environment with PWs of 1.25-1.5 inches
and a ribbon of MUCAPE exceeding 2000 J/kg will produce convection
with heavy rain rates of 1-2"/hr. Although mean 0-6km winds will
be progressive at 15-20 kts, these should remain aligned generally
to the front, suggesting some short duration training, with
effective bulk shear driving multi-cell clusters across the
region. Where repeated rounds of thunderstorms can occur, rainfall
could reach 2-3" as reflected by HREF exceedance probabilities.
Where this rain falls atop more sensitive soils noted by FFG as
low as 1.5"/3hrs, some isolated flash flooding is possible.


...Florida...
Deep trough centered over the Hudson Bay will extend into the Gulf
Coast today, leaving generally flat zonal flow across the Gulf of
Mexico and Southeast. A weak impulse rotating through the flow
will combine with the tail of a jet streak positioned offshore and
an approaching cold front which will stall in the vicinity to
produce scattered convection today. Nearly unidirectional flow
over the Gulf of Mexico will drive convection west to east into
the northern FL Peninsula today, driven by this ascent working
over an airmass characterized by a ribbon of 2" PWs and MUCAPE
above 1000 J/kg. Flow parallel to the approaching front will allow
for training of cells into FL, while Corfidi vectors becoming
increasingly aligned anti-parallel to the mean wind suggests
backbuilding to lengthen rainfall duration. HREF probabilities for
more than 3" peak along the Nature Coast and around the Big Bend
of FL, with rainfall rates likely reaching 2"/hr at times.
Although this area has generally been dry recently, training of
these rates could produce isolated flash flood responses,
especially in any urban areas.


...Southwest/Great Basin...
Expansive mid-level ridge will become entrenched across the
Southwest/Four Corners area, anchoring over New Mexico today. The
placement of this ridge will allow for return flow to spread
across the western portions of the Southwest and into the Great
Basin, noted by PW anomalies on the NAEFS ensemble tables reaching
+1 to +2 sigma from western Arizona northward through NV/UT and
into ID/WY. While subsidence beneath the core of the ridge will
suppress much in the way of convection due to a +18C 700mb cap,
this PW plume around its periphery combined with weak shortwave
impulses lifting around the ridge will likely result in scattered
convection, primarily during peak heating as MUCAPE rises to
750-1000 J/kg.

The inherited MRGL risk was adjusted via an expansion northward to
account for the higher PW plume and at least modest signals in the
HREF for 1"/hr rates. Storms are likely to be moving pretty
quickly on 0-6km mean winds to the northeast at 15-20 kts, but
could still produce 15-min rainfall totals of 0.25-0.5 inches as
reflected by the HRRR sub-hourly. In general this will be less
than FFG, but where storms can move a bit slower (primarily
southern AZ) or where this rain can occur atop more sensitive
terrain features like washes and slot canyons, isolated flash
flood instances are possible.


Weiss


Day 2
Valid 12Z Tue Jul 25 2023 - 12Z Wed Jul 26 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC...

...Mid-Atlantic/Northeast...
A longwave trough will persist across the eastern CONUS extending
from a closed low near the Hudson Bay. Within this trough, an
embedded shortwave will advect eastward while amplifying, moving
across the coastal Mid-Atlantic and into New England during peak
afternoon heating. This ascent will work upon a favorable
thermodynamic environment characterized by PWs reaching above 1.5"
collocated with MUCAPE eclipsing 1500 J/kg. The result of this
will rapid development of thunderstorms during the aftn/eve, with
cells organizing into clusters through 25-35 kts of effective bulk
shear. Storm motions of these cells are likely to remain
progressive to the east/northeast, but some short duration
training is possible. Rain rates within convection will likely
exceed 1"/hr, and may reach 2"/hr as shown by HREF probabilities
and the HRRR sub-hourly fields. This will result in bands of 1-3"
of rainfall, with locally heavier totals possible as reflected by
modest exceedance probabilities in both the HREF and experimental
RRFSe. The greatest risk area for this heavier rainfall in both
ensembles is along the I-95 corridor from around Fredericksburg,
VA to just west of New York City. While confidence here is modest
due to spread in the placement of more organized convection and a
possible MCS in the high-res simulated reflectivity, the greatest
overlap of exceedance probabilities with saturated soils is across
this same region. This prompted the coordination of a small SLGT
risk embedded within the larger MRGL risk extending all the way to
the Canadian border. Scattered instances of flash flooding are
possible, especially where these intense rates track across urban
areas, with a more isolated potential existing elsewhere across
the Northeast.


...Southwest/Four Corners...
Mid-level monsoon ridge will remain entrenched across the Four Corners/Southwest Tuesday with only slight reduction in amplitude
as weak shortwaves traverse the periphery and a more amplified
shortwave trough digs out of the Pacific Northwest. This will
produce slightly greater deep layer ascent, which when combined
with 500-750 J/kg MUCAPE should result in scattered showers and
isolated thunderstorms Tuesday. The PW plume from Monday is
progged to weaken D2 noted by NAEFS ensemble tables showing just
near normal PWs, but this will still be sufficient to produce
rainfall rates of 0.5"/hr, with local rates as high as 1"/hr as
progged by HREF neighborhood probabilities. The guidance has
backed off slightly on total coverage and rainfall amounts, but
the high-res simulated reflectivity still suggests storms will
rotate SW to NE at around 15 kts, with some multi-cell
organization possible within effective bulk shear of 15-25 kts.
The flash flood threat appears modest overall, but if any of these
more intense rain rates can occur atop sensitive features like
burn scars, washes, or urban areas, isolated instances of flash
flooding could be the result.


Weiss

Day 3
Valid 12Z Wed Jul 26 2023 - 12Z Thu Jul 27 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE FOUR CORNERS, AND IN THE VICINITY OF THE GREAT LAKES...

...Upper Midwest through the Ohio Valley...
Dual shortwaves moving across the region on Wednesday will advect
more easterly through the period in response to flattening flow
north of an expanding/elongating mid-level ridge centered near the
Four Corners. The first of these is progged to move from the
Arrowhead of MN early to Upstate NY late, with a secondary impulse
trailing it to move across a similar area the latter half of the
day. Each of these will be accompanied by a modest wave of low
pressure with associated fronts to enhance ascent from west to
east. While uncertainty continues into the timing and latitude of
these features, each one will likely produce scattered convection
through PWs that approach +2 standard deviations overlapped with
MUCAPE that may surge above 3000 J/kg ahead of the fronts.

The simulated reflectivity during this time indicates that
convection may organize into clusters or even MCSs through 30-40
kts of effective bulk shear, driving rain rates that may reach
2"/hr at times. With the mid-level flow becoming increasingly
zonal, mean flow will become aligned to the warm fronts suggesting
an increasing likelihood for training from west to east, with some
backbuilding into the greater instability also possible. This
could result in multiple rounds of heavy rain producing
thunderstorms moving across the region Wednesday, and the MRGL
risk has been expanded from MN east through OH/NY. This was done
to best match the ECENS/GEFS/SREF probabilities, and to account
for spatial spread in the guidance over soils that have
compromised FFG as low as 1.5-2"/3hrs.


...Four Corners...
The monsoon ridge will remain amplified near the Four Corners on
Wednesday, but some elongation is noted in the mass fields from SW
to NE which should allow higher PW anomalies to drape farther
northeast than on previous days. This evolution of the mid-level
ridge is due to a shortwave advecting east from the Pacific which
will drive height falls from the NW, resulting in this elongation
of the ridge. PW anomalies are again progged to be normal to
slightly below according to the NAEFS ensemble tables, but MUCAPE
may reach as high as 1000 J/kg beneath the ridge, especially since
convective debris cloudiness is expected to be minimal on
Wednesday. This results in still favorable thermodynamics for
heavy rainfall, and as forcing increases through subtle height
falls and any weak impulses rotating around the periphery of the
ridge, scattered showers and thunderstorms are likely during the
aftn/eve. Rainfall rates of up to 1"/hr are probable, which
despite generally fast forward motion to the E/NE could produce
locally 0.5-1" of rain as shown by modest ECENS/GEFS
probabilities. Overall the flash flood risk on Wednesday appears
isolated, but a few instances are possible especially over the
most vulnerable soils or terrain features.


Weiss
$$
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* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 252019
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Tue Jul 25 2023

Day 1
Valid 16Z Tue Jul 25 2023 - 12Z Wed Jul 26 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC...

...16Z Update...

...Mid-Atlantic...

No big changes were made to the ERO for today/Tuesday. The Slight
Risk area was expanded a bit towards the north and east, to
include all of the Jersey Shore and all of Delaware, but was
trimmed a bit on the southern end to remove portions of central VA
and southern MD, which are both more difficult to flash flood and
less signal for heavy rainfall rates there, but also expanded to
include all of the NYC metro out to central Long Island and far
southern NY to include Rockland and Westchester Counties. In
addition to the rainfall, CAMs guidance has been in good agreement
that the storms will impact Baltimore through NYC around the
evening rush, which will maximize potential impacts. These changes
were coordinated with OKX/Upton, NY, PHI/Mt. Holly, NJ, and
LWX/Sterling, VA forecast offices.

Elsewhere, there is considerably more uncertainty across northern
VA and also into southern New England as to how much coverage of
storms there will be. Thus, the inherited Marginal Risk remains
unchanged. Of course there's potential for localized flash
flooding, but since the greatest coverage will be in the Slight
Risk area, the potential for flash flooding due to less coverage
of storms is lower in the surrounding Marginal Risk area. The 12Z
HREF guidance is very similar to the previous 06Z guidance, so no
changes were needed on account of the 12Z guidance.

...Northern Plains/Midwest...

Very few changes needed to the inherited Marginal Risk area. There
remains some potential for storms to move across this region,
highlighting far eastern SD and much of southwestern MN. However,
there remains considerable uncertainty as to the coverage and
intensity of those storms, with decent agreement that the storms
will be fast-moving. As mentioned below instability and PWATs will
be quite high in this region, which should allow the storms that
form to become intense and will be capable of high rainfall rates.
Due to the aforementioned uncertainty, rather rural area with lots
of agriculture, and dry antecedent conditions, the Marginal Risk
was determined sufficient with this update.

...Four Corners...

No major changes to the low-end Marginal Risk across this region.
Isolated to widely scattered convection still looks probable this
afternoon, but dry atmospheric conditions and a limited moisture
supply should greatly limit the ability of any of the storms to
produce flash flooding.

Wegman

...Previous Discussion...

...Mid-Atlantic/Northeast...
A longwave trough will persist across the eastern CONUS today,
extending from a closed-low near the Hudson Bay. Within this
trough, an embedded shortwave will advect eastward while
amplifying, moving across the coastal Mid-Atlantic and into New
England during peak afternoon heating. This ascent will work upon
a favorable thermodynamic environment characterized by PWs
reaching above 1.5" collocated with MUCAPE eclipsing 1500 J/kg.
The result of this will be rapid development of thunderstorms
during the afternoon/evening, with cells organizing into clusters
through 25-35 kts of effective bulk shear. Storm motions of these
cells are likely to remain progressive to the east/northeast, but
some short duration training is possible. Rain rates within
convection will likely exceed 1"/hr, and may reach 2"/hr as shown
by HREF probabilities and the HRRR sub-hourly fields. This will
likely result in bands of 1-3" of rainfall with locally heavier
totals possible (as reflected by 00z HREF 40-km neighborhood
exceedance probabilities for the 3" threshold as high as 25-35%).
The greatest risk area for this heavier rainfall is generally
along the I-95 corridor from around Fredericksburg, VA to just
west of New York City (with a modest northward expansion of the
Slight risk for this cycle, incorporating more of northern NJ and
edging into portions of southern NY and NYC). Confidence has
increased a bit with this cycle, and the Slight risk continues to
overlap where the highest exceedance probabilities coincide with
the lowest FFGs and saturated soils is across this same region.
Scattered instances of flash flooding are possible, especially
where these intense rates track across urban areas, with a more
isolated potential existing elsewhere across the Northeast.


...Northern Plains/Midwest...
A Marginal risk has been introduced across portions of
southeastern ND, eastern SD, southwestern MN, far northeastern NE,
and northern IA for this cycle, as the 00z HREF has become
increasingly focused on an area of convection developing later
this evening and into tonight. This convection will largely be
driven by a shortwave trough within the northern stream,
traversing the northern periphery of the very dominant Southwest
ridge. As the trough rounds the ridge and digs southeastward this
evening, thunderstorms are expected to rapidly initiate and
organize amidst impressive deep layer shear of 30-40 kts.
Instability will likely range from 2000-3000+ J/kg (MU CAPE) with
PWs progged to increase to 1.5-1.8" (between the 90th percentile
and max moving average, per ABR sounding climatology) as the
low-level (850 mb) jet strengthens to 15-25 kts. This will result
in strong to severe storms that will be capable of dropping
localized totals of 2-4" over a rather short period (with 00z HREF
neighborhood exceedance probabilities of 3"/3-hr as high as
10-20%). Convection is expected to remain rather progressive,
limiting training potential (and likely keeping any instances of
flash flooding localized).


...Southwest/Four Corners...
Mid-level monsoon ridge will remain entrenched across the Four Corners/Southwest Tuesday with only slight reduction in amplitude
as weak shortwaves traverse the periphery and a more amplified
shortwave trough digs out of the Pacific Northwest. This will
produce slightly greater deep layer ascent, which when combined
with 500-750 J/kg MUCAPE should result in scattered showers and
isolated thunderstorms Tuesday. The PW plume from Monday is
progged to weaken D2 noted by NAEFS ensemble tables showing just
near normal PWs, but this will still be sufficient to produce
rainfall rates of 0.5"/hr, with local rates as high as 1"/hr as
progged by HREF neighborhood probabilities. The guidance has
backed off slightly on total coverage and rainfall amounts, but
the high-res simulated reflectivity still suggests storms will
rotate SW to NE at around 15 kts, with some multi-cell
organization possible within effective bulk shear of 15-25 kts.
The flash flood threat appears modest overall, but if any of these
more intense rain rates can occur atop sensitive features like
burn scars, washes, or urban areas, isolated instances of flash
flooding could be the result.


Churchill/Weiss



Day 2
Valid 12Z Wed Jul 26 2023 - 12Z Thu Jul 27 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF LOWER
MICHIGAN...

...2030Z Update...

...Michigan...

A potent MCS being forced by a strengthening shortwave moving east
across the northern tier of states is likely to develop across
Lower Michigan Wednesday afternoon. The combination of peak
daytime heating and a 40 kt southwesterly jet advecting an over 2
inch PWAT airmass into Lower Michigan will set the stage for
numerous areas of 2 inches or more rainfall amounts to occur as a
result of the MCS, particularly in southeastern Lower Michigan,
including the Detroit area. PWATs above 2 inches is more than 2.5
sigma above normal for this time of year. Antecedent conditions
are a bit wetter than normal, though not extremely so. This should
allow for a somewhat enhanced susceptibility to flash flooding
across the Slight Risk area, especially into Detroit where
urbanization will further lower FFGs. Neighborhood probabilities
remain around 60% for 3 inches of rain through 12Z Thu and up to
20% for 5 inches. There remains considerable but understandable
variability in exactly where those higher rainfall amounts will
occur as the CAMs try to resolve the convective evolution of the
MCS. Nonetheless given the excellent agreement that an MCS will
cross the area Wednesday afternoon and the overall environmental
setup with instability, high PWATs and a strong LLJ, confidence
that flash flooding will occur has increased. The Slight Risk
upgrade was coordinated with DTX/Detroit, MI and APX/Gaylord, MI
forecast offices.

...Rest of the U.S....

Elsewhere, much of southwest FL was removed from the Marginal due
to both dry antecedent soil moisture conditions and the Everglades
being rather flood-proof, and so the Marginal risk was limited to
the I-95 corridor from West Palm Beach south, including the Keys.

Across the Southwest, the guidance has backed off significantly on
convective coverage across the area, likely due to dry mid-level
conditions. Certainly there will still be convection around, but
it should be both weaker and more widely scattered based on the
most recent guidance. The Marginal Risk area was trimmed away from
where the WPC consensus had no meaningful precipitation with this
update. There remains considerable uncertainty in this area,
however so future expansions may be needed.

Wegman

...Previous Discussion...

...Upper Midwest through the Ohio Valley...
Dual shortwaves moving across the region on Wednesday will advect
more easterly through the period in response to flattening flow
north of an expanding/elongating mid-level ridge centered near the
Four Corners. The first of these is progged to move from the
Arrowhead of MN early to Upstate NY late, with a secondary impulse
trailing it to move across a similar area the latter half of the
day. Each of these will be accompanied by a modest wave of low
pressure with associated fronts to enhance ascent from west to
east. While uncertainty continues into the timing and latitude of
these features, each one will likely produce scattered convection
through PWs that approach +2 standard deviations overlapped with
MUCAPE that may surge above 3000 J/kg ahead of the fronts.

The simulated reflectivity during this time indicates that
convection may organize into clusters or MCSs through 30-40 kts of
effective bulk shear, driving rain rates that may reach 2"/hr at
times. With the mid-level flow becoming increasingly zonal, mean
flow will become aligned to the warm fronts suggesting an
increasing likelihood for training from west to east, with some
backbuilding into the greater instability also possible. This
could result in multiple rounds of heavy rain producing
thunderstorms moving across the region Wednesday, and an eventual
Slight risk introduction may be needed (most likely across
portions of the lower peninsula of MI and possibly into portions
of northern IN/OH). Neighborhood (40-km) probabilities for 3" and
5" exceedance are already as high as 30-60% and 15-25%
(respectively) through 00z Thurs (via the latest 00z HREF), but
closer examination of the individual members suggests a high
degree of north/south spread in the QPF axis (with some members
still suggesting QPF magnitudes closer to the Marginal threshold),
so this Slight risk potential will be reevaluated with subsequent
outlooks.


...Four Corners...
The monsoon ridge will remain amplified near the Four Corners on
Wednesday, but some elongation is noted in the mass fields from SW
to NE which should allow higher PW anomalies to drape farther
northeast than on previous days. This evolution of the mid-level
ridge is due to a shortwave advecting east from the Pacific which
will drive height falls from the NW, resulting in this elongation
of the ridge. PW anomalies are again progged to be normal to
slightly below according to the NAEFS ensemble tables, but MUCAPE
may reach as high as 1000 J/kg beneath the ridge, especially since
convective debris cloudiness is expected to be minimal on
Wednesday. This results in still favorable thermodynamics for
heavy rainfall, and as forcing increases through subtle height
falls and any weak impulses rotating around the periphery of the
ridge, scattered showers and thunderstorms are likely during the afternoon/evening. Rainfall rates of up to 1"/hr are probable,
which despite generally fast forward motion to the E/NE could
produce locally 0.5-1" of rain (and often in as little as 15-30
min) as shown by modest ECENS/GEFS probabilities. Overall the
flash flood risk on Wednesday appears isolated, but a few
instances are possible (especially over the most vulnerable soils
or terrain features, including in slot canyons, dry washes, and
burn scars).


...South Florida and Florida Keys...
Storm coverage looks to increase meaningfully across the region on
Wednesday, as tropical moisture with PWs of 2.0"+ (above the 90th
percentile) moves into the region upon increasing
east-southeasterly low-level flow (via the influence of the
westward shifting Atlantic subtropical ridge). While any instances
of rapid-onset flooding are anticipated to remain localized (and
primarily confined to where convection is able to effectively
train), this air mass will be supportive of very high short-term
rainfall totals (with the 00z HREF already indicating 3"/3-hr
neighborhood exceedance probabilities as high as 10-20% through
00z Thurs).

Churchill/Weiss

$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 262027
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Wed Jul 26 2023

Day 1
Valid 16Z Wed Jul 26 2023 - 12Z Thu Jul 27 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE LOWER PENINSULA OF MICHIGAN AND SURROUNDING PORTIONS OF INDIANA AND OHIO, AS WELL AS FOR SOUTHEAST WISCONSIN AND INTO FAR NORTHEASTERN ILLINOIS...

...16Z Update...

...Michigan Area...

Only a few changes were needed for the midday update as the MCS
and associated bowing segments are generally behaving consistent
with some of the CAMs. The 06Z guidance had 2 distinct camps of
models showing the heaviest rains either from the UP to the
northernmost counties of the L.P., with a second camp showing
southern L.P. over to Detroit likely seeing the most rain. Now
with current radar trends and the 12Z CAMs guidance in, the
southern camp appears to have won out as the more correct
solution. That isn't to say there won't be rain further north, as
there's already convection ongoing across the U.P. and northern
WI, but it is not expected to be persistent enough to produce any
more than an isolated flash flooding threat. Thus, the northern
L.P. was downgraded to a Marginal risk with this update. Further
south however, expect more storms to develop behind the initial
bowing segments moving into western MI from off Lake Michigan,
which will result in training storms that are likely to generate
isolated to widely scattered flash flooding. Southeastern MI,
including the Detroit metro area has the highest chances of flash
flooding, especially when considering the locally wet soils from
recent rains from Detroit north.

As the storms push east across the Ontario Peninsula and into
western NY tonight, lack of diurnal support should reduce
instability enough to allow for a gradual weakening trend in the
storms, despite the continued strong upper level support. Thus,
the flooding threat through the very early morning of Thursday
should be isolated due to lower rain rates.

...Florida...

The Marginal Risk area was trimmed out of the Everglades and
confined to the I-95 corridor from roughly Ft. Lauderdale south,
and much of the Keys. Anomalous moisture to over 2.25 inch PWATs
are expected to develop over the area which will support storms
capable of heavy rainfall rates, and a lack of steering flow will
make any storms that form capable of very heavy rainfall rates.
Confidence on where those storms will develop within the Marginal
Risk area is low.

...Four Corners Region...

No changes to the inherited Marginal Risk area were made. Weak
monsoonal flow and moisture will have to overcome a very strong
cap, which will greatly limit storm coverage. However, burn scars
and slot canyon in the area are susceptible to flash flooding with
relatively little rain so the Marginal risk area remains.

Wegman

...Previous Discussion...

...Upper Midwest through the Ohio Valley...
A potent shortwave trough is digging southeastward into MN/WI/IA
early this morning, along the northeast periphery of the southwest
monsoon ridge. Convection is trying to organize into a
consolidated mesoscale convective system (MCS) in association with
this shortwave, and there is certainly a leading bow echo that is
the focus (rapidly translating southeastward across southeast MN
into southwest WI, at the time of writing). However, there are
multiple other clusters of convection to the southwest and
northwest that are complicating things, and the 00z HREF suite is
all over the place its depiction of convection through 12z (and
even more so for 12z and beyond). So there is still ample
uncertainty with regard to how well this developing MCS is able to
hold together through the morning hours, as the low-level jet
supplying moisture transport begins to weaken and veer over the
Middle MS Valley. Many of the HREF members weaken (or even kill
off) this MCS as it moves into southwestern WI and northern IL,
but convection will likely refire by mid-day in association with
the digging shortwave (and potentially enhanced by an MCV) as
instability (via SB CAPE) rapidly increases to 1000-3000+ J/kg
with daytime heating across northern IL and southern WI. Given
this potential (with HREF 40-km neighborhood probabilities for 2"
exceedance over 3-hr increasing to 15-20% after 18z), the
inherited Slight risk area was expanded westward across Lake
Michigan into southwest portions of WI (and far northeast IL,
including northern portions of Chicago). More disorganized
clusters of both surface-based and elevated convection may also be
ongoing farther north and northwest (into north WI and eastern MN)
through morning and mid-day, where a Marginal risk was maintained.
Beyond 18z, the picture becomes less clear as the evolution will
depend largely on mesoscale factors that are unknown at this time,
but the HREF probability-matched mean (PMM) generally depicts the
most likely scenario, which is a QPF streak that favors the
southwestern flank of a developing/maturing MCS. This MCS is
favored to track nearly due east (with the mean storm motion
vectors tending to favor a more northeast track, whereas the
upwind propagation vectors will likely favor a more east-southeast
track towards the LLJ and greater instability). So depending on
where this MCS sets up, any portion of the Lower Peninsula of MI
could favor the highest QPF. The HRRR is the farthest south of the
00z guidance (but shifted a bit north towards the 00z HREF
consensus with the 06z run), suggesting the best totals could
slide southward into far norther IN/OH (where the inherited Slight
risk was also expanded a bit south to include). That said, the
totality of the HREF (and resulting PMM and neighborhood
exceedance probabilities) would most favor southern portions of
MI, which would put southeastern MI (including the Detroit metro
area) into the greatest risk of flash flooding (given relatively
low 3-hr FFGs of 1.5-2.5"). It is here that the HREF PMM is
maximized (depicting a streak of 2-4" totals) with neighborhood
exceedance probabilities for 3" and 5" thresholds of 25-35% and
5-15%, respectively. These totals seem plausible (despite the
relatively fast motion of the MCS), as backbuilding and training
would be favored on the trailing southwest flank of the MCS.
Should the HRRR solution verify, the aforementioned axis of
greatest probabilities would shift south (which could bring a more
direct threat to the Toledo and Cleveland metro areas).


...Four Corners...
The monsoon ridge will remain amplified near the Four Corners
today, but increased elongation from SW to NE continues to be
noted in the mass fields, which should allow higher PW anomalies
to drape farther northeast than on previous days. This evolution
of the mid-level ridge is due to a shortwave advecting rapidly
eastward in the northern stream (from the Pacific) which will
drive height falls from the NW, resulting in this elongation of
the ridge. PW anomalies are again progged to be near normal, but
MUCAPE may reach as high as 1000 J/kg beneath the ridge (per the
HREF), especially since convective debris cloudiness looks to be
minimal today. This results in still favorable thermodynamics for
heavy rainfall, and as forcing increases through subtle height
falls and any weak impulses rotating around the periphery of the
ridge, scattered showers and thunderstorms are likely during the afternoon/evening. Rainfall rates of up to 1"/hr are probable,
which despite generally fast forward motion to the E/NE could
produce locally 0.5-1" of rain (and often in as little as 15-30
min). Overall the flash flood risk looks to remain isolated, but a
few instances are possible (especially over the most vulnerable
soils or terrain features, including in slot canyons, dry washes,
and burn scars).


...South Florida and Florida Keys...
Storm coverage looks to increase meaningfully across the region
today, as tropical moisture with PWs of 2.1"+ (above the 90th
percentile) in association with a Tropical Upper Tropospheric
Trough (TUTT) moving into the region (coinciding with increasing east-southeasterly low-level flow via the influence of the
westward shifting Atlantic subtropical ridge). While any instances
of rapid-onset flooding are anticipated to remain localized (and
primarily confined to where convection is able to effectively
train), this air mass will be supportive of high localized
short-term and longer-term rainfall totals (with the 00z HREF
indicating 3"/3-hr and 5"/24-hr neighborhood exceedance
probabilities as high as 10-20%).


Churchill


Day 2
Valid 12Z Thu Jul 27 2023 - 12Z Fri Jul 28 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NEW ENGLAND AND THE URBAN CORRIDOR OF SOUTHEAST FLORIDA...

...2030Z Update...

...Northeast...

The inherited Slight Risk area was expanded to include much of NH
and southern ME with this update. Antecedent conditions remain
highly favorable for redevelopment of flash flooding Thursday
afternoon and into the overnight across this region. An MCS will
restrengthen on the leading edge of a strong shortwave that will
be translating eastward across New England. A very moist air mass
will advect into the region ahead of the shortwave that will
potentially raise PWATs to over 2 inches. That is over 3 sigma
above normal for northern New England. Thus, while the storms
thankfully will be fast moving, 5-10 kt Corfidi vectors out of the
west will favor backbuilding storms, which will maintain the MCS.
The fast moving storms will certainly keep the flash flooding
threat from becoming much worse, given the plentiful atmospheric
moisture available. However, because of that same abundant
moisture, the storms will be capable of over 2+ inch per hour
rainfall rates. Because streamflows remain very high due to
previous rainfall, expect flash flooding to develop rather quickly
after initiation of the heaviest rain should the storms move over
particularly sensitive areas. The surrounding Marginal Risk into
the Mid-Atlantic was left unchanged with this update, as the
threat for more widely scattered storms that will be largely
detached from the primary forcing going over New England will
likely still have enough energy to cause scattered shower and
thunderstorm development down to the DC area. The expansion of the
Slight was coordinated with the GYX/Gray, ME and CAR/Caribou, ME
forecast offices.

...South Florida...

An easterly wave is slowly pushing westward across the Bahamas
towards south Florida this afternoon. This morning's storms ahead
of the wave have largely dissipated, but storms capable of
producing multiple inches of rain per hour are just offshore.
Sometime overnight tonight the storms are likely to begin to move
ashore into the I-95 corridor of south Florida. There is some
disagreement among the CAMs as to when those storms will begin to
move into the area...however recent rainfall both from previous
days and this morning have left water tables above normal in the
area, meaning less rainfall is needed in order for flooding to
start. In addition to the antecedent soil conditions, the
environmental conditions are both favorable and concerning for
additional flash flooding development. PWATs are about as high as
they can possibly be outside of a tropical cyclone, forecast to
eclipse above 2.25 inches, perhaps approaching 2.5 inches. This is
as much as 4 sigma above normal for this time of year in this
area, which is a truly remarkable statistic. Further, since the
easterly wave is a slow-mover, so too is the associated
thunderstorm activity, as evident by the present depictions by the
Miami radar. Once these storms move ashore, they will be able to
take advantage of those remarkably high PWATs to translate that to
very efficient warm rainfall processes. Rainfall rates in the
strongest storms may be as high as 3 inches per hour. There
remains some uncertainty in the CAMs as to how much coverage of
rain there will be, but there is some potential for additional
upgrades in this area when the coverage of storms becomes more
clear. With the heaviest rain likely holding until well into the
overnight hours, the greatest potential flooding impacts will be
in the Day 2/Thursday morning period. In coordination with
MFL/Miami, FL forecast office, the Slight Risk upgrade was made
with this afternoon's update.

...Upper Midwest and Great Lakes...

No big changes were made to the Marginal Risk area across this
region, as the signal for heavy rains over this area is diffuse at
best. Further, overall antecedent conditions are around normal,
leaning drier than normal, so the soils should be able to handle
most of the rain that falls in this area, and any flooding threat
remains isolated.

...Southwest...

No big changes were made out in the Southwest with the ongoing
monsoonal moisture. There should be storms firing Thursday
afternoon into eastern UT and western CO, so the Marginal was
expanded northward into those areas, but again due to a rather
strong ridge in place across the area, the storms should remain
isolated to widely scattered, which will keep any flash flooding
threat confined to those areas with the most persistent storms or
where the storms form over areas that are more sensitive to
flooding, such as burn scars and slot canyons.

Wegman


...Northeast/New England into the Mid-Atlantic...
A potent shortwave trough (and whatever is left of an associated
MCS/MCV) looks to rapidly translate eastward into the region from
the eastern Great Lakes early Thursday, which will drive
convective development within an anomalously moist (PWs
approaching 2.0", well above the 90th percentile) and increasingly
unstable (SB CAPE rising to 1000-3000 J/kg) air mass. While
significant uncertainty with regard to the ultimate organization
and coverage/placement of convection and resulting QPF remains,
concerns have increased enough to prompt to introduction of a
Slight risk area across all of VT and portions of western NH/MA
and northern CT (in coordination with the local WFOs and largely
owing to wet antecedent conditions and still highly elevated
streamflows). A much broader Marginal risk was maintained and
expanded significantly southward into the Mid-Atlantic, where CAM
guidance is in pretty good agreement regarding at least isolated
to scattered convective coverage (with 00z HREF neighborhood
exceedance probabilities through 00z Friday of 20-40% and 15-25%
for 2" and 3" thresholds, respectively). The Slight risk area with
continue to be refined (with at least a good chance of meaningful
expansion) with the addition of more CAM guidance on the 12z HREF
cycle.


...Upper Midwest and Great Lakes...
Available CAMs continue to suggest the potential for more
scattered areas of convection in association with a shortwave
trough (possibly staying too far north in Canada) in the northern
stream, as well as shortwave impulses rounding the southwest ridge
(in what can be considered the southern stream). While organized
convection is looking a bit less likely due to a lack of amplitude
with the primary northern stream shortwave trough, disorganized
areas of convection may be able to become organized (and would
likely favor portions of south and eastern portions of MN and
northern portions of WI). Maintained a Marginal risk across the
areas most likely to realize isolated to scattered coverage of
convection (with PWs of 1.4-1.8", MU CAPE of 1000-3000 J/kg, and
30-40 kts of deep layer shear expected to support heavy rainfall
and help organize any convection).


...Four Corners...
The increasingly elongated monsoon ridge will remain amplified
near the Four Corners into Thursday, presenting very similar risks
(i.e. highly localized risk of flash flooding) to many of the same
areas as earlier in the week (with the area looking to shrink to
include much of NM, southern CO, and eastern AZ). The diurnally
driven convection will continue to support rainfall rates of up to
1"/hr (with localized sub-hourly accumulations of 0.5-1"). More
vulnerable terrain (burn scars and dry washes) will be most at
risk for any localized flash flooding.


...South Florida and Florida Keys...
Highly anomalous tropical moisture with increased convective
coverage across the region (in association with a lingering TUTT)
will continue (and spread northward a bit) into Thursday. Similar
to Day 1, any instances of rapid-onset flooding will likely remain
confined to localized areas where highly efficient rainfall (with
rates up to 2"/hr) is able to train/repeat. The addition of
high-resolution guidance will provide more clarity in future
outlooks, but even through 00z Friday the HREF neighborhood
exceedance probabilities suggest meaningful odds (15-25%) of
localized 5" exceedance (over 12-hr).

Churchill
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 272020
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Thu Jul 27 2023

Day 1
Valid 16Z Thu Jul 27 2023 - 12Z Fri Jul 28 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH FLORIDA AND NEW ENGLAND...

...16Z Update...

No big changes were made with the midday update. The Slight Risk
areas in both New England and South Florida are unchanged with
this update.

...South Florida...

In South Florida, rainfall moving into the Space Coast has been
largely stratiform, but the persistence of the rain led to a small
expansion of the Marginal there. The Slight Risk area from the
Treasure Coast through Miami remains the same, with the potential
for slow-moving storms to redevelop this afternoon along the urban
corridor, which has been hard hit in recent days so antecedent
conditions favor the development of flooding should there be
enough rainfall from the storms. The highest uncertainty even now
is how much coverage of storms there will be in the area, with
many of the CAMs suggesting coverage will be minimal. Thus, the
Slight Risk area is currently considered a low end threat. That
being said, and as mentioned in previous discussions, the
availability of atmospheric moisture in this area is impressive,
so any stronger storms that form have the potential to cause
locally significant flooding since they will be slow-moving.

...New England...

The well-advertised MCS moving across northern New England so far
has been causing steady rain, but with only isolated instances of
heavy rain. The most significant rainfall associated with it has
ended across northern NY, so the Marginal Risk there was trimmed.
Going forward through this afternoon, however, thunderstorms that
will form as a result of daytime heating and aren't exclusively
tied to the MCS should allow some backbuilding from far eastern
and southern NY eastward through New England. A few of those
storms are already starting to form over the Catskills and Mohawk
Valley since the forcing is stronger closer to the MCS. Further
south across the Mid-Atlantic and the I-95 corridor, the storms
will form a bit later this afternoon once there's been sufficient
daytime heating. The further north you go, the greater coverage of
storms, so the inherited Slight Risk area looks on track.

...Ohio Valley...

A weak upper level trough and the southwestern end of a front
moving across the Northeast may trigger a few storms that may
train along similar paths as each other this afternoon. There
remains some signal for locally enhanced rainfall totals of up to
3 inches, and widely scattered totals to 2 inches. Much of the OH
Valley, generally along the river separating OH and KY have been a
bit wetter than normal, so soil moisture conditions are generally
favorable for flash flooding development should training storms
develop in this area this afternoon.

...Upper Midwest...

The right entrance region of a 90 kt jet over Ontario will move
across the region tonight. A local maximum of moisture on a 15-20
kt southwesterly LLJ will develop, supporting backbuilding storms
over MN and WI through tonight. The storms should be moving fast
enough that any flash flooding risk will remain isolated.

...Four Corners into Wyoming...

Scattered convection is expected to develop this afternoon from
the 4 Corners region northward through Wyoming. The greatest
concentration of storms are expected from southern/southeastern CO
north into WY. While many of the weaker storms are likely to
remain dry due to dry air prevalent in the low to mid levels and
high dew point depressions, the strongest storms, particularly
those that may train from previous convection have the potential
to produce local rainfall totals of 0.5 to 1 inch, which over
particularly sensitive areas such as burn scars and slot canyons
could result in localized flash flooding. As typical with the
monsoon so far this year, the upper level ridging should keep
those scattered storms to distinct areas, and the overall Marginal
risk is considered on the lower side of the Marginal Risk category.

Wegman

...Previous Discussion...

...Northeast/New England into the Mid-Atlantic...
A potent shortwave trough (and the remnants of an MCV) are rapidly
translating eastward early this morning, currently located near
Lake Erie and Lake Ontario. Lift in association with this vort max
(via DPVA), along with upper-level divergence (via the
right-entrance region of a 120 kt jet streak north of Nova
Scotia), will drive convective development within an anomalously
moist (PWs approaching 2.0", well above the 90th percentile) and
increasingly unstable (SB CAPE rising to 1000-3000 J/kg) air mass.
There is still a fair amount of uncertainty with regard to the
ultimate organization and coverage/placement of convection, but it
appears increasingly likely that discrete convection will begin to
initiate around mid-morning across portions of Upstate NY (and
possibly farther south along the I-95 corridor into the
Mid-Atlantic). This discrete convection may intensify and organize
rapidly in the presence of 30-40 kts of effective bulk shear, and
more organized rainfall activity (mostly stratiform initially) may
already be ongoing farther northeast into VT/NH (in association
with a leading MCV/vort max and associated surface low). As
instability increase, discrete convective elements will be capable
of localized hourly accumulations of 1-2", though rather
progressive storm motions of 20-30 kts will should keep any
localized training/repeating to a minimum. Due to the timing of
the shortwave, these more intense cells will likely not come about
until they've left NY State, which will bring the primary threat
of flash flooding to VT and much of NH/ME/MA/RI (where an
inherited Slight risk has been extended south and eastward a bit).
While the overall coverage of cells is more uncertain to the south
of New England, antecedent conditions are rather wet (with as much
as 200-400% of normal rainfall over the past 14 days with most
streamflows still highly elevated). Farther north, there is better
CAM agreement for localized totals in excess of 3" (via HREF 40-km
neighborhood probabilities for 3" exceedance as high as 20-30%,
versus 10-20% farther south). Even so, overall coverage of
excessive rainfall is likely to fall short of the Moderate risk
threshold (40%), and the progressive nature of the system will
keep the peak of the activity confined to a 6-hr window (likely
15-21z across VT/NH, eastern NY and western ME, and between 18-00z
farther south and east). The 12-hr probability-matched mean from
the HREF (ending 00z) captures the expected QPF fairly well,
suggesting localized totals as high as 1-3" across the bulk of the
Marginal risk area, whereas those totals (and some maxima as high
as 4-5") look more common (i.e. scattered) across the Slight risk
area. Chances for flash flooding appear maximized where the
higher-end of these totals (3-5") occur over more vulnerable
terrain (metro and other poor drainage areas).

...Ohio Valley...
Signals have increased for a number of the CAMs (primarily the
ARW, ARW2, and NAM-nest) to support localized 2-3" totals over a
relatively short period this afternoon (primarily from 18-03z).
This looks to occur within an area of low-level convergence with
PWs of 1.6-1.9" with modest deep layer shear of 15-25 kts.
Instability (SB CAPE) is progged to increase to 2000-4000 J/kg to
support convective initiation, and these 2-3" totals could occur
over as little as 2-3 hours (per the HREF PMM). Corresponding FFGs
(over 3-hr) are generally 2.0-3.0", therefore isolated instances
of flash flooding are possible.

...Upper Midwest and Great Lakes...
The 00z HREF continues to suggest the potential for more scattered
areas of convection in association with a more subtle shortwave
trough from late afternoon into the evening and overnight. While
highly organized convection looks less likely with this system
then previous days (due to a lack of amplitude in the trough),
disorganized areas of convection may be able to become more
organized into the evening hours (and would likely favor portions
of MN/WI/IA). Maintained (and expanded a bit) a Marginal risk
across the areas most likely to realize isolated to scattered
coverage of convection (with PWs of 1.4-1.8", MU CAPE of 1000-3000
J/kg, and 30-40 kts of deep layer shear expected to support heavy
rainfall and help organize any convection that is able to develop
into clusters). The 00z HREF signal is still fairly unimpressive
at this point, depicting 2-3" localized totals (mostly over
southeast MN into WI). This signal will be reevaluated later
today.


...Four Corners...
An increasingly elongated monsoon ridge will remain amplified near
the Four Corners today, presenting very similar risks (i.e. highly
localized risk of flash flooding) to many of the same areas as
earlier in the week (though shifting mostly out of UT with
shrinking area over AZ). The diurnally driven convection will
continue to support rainfall rates of up to 1"/hr (with localized
sub-hourly accumulations of 0.5-1.0"). More vulnerable terrain
(burn scars and dry washes) will be most at risk for any localized
flash flooding.


...South Florida and Florida Keys...
Highly anomalous tropical moisture and convective coverage (in
association with an easterly wave and lingering TUTT) will
continue (while spreading northward along the Treasure and Space
coast) today. Similar to yesterday, any instances of rapid-onset
flooding will likely remain confined to localized areas where
highly efficient rainfall (with rates up to 2"/hr) are repeat.
Storm motions will also becoming increasingly slower, which may
result in some localized totals of >2" over the course of an hour.
The addition of high-resolution guidance will provide more clarity
in future outlooks, but even through 00z Friday the HREF
neighborhood exceedance probabilities suggest meaningful odds
(15-25%) of localized 5" exceedance (over 12-hr).


Churchill


Day 2
Valid 12Z Fri Jul 28 2023 - 12Z Sat Jul 29 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST...

...2030Z Update...

...Midwest...

Two rounds of storms are expected to develop in advance of a
southward moving cold front across the upper Great Lakes on
Friday. The first round of storms is likely during the peak
heating of the day, where the storms will take advantage of MUCAPE
values exceeding 3,000 J/kg. Since this round will be well ahead
of the front, any storms that form in this region are likely to be
fairly isolated, but since they'll have plenty of moisture and
instability, they could grow to become intense. The far more
significant round of storms will be the second one, which will
impact the area during the overnight hours Friday night into
Saturday morning. The storms will have the benefit of a stronger
LLJ providing additional moisture, with the front backing up the
storms with good forcing. With a swath of rainfall from yesterday
averaging around 2 inches in the Slight Risk area, the soils are
quite a bit wetter than normal for this time of year in the Slight
Risk area. Thus, think this next round of rain, while areally
expecting a bit less rainfall; a broad 1-3 inches of rain in this
same area is likely to cause more widely scattered flash flooding.
The area of highest threat is along the MI/IN-OH border, and into
northeast OH and northwest PA. There is a bit more uncertainty
both on the north side over central MI and also west of Lake
Michigan over WI and IL, including the Chicagoland area.

...South Florida...

A small expansion of the Marginal was made to include more of the
Gulf coast of the FL Peninsula into the Marginal Risk, including
the Tampa metro. The departing tropical wave and any potential
local sea breezes should focus afternoon convection along the west
coast of the Peninsula. While this area has been quite dry lately,
the abundance of atmospheric moisture should allow any storms that
form to have ample moisture to produce heavy rainfall. The
inherited Marginal also looks good including southeast FL, which
has been much wetter than the west coast, but also is expecting
less coverage and intensity of storms Friday afternoon.

...Four Corners Region...

Based on an analysis of recent verification and the relative
minimum in expected coverage of storms across the Four Corners
region, have opted to drop the Marginal Risk entirely from this
area due to lack of signal. Any very isolated flash flooding
possible will be less than the 5% coverage threshold for a
Marginal Risk.

Wegman

$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 280831
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Fri Jul 28 2023

Day 1
Valid 12Z Fri Jul 28 2023 - 12Z Sat Jul 29 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST...

...Midwest/Ohio Valley/Central and Southern Appalachians...
While numerous differences in the details remain, the general
model consensus shows mid-level energy moving along the top of an
upper level ridge, with ample moisture (PWs at or above 1.5
inches) supporting the development of isolated to scattered
convection across the region during the day today. While likely
to remain unorganized, these storms may still produce locally
heavy amounts, which may result in isolated runoff concerns. More
organized convection, with a greater threat for more widespread
heavy amounts, is likely to develop during the evening and
overnight hours across portions of the Midwest. Strengthening
southwesterly flow ahead of a more-defined wave moving out of the
northern Plains will support increasing moisture (PWs increasing
to at or above 2 inches) along a front sagging south through the
southern Great Lakes region. Each of the 00Z CAMs shows one or
more convective complexes developing and tracking east from the
mid Missouri and upper Mississippi valleys through the southern
Great Lakes, to as far east as the the upper Ohio Valley
overnight. These storms may produce intense rainfall rates, with
the threat for repeating convection increasing the threat for
localized flash flooding concerns. HREF neighborhood
probabilities for amounts exceeding 2 inches are above 50 percent
within much of the Slight Risk area.

...Southeast Coast...
As an area of low pressure over northern Florida drifts into
southern Georgia, increasing onshore flow and low level
convergence, coupled with divergent flow aloft, are expected to
support showers and thunderstorms with heavy amounts along the
Southeast Coast from northern Florida to South Carolina.
Convergent onshore flow is forecast to support PWs of 2.25-2.5
inches along the coast while divergence east of the upper center
further supports ascent across the region. Consensus of the CAMs
indicate that locally heavy amounts are likely, with the HREF
showing high neighborhood probabilities for 3 inches or more
within the Marginal Risk area.

...South Florida...
Southerly flow in the wake of the previously noted low and along
the western periphery of the subtropical ridge will continue to
support above-normal moisture (PWs at or above 2.25 inches) across
the southern peninsula. This is likely to support diurnal
convection, with heavy downpours. This may result in an isolated
runoff concerns, especially across urbanized areas.

Pereira

Day 2
Valid 12Z Sat Jul 29 2023 - 12Z Sun Jul 30 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL APPALACHIANS...

...Ohio Valley/Central and Southern Appalachians to the Northeast
and Mid-Atlantic...
Ongoing storms across portions of the Ohio Valley into the
Northeast are expected to continue through Saturday morning, with intensification possible near the associated wave and cold front
as it moves across southeastern New York and southern New England
during the afternoon hours. Models also show the potential for
storms with heavy rains to develop farther to the south along the
trailing cold front as it drops through the coastal Mid-Atlantic
region during the late afternoon and evening hours. Lacking a
strong model consensus for where and if heavy amounts may occur, a
Marginal Risk was maintained across much of Northeast and
Mid-Atlantic for now.

There remains a pretty good signal for locally heavy amounts
farther to the west across portions of the central Appalachians,
where mid-level energy and favorable upper jet forcing are
expected to interact with the deep moisture pool along the front
to produce at least locally heavy amounts. Given the relatively
wet soil conditions and low flash flood guidance values, a Slight
Risk was maintained across portions of West Virginia into
southwestern Virginia.

...Eastern and south-central Colorado and the Sangre de Cristo
Mountains of New Mexico...
Easterly low level inflow is expected to contribute to an increase
in moisture and the development of showers and thunderstorms over
the mountains. Weak flow aloft is expected to increase the
potential for slow-moving storms, which may result in locally
heavy amounts with isolated flash flooding concerns.

...Southeastern Arizona/Southwestern New Mexico...
Increasing moisture ahead of a easterly inverted trough moving
across northern Mexico is forecast to help support increasing
coverage of showers and thunderstorms across the region. Models
shows PWs returning to 1-1.5 standard deviations above normal
across portions of the region, increasing the threat for showers
and thunderstorms with locally heavy amounts and isolated runoff
concerns.

Pereira


Day 3
Valid 12Z Sat Jul 29 2023 - 12Z Sun Jul 30 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST AND THE NORTHERN PLAINS...

...Southwest...
An inverted upper trough will continue to lift northeast from
northern Mexico into the southwestern U.S., pushing the
subtropical ridge farther to the east and allowing deeper
moisture to advect across Arizona through the Four Corners region
and into the central Rockies. Both the NAM and GFS show PWs
increasing to over 1.5 standard deviations above normal across
portions of southern Arizona and the Four Corners region on
Sunday. This moisture along with decreasing heights aloft is
likely to support more widespread afternoon and evening
thunderstorms and an increased threat for locally heavy amounts
and isolated flash flooding.

...Northern Plains...
Mid level shortwaves moving off of the top of the ridge may pose a
localized heavy rainfall threat as they interact with a plume of
deeper moisture fed by southeasterly low level flow. These storms
may pose an isolated flash flooding threat across the northern
High Plains into central South Dakota before moving into the less
flood prone areas of central Nebraska. While most of the guidance
indicates that storms with locally heavy amounts can be expected,
they provide very little agreement as to where the heaviest
amounts may occur.

Pereira
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 281929
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
328 PM EDT Fri Jul 28 2023

Day 1
Valid 16Z Fri Jul 28 2023 - 12Z Sat Jul 29 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST...

...16z Update...
No major changes were needed for the 16z update. Minor changes
included expanding the slight risk slightly more into
south-central Wisconsin as well as combination of the marginal
risk areas along the Southeast Coast and Southwest Florida to
include the central Peninsula region. Both changes were based on
latest high-res guidance indicating higher totals in these regions
with support from HREF probablilty fields as well.

Santorelli

...Midwest/Ohio Valley/Central and Southern Appalachians...
While numerous differences in the details remain, the general
model consensus shows mid-level energy moving along the top of an
upper level ridge, with ample moisture (PWs at or above 1.5
inches) supporting the development of isolated to scattered
convection across the region during the day today. While likely
to remain unorganized, these storms may still produce locally
heavy amounts, which may result in isolated runoff concerns. More
organized convection, with a greater threat for more widespread
heavy amounts, is likely to develop during the evening and
overnight hours across portions of the Midwest. Strengthening
southwesterly flow ahead of a more-defined wave moving out of the
northern Plains will support increasing moisture (PWs increasing
to at or above 2 inches) along a front sagging south through the
southern Great Lakes region. Each of the 00Z CAMs shows one or
more convective complexes developing and tracking east from the
mid Missouri and upper Mississippi valleys through the southern
Great Lakes, to as far east as the the upper Ohio Valley
overnight. These storms may produce intense rainfall rates, with
the threat for repeating convection increasing the threat for
localized flash flooding concerns. HREF neighborhood
probabilities for amounts exceeding 2 inches are above 50 percent
within much of the Slight Risk area.

...Southeast Coast...
As an area of low pressure over northern Florida drifts into
southern Georgia, increasing onshore flow and low level
convergence, coupled with divergent flow aloft, are expected to
support showers and thunderstorms with heavy amounts along the
Southeast Coast from northern Florida to South Carolina.
Convergent onshore flow is forecast to support PWs of 2.25-2.5
inches along the coast while divergence east of the upper center
further supports ascent across the region. Consensus of the CAMs
indicate that locally heavy amounts are likely, with the HREF
showing high neighborhood probabilities for 3 inches or more
within the Marginal Risk area.

...South Florida...
Southerly flow in the wake of the previously noted low and along
the western periphery of the subtropical ridge will continue to
support above-normal moisture (PWs at or above 2.25 inches) across
the southern peninsula. This is likely to support diurnal
convection, with heavy downpours. This may result in an isolated
runoff concerns, especially across urbanized areas.

Pereira




Day 2
Valid 12Z Sat Jul 29 2023 - 12Z Sun Jul 30 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST AND THE CENTRAL APPALACHIANS...

...Ohio Valley/Central and Southern Appalachians to the Northeast
and Mid-Atlantic...
Ongoing storms across portions of the Ohio Valley into the
Northeast are expected to continue through Saturday morning, with intensification possible near the associated wave and cold front
as it moves across southeastern New York and southern New England
during the afternoon hours. The latest model QPF amounts, along
with HREF exceedence probabilities support the addition of a
slight risk area across this region for the update this afternoon.
Models also show the potential for storms with heavy rains to
develop farther to the south along the trailing cold front as it
drops through the coastal Mid-Atlantic region during the late
afternoon and evening hours. Lacking a strong model consensus for
where and if heavy amounts may occur, a Marginal Risk was
maintained across the Mid-Atlantic for now.

There remains a pretty good signal for locally heavy amounts
farther to the west across portions of the central Appalachians,
where mid-level energy and favorable upper jet forcing are
expected to interact with the deep moisture pool along the front
to produce at least locally heavy amounts. Given the relatively
wet soil conditions and low flash flood guidance values, a Slight
Risk was maintained across portions of West Virginia into
southwestern Virginia/northeast Tennessee.

...Southeast Coast...
An area of low pressure will continue drifting slowly northward
continuing to fuel numerous showers and thunderstorms across the
Southeast coastal region into Saturday. Ample moisture and
instability will continue to support a localized flash flood
threat across this region. New model guidance today allowed for
the addition of a marginal risk area here for the afternoon update.

...Eastern and south-central Colorado and the Sangre de Cristo
Mountains of New Mexico...
Easterly low level inflow is expected to contribute to an increase
in moisture and the development of showers and thunderstorms over
the mountains. Weak flow aloft is expected to increase the
potential for slow-moving storms, which may result in locally
heavy amounts with isolated flash flooding concerns.

...Southeastern Arizona/Southwestern New Mexico...
Increasing moisture ahead of a easterly inverted trough moving
across northern Mexico is forecast to help support increasing
coverage of showers and thunderstorms across the region. Models
shows PWs returning to 1-1.5 standard deviations above normal
across portions of the region, increasing the threat for showers
and thunderstorms with locally heavy amounts and isolated runoff
concerns.

Santorelli/Pereira



Day 3
Valid 12Z Sun Jul 30 2023 - 12Z Mon Jul 31 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST AND THE NORTHERN PLAINS...

...Southwest...
An inverted upper trough will continue to lift northeast from
northern Mexico into the southwestern U.S., pushing the
subtropical ridge farther to the east and allowing deeper
moisture to advect across Arizona through the Four Corners region
and into the central Rockies. Both the NAM and GFS show PWs
increasing to over 1.5 standard deviations above normal across
portions of southern Arizona and the Four Corners region on
Sunday. This moisture along with decreasing heights aloft is
likely to support more widespread afternoon and evening
thunderstorms and an increased threat for locally heavy amounts
and isolated flash flooding.

...Northern Plains...
Mid level shortwaves moving off of the top of the ridge may pose a
localized heavy rainfall threat as they interact with a plume of
deeper moisture fed by southeasterly low level flow. These storms
could pose an isolated flash flooding threat across the northern
High Plains into central South Dakota before moving into the less
flood prone areas of central Nebraska. While most of the guidance
indicates that storms with locally heavy amounts can be expected,
there is still considerable uncertainty on exactly where storms
may set up.

Santorelli/Pereira
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 290818
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sat Jul 29 2023

Day 1
Valid 12Z Sat Jul 29 2023 - 12Z Sun Jul 30 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST AND THE CENTRAL APPALACHIANS/OHIO VALLEY...

...Northeast and Mid-Atlantic...
A well-defined shortwave associated with ongoing convection over
the Great Lakes region is forecast to move east of the lower
Lakes, with storms reintensifying as the wave interacts with
deepening moisture and increasing instability supported by daytime
heating and amplifying southwesterly flow. Recent runs of the RAP
show PWs increasing to at or above 2 inches within an area of
strong ascent afforded in part by favorable upper forcing. Ahead
of the system, a downstream wave now moving across Pennsylvania
may bring some scattered showers and storms to parts of southern
New England later this morning. This will likely be followed by
more organized strong storms, with intense rainfall rates likely,
and training possible from eastern New York and Pennsylvania to
southern and central New England during the afternoon into the
evening hours. The Slight Risk area reflects where the HREF
guidance centers its heaviest amounts from the Catskills eastward
into southern and central New England, with significant
probabilities for amounts exceeding 3 inches.

Some locally heavy amounts are possible farther south as well as
storms are expected to develop along the trailing cold front as it
presses south through Mid-Atlantic this afternoon and evening.

...Central Appalachians to the central Plains...
A Slight Risk area was maintained across portions of the central
Appalachians and extended west into portions of the Ohio Valley.
Storms developing along the previously noted cold front may create
at least isolated flash flood concerns, especially across areas in
West Virginia and eastern to central Kentucky that have recently
been impacted by heavier rains and where FFGs are relatively low.
Some of the CAMs show a fairly good signal for
slow-moving/training storms, especially from central Kentucky into
the Appalachians, increasing the threat for heavy amounts and
flash flooding concerns.

Meanwhile, convection developing over the central Plains may
intensify and spread southeast across the mid Mississippi and into
the lower Ohio Valley later today. While these storms are
expected to be generally progressive, some locally heavy amounts
producing isolated flash flooding cannot be ruled out.

...South Carolina...
An area of low pressure will continue drifting slowly northward,
producing additional showers and thunderstorms along the South
Carolina Coast. Ample moisture and instability will continue to
support a localized flash flood threat across this region before
the wave begins to turn to the east early Sunday ahead of the
amplifying trough to the north.

...Eastern and south-central Colorado and the Sangre de Cristo
Mountains of New Mexico...
Easterly low level inflow is expected to contribute to an increase
in moisture and the development of showers and thunderstorms over
the mountains. Weak flow aloft is expected to increase the
potential for slow-moving storms, which may result in locally
heavy amounts with isolated flash flooding concerns.

...Southeastern Arizona/Southwestern New Mexico...
Deepening moisture ahead of an inverted upper trough tracking west
across northern Mexico is forecast to help support increasing
coverage of showers and thunderstorms across the region. Models
shows PWs returning to 1-1.5 standard deviations above normal
across portions of the region, raising the threat for showers and
thunderstorms with locally heavy amounts and isolated runoff
concerns.

Pereira





Day 2
Valid 12Z Sun Jul 30 2023 - 12Z Mon Jul 31 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST AND THE NORTHERN PLAINS...

...Southwest...
An inverted upper trough will continue to lift northeast from
northern Mexico into the southwestern U.S., pushing the
subtropical ridge farther to the east and allowing deeper
moisture to advect across Arizona through the Four Corners region
and into the central Rockies. The general consensus of the
guidance continues to show PWs increasing to over 1.5 standard
deviations above normal across southern California and along the
southern Colorado Basin into the Four Corners region on Sunday.
This moisture along with decreasing heights aloft is likely to
support more widespread afternoon and evening thunderstorms and an
increased threat for locally heavy amounts and isolated flash
flooding.

...Northern Plains...
Mid-level shortwaves moving off of the top of the ridge may pose a
localized heavy rainfall threat as they interact with a plume of
deeper moisture fed by southeasterly low level flow. These storms
could create isolated flash flooding concerns across the northern
High Plains into central South Dakota before moving into the less
flood prone Sandhills of central Nebraska. Guidance continues to
differ significantly on the magnitude and location of any heavy
amounts that may develop across this region.

Pereira



Day 3
Valid 12Z Mon Jul 31 2023 - 12Z Tue Aug 01 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST TO THE NORTHERN HIGH PLAINS, THE MID MISSOURI TO THE
LOWER OHIO VALLEY, AND OVER PORTIONS OF NORTHEAST FLORIDA...

...Southwest to the northern High Plains...
A favorable monsoon pattern will continue to support storms across
portions of the Southwest, especially Arizona and interior
southern California, while expanding farther north and west across
the Great Basin and the central Rockies. Deepening moisture and
energy moving along the western periphery of an eastward-shifting
upper high is likely to support the increasing coverage of showers
and thunderstorms across southern Nevada, Utah, and western
Colorado. Energy moving over the top of the ridge may interact
with low level easterly flow over the central Plains, supporting
storms and potentially locally heavy amounts spreading east of the
mountains into eastern Wyoming and Black Hills region. Storms
that develop across the region will have the potential to produce
intense rainfall rates and raise flash flooding concerns,
especially in urbanized and areas of complex terrain.

...Mid Missouri to the lower Ohio Valley...
Mid-level energy embedded within northwesterly flow interacting
with a deep moisture pool associated with a low level front may
produce rounds of convection with locally heavy amounts. While
the signal for potentially heavy amounts remain, models continue
to disagree on their magnitude and location.

...Northeast Florida...
Deep moisture (PWs around 2 inches) associated with a stalled
frontal boundary is likely to enhance shower and thunderstorm
coverage and the potential for heavy rainfall amounts across the
area.

Pereira
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 300819
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Sun Jul 30 2023

Day 1
Valid 12Z Sun Jul 30 2023 - 12Z Mon Jul 31 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, THE NORTHERN PLAINS TO CENTRAL PLAINS/LOWER
MISSOURI VALLEY, AND SOUTHERN APPALACHIANS/SOUTHEAST...

...Southwest...
An inverted upper trough will continue to lift northeast from
northern Mexico into the southwestern U.S., pushing the
subtropical ridge farther to the east and allowing deeper
moisture to advect across Arizona through the Four Corners region
and into the central Rockies. The general consensus of the
guidance continues to show PWs increasing to over 1.5-2 standard
deviations above normal across southern California and along the
southern Colorado Basin into the Four Corners region today. This
moisture along with decreasing heights aloft is likely to support
more widespread afternoon and evening thunderstorms and an
increased threat for locally heavy amounts and isolated flash
flooding.

...Northern to central Plains and lower Missouri Valley...
Mid-level shortwaves moving off of the top of the ridge may pose a
localized heavy rainfall threat as they interact with a plume of
deeper moisture fed by southeasterly low level flow. While there
are differences in the details, the overnight CAM guidance is
highlighting two areas of storm development later today. One over
southern Montana and central Wyoming, with storms spreading east
into western South Dakota and Nebraska. The highest probabilities
from the HREF for heavy amounts (greater than 2 inches) currently
centers northeastern Wyoming into southwestern South Dakota. This
may create isolated flash flooding concerns across the region
before these storms move into the less flood prone Sandhills of
central Nebraska. Storms that continue or develop farther
downstream may also produce locally heavy rain and pose an
isolated threat for flash flooding across portions of the lower
Missouri Valley as well.

Guidance is also showing storms developing farther to the north
across northern Montana and southern Canada before moving into
western North Dakota, with some of the CAMs showing localized
heavy amounts across portions of northeastern Montana into western
North Dakota.

...Southern Appalachians/Southeast...
Plenty of moisture (PWs around 1.75 inches or higher) is expected
to remain along and south of a frontal boundary as it continues to
dip south through the Carolinas today. This moisture interacting
with energy associated with overnight convection over the Ohio and
Tennessee valley may produce some locally heavy amounts as it
moves across the southern Appalachians and into the Southeast
later today. This deep moisture will extend south to the Gulf
Coast and across the Florida peninsula, bolstering the potential
for heavy downpours and localized runoff concerns as the more
typical sea breeze convection develops during the afternoon.

Pereira




Day 2
Valid 12Z Mon Jul 31 2023 - 12Z Tue Aug 01 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST TO THE CENTRAL HIGH PLAINS, THE MID MISSOURI TO THE MID
MISSISSIPPI VALLEYS, AND OVER PORTIONS OF SOUTHERN GEORGIA AND
FLORIDA...

...Southwest to the central High Plains...
A favorable monsoon pattern will continue to support storms across
portions of the Southwest, especially for Arizona and interior
southern California, while expanding farther north and west across
the Great Basin and the central Rockies. Deepening moisture and
energy moving along the western periphery of an eastward-shifting
upper high is likely to support the increasing coverage of showers
and thunderstorms across southern Nevada, as well as much of Utah,
Colorado, and Wyoming. PWs of 1-2 standard deviations above
normal are expected across much of this area. Storms that develop
across the region will have the potential to produce intense
rainfall rates and raise flash flooding concerns, especially
across urbanized and areas of complex terrain.

...Mid Missouri to the Mid Mississippi valley...
Mid-level energy embedded within northwesterly flow interacting
with a deep moisture pool associated with a low level front may
produce rounds of convection with locally heavy amounts. The 00Z
guidance continued to show a fair amount of spread with regard to
the magnitude of amounts, but those that did show heavy amounts
generally agreed on an axis extending from southwestern Iowa
through Missouri.

...Southeast Georgia and northern to central Florida...
Deep moisture (PWs around 2 inches) associated with a stalled
frontal boundary is likely to enhance shower and thunderstorm
coverage and the potential for heavy rainfall amounts across the
area.

Pereira




Day 3
Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL ROCKIES...

...Southwest to the northern High Plains...
While drier air is expected to begin limiting the potential for
heavy rainfall and flash flooding concerns across southern Arizona
and southern California, deep monsoon moisture and shortwave
energy will continue to support the threat farther northeast
through the Great Basin and into the central Rockies and High
Plains. By Tuesday, some of the greater PWs anomalies (2+
standard deviations) are forecast to move into eastern Wyoming and
Colorado. This includes recent burn scars and areas with
relatively low flash flood guidance values. A Slight Risk was
added to highlight the greater flash flood threat for this area.
Additional upgrades may be needed, especially if confidence in
heavy amounts across other vulnerable areas increases.

...Mid Missouri to the lower Ohio Valley...
The pattern from Day 2 into Day 3 remains much of the same, with a
similar threat for southeastward propagating storms, producing
heavy amounts along much the same axis. Deeper moisture (PWs
reaching above 2 inches) bolstered by strong westerly inflow into
the low level front is expected to increase the threat for heavy
amounts this period. Upgrades above a Marginal Risk may be
forthcoming, especially if the models start to show a stronger
signal for repeating heavy amounts across the same area.

Pereira
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 310820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Mon Jul 31 2023

Day 1
Valid 12Z Mon Jul 31 2023 - 12Z Tue Aug 01 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHWEST TO CENTRAL HIGH PLAINS, THE MID MISSOURI TO MID
MISSISSIPPI VALLEYS, AND OVER PORTIONS OF THE FLORIDA PENINSULA...


...Southwest to the central High Plains...
Above normal moisture will continue to be present across portions
of the Southwest, particularly portions of Arizona today but also
extending northward toward portions of the Central High
Plains/Central Rockies. First, moisture anomalies peak near 2-3
standard deviations above normal today across portions of Arizona
which combined with expected daytime heating/instability should
produce higher coverage of storms and intensity compared to recent
days. This is supported by the various CAMs and the 00Z HREF which
shows peak 0.5" hourly total probabilities this afternoon at 60-80
percent from northern AZ into far southern UT. This raises the
concern for isolated flash flooding for both urbanized areas and
complex terrain. Across portions of Colorado and Wyoming,
shower/thunderstorm activity is expected to be enhanced with
approaching shortwave energy timing during the peak of the
afternoon. Localized hourly totals up to 1-1.5" will be possible,
especially by early evening as storms move east off the terrain
into the foothills and Plains.

...Mid Missouri to the Mid Mississippi valley...
Current convection across portions of the Plains early this
morning will continue to move southeast in the northwest flow
regime, rounding the periphery of the strong upper ridge axis
centered over Texas. This activity will move along/northeast of a
stalled frontal boundary in the region and act on the
southwesterly low-level jet with upwards of 30-35 kts of inflow at
850 mb. As the main complex of storms moves southeast, additional
convection is likely to develop ahead of it across portions of far
eastern KS and much of Missouri. Hourly totals exceeding 1" will
be possible with the strongest cores this morning, resulting in a
localized flash flood threat. Additional convection is expected to
develop late this evening into tonight (early Tuesday morning)
across portions of the region, as the low-level jet increases, and
some of this rainfall may overlap, particularly across portions of
northern MO as supported by the 24-hr HREF probabilities of 3"+
which reach 30-40 percent. The environment will be certainly
conducive for training, highly efficient rain producing
thunderstorms that could produce localized but higher totals. If
the trends toward a higher end heavy rainfall threat continue to
grow for tonight across central Missouri, an upgrade to Slight
Risk could be needed.

...Eastern Florida Peninsula...
Continued deep moisture present across Florida, characterized by
precipitable water values exceeding 2" (near 1.5-2 standard
deviations above normal) will interact with a stalled frontal
boundary to the north to enhance the daily afternoon/evening
thunderstorm coverage and intensity today. Hourly totals between
1-2" will be possible during the peak heating this
afternoon/evening, supported by the 00Z HREF neighborhood
probabilities for 2" in 1-hr peaking at 40-70 percent across the
eastern Florida Peninsula. This supports the potential for
localized flash flooding for mainly the urban locations.

Taylor



Day 2
Valid 12Z Mon Jul 31 2023 - 12Z Tue Aug 01 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
MISSOURI AND THE CENTRAL ROCKIES...

...Southwest to the northern High Plains...
The anomalous deep monsoonal moisture begins to move out of the
Southwest during the period, reaching further north into northern
Utah, Wyoming, and northern Colorado. This will begin to limit the
threat of flash flooding for portions of Arizona and Utah.
However, shortwave energy lifting through the flow will bring
another day of unsettled/active weather and the threat of intense
rain rates and heavy rainfall, particularly for portions of
Colorado and southern Wyoming where rain rates of 1-2"/hr may fall
over recent burn scars and areas of relatively low flash flood
guidance.

...Northern Plains...
Localized heavy rainfall will be possible during the period as an
axis of higher moisture surges ahead of an approaching front
combined with favorable forcing and enough instability to produce
localized intense thunderstorms that may lead to a few instances
of training thunderstorms that could produce flash flooding.

...Mid Missouri to the lower Ohio Valley...
Repeating shortwave energy rounding the periphery of the strong
upper ridge axis centered over Texas will keep an active pattern
in place from portions of the Plains through the Mid Missouri to
Mid Mississippi Valley. In the northwest flow regime, there is
greater than normal uncertainty with how convection will develop
and evolve but the guidance continues to suggest activity will be
ongoing at the start of the period (Tuesday morning) across
portions of southern Iowa into Missouri. Depending on how that
plays out, additional convection may develop later in the period,
followed by another round of overnight convection into early
Wednesday morning. Each round could produce locally heavy rainfall
and intense rain rates, and the nighttime convection has the
synoptic setup that bears watching for localized intense rain
rates and training convection that could produce localized higher
rainfall totals. As a result, with growing support from the 00Z
deterministic and ensemble guidance, a Slight Risk was introduced
this cycle.

Taylor

Day 3
Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MISSISSIPPI VALLEY, GREAT BASIN, AND CENTRAL ROCKIES...

...Mid Mississippi Valley...
Another complex of thunderstorms is likely to be ongoing at the
start of the period (Wednesday morning) across the outlook area,
associated with shortwave energy moving within the northwest flow
regime, generally along/east of a quasi-stationary frontal
boundary draped in the region, and interacting on the nose of the
nighttime southwesterly low level jet that is expected to reach
30-40 kts at 850 mb. Precipitable water values are likely
anomalous, 2 standard deviations above normal, helping to fuel the
intense rain rates. Then another round of thunderstorms will be
possible late in the period (early Thursday morning) developing on
the nose on the increasing low level jet again over portions of
eastern MO, southern IL, and southwest KY. The synoptic setup will
remain conducive for training/backbuilding convection and this
would likely be the third consecutive day of potentially
overlapping rainfall events, so the cumulative totals may begin to
saturate some soils and heighten the flash flood risk. The 00Z
deterministic and ensemble guidance shows potential for higher
rainfall this period compared to today and Tuesday, and as the
mesoscale details become clearer in the next couple of days, some
localized higher rainfall totals may materialize somewhere in the
region.

...Great Basin to North/Central Rockies and High Plains...
Widespread showers and thunderstorms are expected Wednesday into
Wednesday night aided by shortwave energy and right entrance
dynamics with a strengthening jet streak over the northern
Rockies. Moisture will very anomalous, upwards of 1-1.25" of
precipitable water and nearly 3 standard deviations above normal,
based on the latest model guidance and with the daytime heating
and terrain influences, localized intense rain rates exceeding
0.5"/hr will be possible. The 00Z guidance continues to highlight
portions of eastern Idaho, western Wyoming, and northern Utah with
the grater probabilities of seeing the higher rainfall totals as
well as portions of Colorado and southeast Wyoming where Slight
Risks were maintained from the previous forecast cycle.

Taylor
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 010810
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 AM EDT Tue Aug 01 2023

Day 1
Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MISSOURI AS
WELL AS THE CENTRAL ROCKIES AND PLAINS...

...Southwest to the central High Plains...
Another round of afternoon to evening convection expected today
with the monsoonal moisture axis primarily confined to portions of
eastern Nevada through much of Utah and into Wyoming/Colorado.
Probabilities of seeing 0.5-1" hourly totals ramp up considerably
by afternoon, peaking upwards of 30-50 percent across portions of
NV/UT and above 60-70 percent for the CO foothills eastward
through the central High Plains by late tonight. Some of these
areas have received heavy rainfall Monday and early this morning,
so soils will become increasingly saturated, especially across
portions of CO. So the combination of intense rain rates
(1-2"/hr), isolated rainfall totals of 3-4", and wetter antecedent
conditions may lead to another round of scattered flash flooding.

...Northern Plains...
Localized heavy rainfall will be possible during the period as an
axis of higher moisture surges ahead of an approaching front
combined with favorable forcing and enough instability to produce
localized intense thunderstorms that may lead to a few instances
of training thunderstorms that could produce flash flooding.

...Mid Missouri to the lower Ohio Valley...
Very active, unsettled, and potentially significant heavy rainfall
event beginning to set up across portions of Missouri the next
couple of days. Upper ridge axis remains anchored over Texas early
this morning with the ring of fire convection developing along its
periphery. The northwest flow aloft positioned over the Mid
Missouri to Mid Mississippi Valleys will interact with the
strengthening low level jet early this morning to produce a
narrow/localized swath of heavy rainfall that is just beginning to
develop per recent radar and satellite imagery. This activity is
likely to train/repeat as it slowly drifts southeast across
central Missouri. The 00Z HREF suggests localized higher totals
exceeding 3-5" will be possible through late morning before the
low level jet wanes. This generally accounts for the western half
of the Slight Risk area, which is on the higher end of the Slight
Risk probabilities.

After a break/lull expected mid day to early evening, another
round of convection will develop across portions of
central/eastern Missouri after midnight tonight through Wednesday
mid morning. The setup continues to look very conducive for heavy
rainfall with anomalous moisture in place, sufficient elevated
instability, and a strengthening low level jet positioned
favorably into the region. The 00Z HREF and CAMs show potential
for another localized/significant heavy rainfall swath, which is
likely to be displaced just enough eastward of this morning's
heavy rainfall, limiting the overlap and this threat accounts for
the eastern area of the Slight Risk outlook. That threat will
continue into the Day 2 period with additional heavy rainfall
going beyond 12Z Wednesday.


Taylor


Day 2
Valid 12Z Tue Aug 01 2023 - 12Z Wed Aug 02 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID MISSISSIPPI VALLEY...

...Mid-Mississippi and Lower Ohio Valleys...
Convection will be ongoing at the start of the period across
portions of Missouri, associated with the nighttime low level jet
interacting with a frontal boundary draped in the region and
shortwave energy moving through the northwest flow aloft.
Favorable moisture axis and sufficient elevated instability will
continue the threat of locally significant heavy rainfall and
flash flooding early on in the period Wednesday morning where the
latest deterministic and ensemble guidance supports potential for
several inches. This should fall slightly displaced from the heavy
rainfall on Tuesday morning, but also could be more significant
given the more favorable environmental ingredients. In
coordination with WFO St. Louis, a Moderate Risk was introduced to
account for the potential for locally but significant flash
flooding early Wednesday. Another round of heavy rainfall is also
then likely to develop again Wednesday night into Thursday
morning, though with a warm front beginning to advance more
eastward, this axis of heavy rainfall looks to be displaced
eastward yet again but could fall near or around the St. Louis
metro area and into southern Illinois and with similar
environmental ingredients (high PWs, warm cloud depths,
strengthening low level jet), swaths of heavy rainfall will be
likely at the end of the period into Thursday morning.

...Great Basin to North/Central Rockies and High Plains...
Widespread showers and thunderstorms are expected Wednesday into
Wednesday night aided by shortwave energy and right entrance
dynamics with a strengthening jet streak over the northern
Rockies. Moisture will be very anomalous, upwards of 1-1.25" of
precipitable water and nearly 3 standard deviations above normal,
based on the latest model guidance and with the daytime heating
and terrain influences, localized intense rain rates exceeding
0.5"/hr will be possible. The 00Z guidance continues to highlight
portions of eastern Idaho, western Wyoming, and northern Utah with
the greater probabilities of seeing the higher rainfall totals as
well as portions of Colorado and southeast Wyoming where Slight
Risks were maintained from the previous forecast cycle.

Taylor

Day 3
Valid 12Z Wed Aug 02 2023 - 12Z Thu Aug 03 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE TENNESSEE VALLEY AND NORTHERN HIGH PLAINS...

...Tennessee Valley...
A pair of robust shortwave troughs are expected to move through
portions of the Mid Mississippi to Tennessee Valleys Wednesday
night (early Thursday) and then again Thursday night (early
Friday), bringing a couple rounds of heavy rainfall to the area.
Convection is expected to ramp up late in the evening and
overnight hours associated with the strengthening low level jet
where the latest model guidance shows upwards of 30-40 kts at 850
mb impinging on a boundary draped across the region. With the mean
flow oriented nearly parallel to the expected storm motions, some
repeating rounds and backbuilding will be possible and given the
anomalous moisture expected (PWs 1.75-2"), the highly efficient
rain producing thunderstorms combined with potential longer
duration could bring localized swaths of higher end rainfall
totals. There still remains large model spread, particularly
spatially with some north/south uncertainties but the consensus
and ensemble data point toward portions of southern Kentucky
through middle and eastern Tennessee having the grater threat for
both rounds to overlap for the period, so a Slight Risk was
introduced this forecast cycle.

...Great Basin to Northern High Plains...
Shortwave energy lifting through the region combined with
favorable right entrance jet dynamics will support a broad area of
forcing for ascent from portions of the Great Basin to the
Northern High Plains. Moisture anomalies will remain above normal,
nearly 2-2.5 standard deviations above normal, particularly across
portions of northeast WY, southeast MT and western SD. Convection
is expected to develop initially over portions of the Great Basin
before moving eastward through WY/MT and pushing into the Plains
later in the period where it'll interact with the greater
instability/moisture. Localized rain rates 0.5-1"/hr will be
possible across ID while further east rain rates up to 1.5"/hr
will be possible in the Slight Risk area, where the 00Z guidance
suggests localized rainfall totals over 2" will be possible. The
inherited Slight Risk continues to look reasonable with only minor
adjustments made.


$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 020820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Wed Aug 02 2023

Day 1
Valid 12Z Wed Aug 02 2023 - 12Z Thu Aug 03 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MID MISSISSIPPI VALLEY...

...Mid Mississippi to Lower Ohio Valleys...
A significant flash flooding event is expected across portions of
Mid Mississippi Valley this morning and then again late tonight
into Thursday morning, impacting much of north-central to
east-central Missouri with a narrow but potentialy extreme amount
of rainfall in short period of time. This includes the greater St.
Louis metro area, particularly for tonight/early Thursday morning.

Early this morning, convection has continued to blossom and expand
in coverage from southern Iowa through central Missouri at the
nose of the strengthening low level jet. The latest RAP analysis
showed upwards of 40 kts overrunning a frontal boundary draped
across the region. This coincides with favorable forcing for
ascent from shortwave energy rounding the periphery of the strong
upper ridge axis. Finally, moisture anomalies are quite high this
morning, where the blended TPW product shows values exceeding 2",
and likely to increase further. The current convection will slowly
drift south/southeast through central/east-central Missouri
through mid to late morning. High warm cloud depths and an axis of
2000-2500 J/kg MUCAPE this morning will support higher end rain
rates exceeding 2-3"/hr (localized 3"+ hourly totals possible) and
with backbuilding/training convection, several hours of these
intense rain rates in a narrow/localized swath is likely to
produce a swath of extreme rainfall totaling 4-8"+ (localized
8-10") through the morning, which is supported by the 00Z HREF
showing 50-60 percent probabilties for 5"+. This activity will
begin to wane after 15Z (most intense rain rates through 15Z) but
may not completely subside until early afternoon.

Then later tonight into Thursday morning, another round of locally
significant heavy rainfall is expected to develop across eastern
Missouri into southern Illinois. Continued anomalous moisture,
another surging low level jet, favorable instability, and high
warm cloud depths all point toward intense rain rates (2-3"/hr)
and localized higher end rain totals. The axis of heaviest rain
may fall just east/southeast of this morning's heavy rainfall
footprint, but very close overlap and also in and around the St.
Louis metro. The 00Z HREF probs for 5"+ again reach 50-60 percent
for tonight/early Thursday morning, maximized just west of St.
Louis which shows even a near 20 percent chance for 8" totals. The
00Z CAMs generally all point to an higher end Moderate Risk flash
flood event event and with the proximity to the STL metro, higher
end flash flooding will be possible due to the intensity of
rainfall for several hours over a more urbanzed location.

...Great Basin to North/Central Rockies and High Plains...
Widespread showers and thunderstorms are expected Wednesday into
Wednesday night aided by shortwave energy and right entrance
dynamics with a strengthening jet streak over the northern
Rockies. Moisture will be very anomalous, upwards of 1-1.25" of
precipitable water and nearly 3 standard deviations above normal,
based on the latest model guidance and with the daytime heating
and terrain influences, localized intense rain rates between
0.5-1"/hr will be possible. The 00Z guidance continues to
highlight portions of eastern Idaho, western Wyoming, and northern
Utah with the greater probabilities of seeing the higher rainfall
totals (localized 1-2") as well as portions of far northern
Colorado and southeast Wyoming where Slight Risks were maintained
from the previous forecast cycle.

Taylor


Day 2
Valid 12Z Thu Aug 03 2023 - 12Z Fri Aug 04 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE LOWER OHIO VALLEY TO SOUTHERN APPALACHIANS AND NORTH/CENTRAL
HIGH PLAINS...

...Lower Ohio Valley, Tennessee Valley into Southern
Appalachians...
The setup remains favorable for heavy rainfall and flash flooding
both Thursday morning and again late Thursday night into Friday
morning as shortwave energy moves through the region within the
northwest flow regime. At the surface, an area of low pressure
slide southeastward, advancing a warm front into the region.
Strong southwesterly low level flow will bring a surge of moisture
into the region, characterized by precipitable water values
increasing from around 1.5" to over 2" from southern Kentucky
through Tennessee and western NC. A complex of storms is expected
to be moving through portions of southern Kentucky and middle
Tennessee early in the period followed by another round of
convection igniting late in evening and overnight into Friday
across middle/eastern Tennessee into western North Carolina. The
repeating rounds could bring several inches of rainfall for the
24-hr period (2-4"+) and given the forcing, highly anomalous
moisture, and favorable instability setup, intense rain rates
between 1-2"/hr will be possible. There remains still some spatial
uncertainty in where the axis of heaviest rainfall falls (global
models further north/northeast compared to the southern CAMs) and
the axis of heavy rainfall is likely to be rather narrow but could
be locally significant, especially for the more susceptible
locations and complex terrain areas.

...Great Basin to Northern High Plains...
Shortwave energy lifting through the region combined with
favorable right entrance jet dynamics will support a broad area of
forcing for ascent from portions of the Great Basin to the
Northern High Plains. Moisture anomalies will remain above normal,
nearly 2-2.5 standard deviations above normal, particularly across
portions of northeast WY, southeast MT and western SD. Convection
is expected to develop initially over portions of the Great Basin
before moving eastward through WY/MT and pushing into the Plains
later in the period where it'll interact with the greater
instability/moisture. Localized rain rates 0.5-1"/hr will be
possible across ID while further east rain rates up to 1.5"/hr
will be possible across portions of northeast WY, southeast MT,
and western SD. 00Z deterministic and ensemble guidance point
toward localized 2"+ totals possible in that Slight Risk. Further
south into northeast CO, northwest Kansas, and southern Nebraska,
while rain rates and totals may not be as intense, the much wetter
antecedent conditions combined isolated/scattered convection
producing upwards of 1"/hr rain rates and localized 1-2" totals
may produce more scattered instances of flash flooding and the
inherited Slight Risk was maintained.

...Interior Northeast...
Broad forcing for ascent associated with an approaching shortwave
energy embedded within the larger troughing will overspread the
region while at the surface, a cold front begins to advance from
the west. Ahead of that, southerly flow will bring modest moisture characterized by precipitable water values between 1-1.25". With
daytime heating producing favorable MLCAPE values, scattered to
numerous showers and thunderstorms are expected which could
produce localized intense rain rates in excess of 1"/hr and may
lead to isolated instances of flash flooding across portions of
the interior Northeast.

Taylor

Day 3
Valid 12Z Thu Aug 03 2023 - 12Z Fri Aug 04 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN HIGH PLAINS...

...Northern Rockies to Central Plains...
Fairly potent shortwave energy lifting through the Great Basin is
expected to become closed off at 500 mb across eastern Montana and
the western Dakotas. This will wrap very anomalous moisture into
the region, especially across southeast Montana into western South
Dakota where PWs > 1.5" are expected, which is over 3 standard
deviations above normal. As low pressure develops and moves from
WY to the Dakotas, a very favorable frontogenetical band of
precipitation is expected which could produce a longer duration
moderate to heavy rainfall event. The signal for 1-2"+ looks
pretty strong such that the Slight Risk was maintained across
western SD but also expanded into southeast MT. There remains some
uncertainty in how quickly convection progresses southeast into
the Plains but the Marginal Risk was generally maintained through
eastern Nebraska and western Iowa.

...Southeast...
Anomalous moisture (PWs > 2") combined with favorable forcing for
ascent associated with shortwave energy moving through the region
will keep an active/unsettled pattern in place and the threat of
heavy rainfall and flash flooding. There remains uncertainties in
the placement and evolution of the convection during the period
but the environmental ingredients from the 00Z guidance all point
toward some threat of flash flooding so the Marginal Risk was
broadly placed from the Southern Appalachians toward the coastal
areas of North Carolina and South Carolina.

...Northeast...
Longwave troughing becomes increasingly neutrally tilted during
the period with the trough axis centered through Upstate New York
late Friday afternoon and evening. This will allow for a longer
period of southerly flow to bring anomalous moisture northward
across the Northeast, particularly across the coastal regions of
southern New England through coastal Maine with the latest
guidance pointing toward a narrow axis of 1.5"+ PW setting up.
Depending on the timing of the advancing cold front, enough
destabilization will be possible during the afternoon/evening to
produce scattered to numerous showers and thunderstorms across the
region before quickly moving east. The modest moisture and
instability and favorable forcing will support localized intense
rain rates (1-1.5"/hr) with some localized totals near 2"
possible. A Marginal Risk was maintained but as mesoscale details
become clearer, an upgrade could be needed across portions of
southern New England where antecedent conditions are much wetter
(14-day rainfall departures are running 200-400 percent of
normal).

Taylor

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt__;!!DZ3fjg!-7Wj4FLsfM2N-fHh8-uDnkR88bFNzOdQC8wg7FOpa-H9DBVkAscU4ira6Syxbga-ROlEKeALIgWqZFJOwGrJ78MY_Fg$

Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt__;!!DZ3fjg!-7Wj4FLsfM2N-fHh8-uDnkR88bFNzOdQC8wg7FOpa-H9DBVkAscU4ira6Syxbga-ROlEKeALIgWqZFJOwGrJXcHQVfA$

Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt__;!!DZ3fjg!-7Wj4FLsfM2N-fHh8-uDnkR88bFNzOdQC8wg7FOpa-H9DBVkAscU4ira6Syxbga-ROlEKeALIgWqZFJOwGrJz3YpPD4$


$$
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FOUS30 KWBC 030833
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Thu Aug 03 2023

Day 1
Valid 12Z Thu Aug 03 2023 - 12Z Fri Aug 04 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MISSISSIPPI, LOWER OHIO, AND TENNESSEE VALLEYS TO THE
SOUTHERN APPALACHIANS, AS WELL AS THE NORTH/CENTRAL PLAINS INTO
EASTERN PORTIONS OF THE INTERMOUNTAIN WEST...

...Mid Mississippi Valley, Lower Ohio Valley, Tennessee Valley,
Southern Appalachians...
The setup remains favorable for heavy rainfall and flash flooding
both Thursday morning and again late Thursday night into Friday
morning as shortwave energy moves through the region within the
northwest flow regime. The MCS and attendant MCV track across
southeast MO and southern IL early this morning support a westward
shift in the back edge of both the Slight and Marginal Risk areas
across the western TN and mid MS Valleys. Elevated MUCAPEs between
1000-2000+ J/Kg along with PWs aoa 2.25" within the convective
environment (MCS) will support peak hourly rainfall rates of 2+
inches through 13-15Z, as the southwesterly LLJ (30-35 kts at
850mb) veers more westerly towards 12Z, becoming more parallel to
the mean 850-300 mb flow and thus favoring continued upwind
propagation and cell training for a while longer.

Initial MCS remnants/MCV will track into the TN Valley and
eventually the southern Appalachians. Surge of PWs between 2-2.25+
inches along with anomalous 850-700 mb moisture flux (3 to 4+
standard deviations above normal for early August) will make for
optimal warm rain processes, even as the deep-layer CAPEs struggle
to reach 1000 J/Kg farther east across the southern Appalachians.
CSU GEFS-FV3 based first guess fields support a Slight Risk into
western NC/Upstate SC and across northern-central GA. CAMs show
the next round of convection developing along or just north of the
surface stationary boundary near the KS-MO border late this
evening, then evolve into another MCS overnight into Friday
morning across south-central and southeast MO, far southern IL,
and western KY-TN. Still a bit more spread in the QPF guidance
than one would like to see at the tail end of the day 1 forecast
period,
however it is likely that some areas will experience repeating
rounds of convection early and late in the D1 period, which could
bring several inches of rainfall for the 24-hr period (2-4"+) and
given the forcing, highly anomalous moisture, and favorable
instability setup, intense rain rates between 1-2"/hr will be
possible.

...Eastern portions of the Great Basin into Wyoming and parts of
the Central Plains...
Shortwave energy lifting overtop dampening upper ridge combined
with favorable right entrance jet dynamics will support a broad
area of forcing for ascent from eastern portions of the Great
Basin across much of WY and into the Plains. Moisture anomalies
will remain above normal, nearly 2-2.5 standard deviations above
normal, particularly across northern UT-eastern ID into northern
WY and western SD. Convection is expected to develop initially
over portions of the Great Basin before moving eastward through
WY/MT and pushing into the Plains later in the period where it'll
interact with the greater instability/moisture. Localized rain
rates 0.5-1"/hr will be possible across ID while further east rain
rates up to 1.5-2.0"/hr will be possible across portions of
eastern WY and western SD. 00Z deterministic and ensemble guidance
point toward localized 2"+ totals possible in that Slight Risk.
Further south into northeast CO, northwest Kansas, and southern
Nebraska, localized rain rates and totals may be just as intense.
This along with the wet antecedent conditions may produce more
scattered instances of flash flooding, and as such the inherited
Slight Risk was maintained.

...Interior Northeast...
Broad forcing for ascent associated with an approaching shortwave
energy embedded within the larger troughing will overspread the
region while at the surface, a cold front begins to advance from
the west. Ahead of that, southerly flow will bring modest moisture characterized by precipitable water values between 1-1.25". With
daytime heating producing favorable MLCAPE values, scattered to
numerous showers and thunderstorms are expected which could
produce localized intense rain rates in excess of 1"/hr and may
lead to isolated instances of flash flooding across portions of
the interior Northeast.

Hurley/Taylor


Day 2
Valid 12Z Fri Aug 04 2023 - 12Z Sat Aug 05 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN HIGH PLAINS AS WELL AS MUCH OF NEW ENGLAND...

...Northern Rockies to Central Plains...
Fairly potent shortwave energy lifting through the Great Basin is
expected to become closed off at 700-500 mb across eastern Montana
and the western Dakotas by early Saturday. This will wrap very
anomalous moisture into the region, especially across southeast
Montana and far northeastern WY into central-western South Dakota
where PWs of 1.3 to 1.5"+ are expected, which is over 3 standard
deviations above normal. As low pressure develops and moves from
WY to SD, a very favorable frontogenetical band of precipitation
is expected which could produce a longer duration moderate to
heavy rainfall event. The signal for 1.5 to 3"+ looks pretty
strong, especially with hints of coupled upper jet streaks, such
that the Slight Risk was maintained across parts of the northern
High Plains (south-central and southeast MT into north-central and
northeast WY) into southwest ND, western and central SD, and
northern portions of NE. Per the QPF spread in the guidance, there
continues to be some uncertainty in how quickly convection
progresses southeast into the Plains but the Marginal Risk was
generally maintained through eastern Nebraska and western Iowa.

...Southeast...
Anomalous moisture (PWs > 2") combined with favorable forcing for
ascent associated with shortwave energy moving through the region
will keep an active/unsettled pattern in place and the threat of
heavy rainfall and flash flooding. The notable difference however
from yesterday's Day 2 ERO was a southward shift in the Marginal
Risk area, based on the model consensus of a noticeable southward
trend in the location of the surface boundary and thus the
deep-layer instability/moisture pooling ahead of the fronts.

...Northeast...
Longwave troughing becomes increasingly neutrally tilted during
the period with the trough axis centered through Upstate New York
late Friday afternoon and evening. This will allow for a longer
period of southerly flow to bring anomalous moisture northward
across the Northeast, particularly across the coastal regions of
southern New England through coastal Maine with the latest
guidance pointing toward a narrow axis of 1.5"+ PW setting up.
Depending on the timing of the advancing cold front, enough
destabilization will be possible during the afternoon/evening to
produce scattered to numerous showers and thunderstorms across the
region before quickly moving east (00Z ECMWF shows mixed-layer
CAPEs between 1000-1500 J/KG). The modest moisture and instability
and favorable forcing will support localized intense rain rates
(1-1.5"/hr) with some localized totals near 2" possible. The broad
Marginal Risk was maintained, but so too was the Slight Risk
across eastern NY into much of New England given the combination
of higher exceedance probabilities (e.g. more widespread HREF
probs aoa 50% of QPF>2" in the 12hr period ending 00Z Sat), along
with the wet antecedent soils.

Hurley

Day 3
Valid 12Z Sat Aug 05 2023 - 12Z Sun Aug 06 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN PLAINS INTO PARTS OF THE UPPER MIDWEST...

...Northern Rockies and Plains into the Midwest...
Aforementioned vigorous mid/upper level shortwave, along with the
eastward progression of coupled upper level jet streaks
(right-entrance region of the jet streak near the U.S.-Canadian
border and left-exit region of the upper jet axis nudging into the
central High Plains) will maintain a favorable synoptic setup for
more organized areas of heavy rainfall. This as the 850-700 mb
moisture flux anomalies remain 3 to 4 standard deviations above
normal. Much stronger deep-layer instability south of the best
frontogenetical forcing will likely result in organized convection
(and heavy rainfall footprint) sliding farther south of the
current ensemble mean of global guidance, and as such have altered
the Slight Risk accordingly -- not only across the Dakotas (mainly
SD), but also downstream into southern MN, northeast NE, and
northern IA. The UFVS-verified version of the CSU first-guess
field supports this shift with the Slight Risk area, as the 00Z
non-CAM guidance already hints at areas of 2-4"+ max totals.

Hurley
$$
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FOUS30 KWBC 040859
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
458 AM EDT Fri Aug 04 2023

Day 1
Valid 12Z Fri Aug 04 2023 - 12Z Sat Aug 05 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHERN HIGH PLAINS...

...Portions of the Northern High Plains...

Maintained the Moderate risk area for portions of the northern
High Plains with anomalous moisture and persistent upper level
energy helping to focus and sustain convection. The global
models, high resolution CAMs...ensembles and the NBM all point to
potential for 1 to 3 inches across parts of Montana into the
western Dakotas on Friday and Friday night. A closed mid and
upper level low located he right rear entrance region of an upper
level jet over Canada and the nose of a 90 to 110 kt upper level
jet moving on-shore and progressing across California/Nevada
during the day and into the Great Basin this evening will tap into
a moisture atmosphere for a numerous showers and thunderstorms
capable of producing intense rainfall rates and potentially
excessive rainfall amounts. The ECMWF ensemble mean shows
Precipitable Water values more than 3 standardized anomalies
greater than climatology for this time of year in southeast MT and
southwest ND by 05/00Z...placing it in the top few climatological percentiles...and the GEFS paints a similar picture with only
slightly less magnitude. Latest 40 km neighborhood probabilities
still showed values at or somewhat above 40 pct for the 5 inch
threshold and nearly 15 pct chance of exceedance of 8 inches. The
expectation is that convection with the most intense rainfall rate
will develop during peak heating across the Moderate Risk area,
but will continue into Thursday night with somewhat diminished
rainfall rates as the area builds towards the better instability
farther south.as more stratiform rain that very slowly sags
southward towards the Black Hills. This pattern will favor more
widespread flash flooding since the rainfall is expected to
persist much longer and over a larger area than previous days.

...Northeast...

Reintroduced a small portion of the Slight Risk area over
hydrologically sensitive portions of New England...in part given
the expected on-going rainfall early. Satellite and radar imagery
showed the convection to be fast moving which should help mitigate
many problems. However, the latest CAMs show a subtle increase in
the probabilities of rainfall exceeding flash flood guidance in
the short term...perhaps tied to steepening low level lapse rates
which lead to somewhat enhanced rainfall rates. Kept the Slight
Risk fairly targeted thinking that much of the area has been drier
than normal and more likely to handle the expected rainfall
amounts.

,,,Mid Mississippi Valley...
Introduced a Slight Risk in portions of the Mississippi Valley
where on-going convection may still be overlapping with the lowest
flash flood guidance.



...Southeast...

Anomalous moisture (PWs > 2") combined with favorable forcing for
ascent associated with shortwave energy moving through the region
will keep an active/unsettled pattern in place and the threat of
heavy rainfall and flash flooding. The notable difference however
from yesterday's Day 2 ERO was an uptick in the rainfall along the
NC/VA coastline as a weak shortwave approaches from the west later
today. Extended the Marginal risk area to cover the potential
that some flooding or run-off problems occur. On the other
hand...flash flood guidance is very high which should help
mitigate any concerns.

Bann


Day 2
Valid 12Z Sat Aug 05 2023 - 12Z Sun Aug 06 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN PLAINS INTO PARTS OF THE UPPER MIDWEST...

...Northern Rockies and Plains into the Midwest...
A vigorous mid/upper level shortwave, along with the eastward
progression of coupled upper level jet streaks will maintain a
favorable synoptic setup for additional organized areas of heavy
rainfall. This occurs as the 850-700 mb moisture flux anomalies
remain 3 to 4 standard deviations above normal. Much stronger
deep-layer instability south of the best frontogenetical forcing
will likely result in organized convection. The anticipated heavy
rainfall being a bit farther south of the current ensemble mean of
global guidance has already been accounted for and still looks
reasonable. The UFVS-verified version of the CSU first-guess
field was less expansive with its Slight risk area than
WPCs...apparently as a result of the spread in placement of the
maximum QPF. But inspection of the spaghetti plots for 2 and 3
inch contours suggests a broader Slight was warranted.

Bann


Day 3
Valid 12Z Sun Aug 06 2023 - 12Z Mon Aug 07 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE GREAT LAKES...

The potential for heavy rainfall will continue to spread eastward
from the Upper Midwest on Saturday into the Great Lakes region on
Sunday/Sunday night. Growing spread in the guidance makes zeroing
in on a specific area...with the SREF being considerably slower
than the GEFS (the difference spanning from Wisconsin and far
eastern Minnesota across southern Lower Michigan. There was
agreement between the various ensembles that 2+ inch amounts are
possible. A shift in the 04/00Z ECMWF did give somewhat more
support to the GEFS QPF placement over eastern Wisconsin into
western Michigan. Opted to place a broad and rather unfocused
Slight Risk over the western Great Lakes which was aligned with
northeast to southwest fgen associated with a deepening surface
low. The expectation is that that additional shifts and
modifications will be needed as the guidance becomes clearer.

...Tennessee Valley...
Locally heavy rainfall may occur in proximity to a
quasi-stationary front during the afternoon and evening.
Spaghetti plots from the GEFS show at least some threat of 2+ inch
rainfall amounts where both the ECMWF and the GFS started to pool
moisture along the front. Saw little reason to go any more than a
Marginal at this point.

...Northern Rockies...
Introduced a Marginal risk area in parts of the Northern Rockies
where the operational model/ensemble consensus was for 0.5 to
0.75+ inch amounts. With steepening lapse rates combined with low
level upslope flow, the thinking is that enough of the rainfall
amounts could be delivered quickly enough to cause isolated
problems,

Bann
$$
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FOUS30 KWBC 060851
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
450 AM EDT Sun Aug 06 2023

Day 1
Valid 12Z Sun Aug 06 2023 - 12Z Mon Aug 07 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST/NORTHERN PLAINS...

...Northern Plains...
A Slight Risk remains in place across parts of the Upper
Midwest/Northern Plains as a vigorous vertically-stacked low makes
its way eastward ever so slowly across the region. An axis of
1.75-2" precipitable moisture plume will continue to wrap into the system...providing a sounding profile favorable for heavy rainfall
and rainfall rates. Latest guidance shows an axis of 1 to 3 inches
in a northwest to southeast axis from Iowa into Illinois...with
some of the models with higher resolutions showing embedded higher
maximum amounts. HREF 40 km neighborhood probabilities for 1 inch
per hour amounts are at or above 30 percent along this axis by
afternoon and maximize over 40 percent in parts of southeast Iowa
by late afternoon/early evening and then begin to taper off by
late evening. There is a better signal from the HREF that 3 hour
QPF may exceed 3 hour flash flood guidance than 1 hour QPF
exceeding 1 hour QPF...mainly across parts of Iowa that received
heavy to excessive rainfall earlier on Saturday.


...Northeast IL/Northwest IN Eastward to Parts of New York and the
Mid Atlantic...
A compact mesolow has occluded while tracking east into northern
Indiana by early this morning with generally light to moderate
rainfall amounts. With the models still pointing to the
possibility of an uptick in rainfall rates later
today...re-aligned the Marginal risk area a bit in lower Michigan.
Still not expecting much in the way of backbuilding, cell merger
activity, or training. As the system continues to push
eastward...precipitable water values of 1.8 to 2,2 inches get
drawn into the system by this evening with growing coverage of
showers and thunderstorms. Some of the convection should be able
to produce some downpours given that environment. Opted to
introduce a Slight Risk area over parts of the Mid-Atlantic/I-95
corridor where convection from the late afternoon into the late
evening or or early Monday morning will be force-fed the moisture
by an increasing low level jet...at least enhancing the risk of
excessive rainfall totals. A greater concern this far east occurs
after the end of the Day 1 period.

...Pacific Northwest...

Highly anomalous moisture also persists across portions of the
interior Pacific Northwest, with PWATs to 1.25 inches running more
than 3 sigma above normal for this time of year. The concern is
with daytime heating Sunday afternoon that the storms that form
are likely to train westward across portions of WA that are not
equipped to handle much heavy rain all at one time. Since the
forcing is weak on Sunday, the area remains in a Marginal Risk.

Bann



Day 2
Valid 12Z Mon Aug 07 2023 - 12Z Tue Aug 08 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
NORTHEAST AS WELL AS NORTHEASTERN WASHINGTON AND NORTHERN IDAHO...

...Much of the Northeast...

Guidance continues to struggle with the placement of heaviest
rainfall across this region on Monday. A developing surface low
starts the day near Chicago and will track to near Lake Ontario by
Tuesday morning. Along this track, the areas likely to be hardest
hit with the rain are from the I-95 corridor from Baltimore to NYC
northward into northern NY and New England. Isolated rainfall
totals to 4 inches are possible. This encompasses areas which are
still hydrologically sensitive following heavy rainfall and
flooding over the past few weeks...particularly northern NY into
northern New England. Some expansion was done to the Slight risk
area in eastern Pennsylvania given an increase in QPF with little
need for changes in areas where orographics influence rainfall
totals compared with the previous outlook. Expect more widely
scattered convection from the southern Appalachians north through
the Midwest.

...Northeastern WA through Northern ID...

A strong, but slow-moving and shearing shortwave in the upper
levels will track across central WA on Monday. With highly
anomalous atmospheric moisture already in place, it's likely this
upper level forcing will enhance local rainfall totals across this
region. This area has been hit with rainfall yesterday, today, and
is expected tomorrow, so the likely wettest day of the group being
the last day of the rain will mean FFGs will be at their lowest in
the area when the heaviest rain falls. T

Bann


Day 3
Valid 12Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NEW ENGLAND AND NEW YORK ON TUESDAY...

...Northeast...
Increasing dynamics associated with low pressure that will have
made its way eastward from the Upper Midwest and Great Lakes
region will aid increasing coverage of showers and thunderstorms
capable of producing heavy rainfall rates and excessive rainfall.
A plume of precipitable water values in excess of 1.75 inches
should already by in place from the Mid-Atlantic region
north/northeastward that can be tapped by the vertically stacked
system. The threat of excessive is expect to be driven by intense
rainfall rates...especially more in the south and in the
east...while concern for portions of upstate New York into Vermont
will be a combination of heavy rates with the deep moisture
followed by the upper system over Canada that begins to change
course and possibly bring renewed convection later in the period.
This area in particular is still recovering from recent flooding.
There was too much spread in the guidance to place a Moderate with
any confidence. The 06/00Z NAM and UKMET depict 2 to 4 inch
amounts just north of the international border while the ECMWF is
consistent in 4 or 5 inches along coastal Maine. If the NAM
solution verifies a bit farther east...the concern for flash
flooding will be considerably higher given their hydrologic
sensitivity. Surrounding the Slight is a broader apron of
Marginal Risk given uncertainty in how quickly rainfall will be
moving away and how much will have already fallen by then.

...Southeast U.S....
Maintained the Marginal Risk area along and immediately south of
an unusually strong cold front for this time of year where
precipitate water values at or above 2 inches are expected to be
in place. Exactly where there will be interaction between any
mesoscale boundaries...such as sea breezes or outflow from nearby convection...makes it difficult to pinpoint the most likely area
to receive excessive rainfall.

...Western Plains...
Another day of late day and evening convection...with at least
some threat of excessive rainfall...is possible as return flow
draws moisture farther into the western Plains on the west side of
a quasi-stationary front. Thinking is that the convection will
initiate over the Western High Plains with the concern for
excessive rainfall increasing as the convection moves
east/southeastward towards deeper moisture as shown by
precipitable water values of 1.5 inches.

Bann
$$
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FOUS30 KWBC 070828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Mon Aug 07 2023

Day 1
Valid 12Z Mon Aug 07 2023 - 12Z Tue Aug 08 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
NORTHEAST INTO THE MID-ATLANTIC REGION AS WELL AS NORTHEASTERN
WASHINGTON AND NORTHERN IDAHO...

...Much of the Northeast U.S....
A potent mid/upper level shortwave trough will make its way
eastward from Illinois to portions of the central Appalachians
during the day while an anomalously strong jet for the season
makes its way from the Ohio Valley into parts of the
Mid-Atlantic...helping to keep a feed of deep moisture over the
eastern U.S.. Exactly how this interacts with a shortwave trough
initially over Michigan before heading north of the border...and
any associated height falls over the eastern Great Lakes and
Northeast U.S. continues to result in poor run to run
consistency...model QPF spread...and associated excessive rainfall
potential. With the event moving into the Day 1 timeframe...an
increasing amount of CAM guidance is available. The HREF
paired-matched mean showed one wave of QPF moving quickly into New
England early today before an additional round of active
convection capable of intense downpours develops in the afternoon
focused farther south which eventually translates/spread north and
east. 40 km neighborhood probabilities from the 07/00Z HREF show
the highest probabilities for 1-hour QPF to exceed one/two
inch(es) developing late afternoon/early evening over parts of the
Mid-Atlantic region in proximity to the potent mid/upper level shortwave...although the signal for 1 hour QPF to exceed flash
flood guidance/3 hour QPF to exceed flash flood guidance is
greater in portions of New York state where Flash Flood Guidance
is lowest. For this reason...opted not to make too many
adjustments to the northern portion of the Slight Risk area but
pulled the southern portion of the Slight Risk in response to the
potential for downpours with 2+ inch per hour rates. With so much
interaction at the synoptic scale still to be sorted
out...additional adjustments one way or the other are still
possible.

...Portions of the Northwest U.S....

Maintained the Slight Risk over portions of
Washington/Idaho/Montana and Oregon with a fairly subtle southward
expansion towards the Blue Mountains. Low pressure prior to the
start of Day 1 has drawn moisture into the region from the east.
This moisture combined with the slow moving surface low will help
fuel another round of afternoon and evening convection. Given the
anomalous amount of moisture...with precipitable water values some
150 to 200 percent of normal in northern Idaho...locally heavy
downpours that result in flooding are certainly possible. The
most vulnerable areas being over and near recent burn
scars...although instances of flooding can also occur elsewhere.
Thinking is that the NAM is too aggressive with its QPF...a bias
with this type of flow pattern...

...Western High Plains...

Scattered showers and thunderstorms will develop over parts of
Colorado and Wyoming before spreading eastward. Storms will be
moving into an airmass with deeper moisture which should allow for
an uptick in rainfall rates with time. Locally heavy rainfall
rates and isolated heavy rainfall amounts may lead to isolated
instances of flooding or at least run-off problems in regions of
poor drainage.

Bann



Day 2
Valid 12Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHERN NEW
ENGLAND AND NORTHERN NEW YORK AND THE CENTRAL PLAINS...

...Northeast...
Convection capable of producing heavy rainfall rates and excessive
rainfall amounts should be on-going mainly across northern New
York and northern New England as Monday's system continues on its
northward track. One area of surface low pressure is forecast to
be located just north of New York while a second area of low
pressure begins to organize south of Maine. Instability appears
to be on the weak side but precipitable water values across the
region should still be in excess of 1.5 inches...and on the order
of 1.75 inches closer to the coast. That should still be
sufficient to produce locally heavy rain rates early in the
period...with the HREF neighborhood probabilities showing the
potential for highest rainfall rates getting shunted eastward
during the afternoon. The UKMET and the CMC both attempt to
develop a decent band of deformation rainfall north of the border
which stays north of the border...while the GFS was tended to
rotate a second round of precipitation across northern New England
later in the day. Kept a Slight Risk area in the area...trimming
some area in deference to a model signal for a somewhat faster
departure while keeping the risk across the north until it becomes
more clear how energy north of the border will evolve.

...Southeast U.S....
Maintained the Marginal Risk area along and immediately south of
an unusually well-defined front for this time of year where
precipitate water values at or above 2 inches are expected to be
in place. Exactly where a morning MCV will be located over
Mississippi or Alabama...and how much interaction between any
mesoscale boundaries...such as sea breezes or outflow from nearby convection...makes it difficult to pinpoint the most likely area
to receive excessive rainfall.

...Western Plains...
Another day of late day and evening convection...with at least
some threat of excessive rainfall...is possible as return flow
draws moisture farther into the western Plains on the west side of
a quasi-stationary front. Thinking is that the convection will
initiate over the Western High Plains with the concern for
excessive rainfall increasing as the convection moves
east/southeastward towards deeper moisture as shown by
precipitable water values of 1.5 inches. There was a fair amount
of spread in the QPF placement over the region...tended to favor
the more southern solutions like the ECMWF/CMC/UKMET (ignoring its
8+ inch convective-feedback bulls eye over northeast Nebraska) due
to the better instability.

Bann


Day 3
Valid 12Z Wed Aug 09 2023 - 12Z Thu Aug 10 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY INTO THE SOUTHEAST U.S....

...Illinois into the Tennessee Valley and portions of the
Southeast...
Convection is expected to develop during the late
afternoon/evening and grow up-scale as shortwave energy initially
over Kansas/Nebraska heads eastward...leading to height falls
aloft overspreading the Mid-Mississippi Valley by 10/00Z. Gulf
moisture gets pulled northward during the day with both the GFS
and NAM showing precipitable water values exceeding 2 inches from
the southeast corner of Missouri into parts of Arkansas,
Mississippi and Alabama by 09/18Z while the flow aloft ahead of
the strengthening shortwave becomes increasingly difluent. Models
tend to develop convection during the first part of the Day 3
period...but the expectation is for the risk of highest rainfall
rates and accumulations will not occur until the later part of the
period due to the better thermodynamics and the arrival of 70+ kt
speed maximum aloft. Given the time of year, the precipitable
water values are above normal but not terribly so...only 2 to 2.5
standardized anomalies above climatology in Missouri/western
Tennessee by Wednesday evening. However...the moisture flux is
3.5 to 4+ standard anomalies above climatology. The NBM and
Ensemble Bias Corrected QPF focused 2 or 3 inch amounts along an
axis from far southern Illinois into northern Tennessee...and the
07/00Z NAM and 06/12Z UKMET QPF axis was farther north while the
GFS was farther south (supporting the GEFS idea of 3 inch contours
into northern Georgia. Despite the spread...felt confident enough
to begin realigning the axis a bit but left it fairly broad to
deal with the spreads of QPF solutions and related excessive
rainfall threat.

Bann
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 072049
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
448 PM EDT Mon Aug 07 2023

Day 1
Valid 01Z Tue Aug 08 2023 - 12Z Tue Aug 08 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
NORTHEAST, THE MID-ATLANTIC REGION, EASTERN WASHINGTON, & NORTHERN
IDAHO...

...Northeast...
Based on the latest CAMs guidance, the consensus has been for the
DC/MD/VA/DE region to likely see one very fast moving line of
storms race across the area this afternoon, with very little to no
rainfall activity occurring behind it. This presents a scenario
where flash flooding is highly unlikely to occur, other than some
nuisance flooding in poor drainage areas. The NAMnest is a notable
outlier on this scenario, showing a much slower cell tracking
across central MD and into the Baltimore region behind the line
this evening, but this solution was discounted for lack of support
among the other guidance. Thus, in coordination with LWX/Sterling,
VA forecast office, have downgraded to a Marginal with this
forecast update.

The rest of the Northeast remains in steady state. Even here the
primary threat by far will be the severe potential. While the flow
becomes more parallel to the front the further north you go, and
thus increasing the flash flooding threat broadly, the fast moving
storms and the requirement for multiple rounds should keep the
flooding threat relatively low in most areas. Particularly
sensitive areas of northern NY and through the Philadelphia and
NYC metros are at higher risk of flash flooding due to less
rainfall needed.

There was also a notable signature in much of the guidance across
central and western WV with the storms now entering the area for
increased rainfall totals as compared to the surrounding area.
Training storms are beginning to develop with the event just
getting underway. Either way the storms will be fast-moving, so it
will take multiple rounds of storms to result in flooding. Thus,
the area was kept in a Marginal.


...Pacific Northwest...
The inherited Slight Risk area was expanded in coordination with
PDT/Pendleton, OR forecast office. Stationary/very slow moving
storms are ongoing just east of the Cascades, and a second line
just east of the Tri-Cities, extending south into portions of
northeast OR. The forcing will remain highest across WA today, so
it's this region which is already seeing nearly stationary storms
that are actively congealing into an MCS that is at the greatest
risk of flash flooding in the now-term. Later this afternoon
expect more widespread convection to break out across ID, with low
FFGs contributing to flooding potential, especially in burn scars,
canyons, any urbanized areas such as Spokane and Coeur d'Alene,
and other poor drainage areas.


...Western High Plains...
Scattered showers and thunderstorms will develop over parts of
Colorado and Wyoming before spreading eastward. Storms will be
moving into an airmass with deeper moisture which should allow for
an uptick in rainfall rates with time. Locally heavy rainfall
rates and isolated heavy rainfall amounts may lead to isolated
instances of flooding or at least run-off problems in regions of
poor drainage.


...Southwest...
The combination of moisture rotating around the east side of now
Post-Tropical Cyclone Eugene, a weak disturbance crossing
northwest Mexico, and a shortwave across northwest AZ has led to a
bit of convective expansion across portions of southeast AZ and
central NM. Regionally, there is a broad area of 1000-4000 J/kg
of ML CAPE and effective bulk shear in northern AZ and portions of
NM is 25+ kts, enough for convective organization. The area with
the most impressive moisture in southeast AZ, where convection has
shown the coldest cloud tops. Hourly rain totals to 2" would not
be a surprise where cells manage to backbuild, train, or merge. A
mesoscale precipitation discussion is being considered for
portions of the area, so a Marginal Risk area has been introduced
in this special update.


Wegman/Bann



Day 2
Valid 12Z Tue Aug 08 2023 - 12Z Wed Aug 09 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN NEW ENGLAND...

...19Z Update...

...Northeast...

Guidance continues to come into agreement on a potentially
significant heavy rainfall event across portions of New England on
Tuesday. A slow moving but deepening low north of New York will
track northeast to the west of Maine through the period. A 50 kt
low level jet will advect unstable and unusually moist air from
the Atlantic north into New England. This will support training
storms as they move northward along with the flow. The storms will
impact the terrain, particularly the White Mountains of New
Hampshire into western Maine. Upslope support in these regions are
likely to wring out additional rainfall to the south and east of
the range, further enhancing totals. This area has been hard-hit
in recent days and weeks, so streams and rivers are running high,
with little space for additional rainfall on the order of 2 to 4
inches with locally higher amounts. The Moderate Risk area is
focused in the region that has the most favorable antecedent
conditions, with instability and persistent strong LLJ flow likely
to result in training storms. Areas with the greatest persistence
of training storms have the potential to locally exceed 6 inches.
In coordination with BTV/Burlington, VT; GYX/Gray, ME; and
CAR/Caribou, ME forecast offices, the Moderate Risk was introduced
with this update.

...Central Plains...

The Slight Risk area was expanded westward to include more of the
Plains of northeast CO with this update. There has been increasing
signal for more concentrated storms in this area, which will
expand southeastward across far southern NE/KS through the period.

...Deep South..

No significant changes were made. Any flash flooding in this area
will be very localized due to high FFGs and limited coverage of
storms. The greatest threats are in urbanized and other poor
drainage areas.

Wegman

...Previous Discussion...

...Northeast...
Convection capable of producing heavy rainfall rates and excessive
rainfall amounts should be on-going mainly across northern New
York and northern New England as Monday's system continues on its
northward track. One area of surface low pressure is forecast to
be located just north of New York while a second area of low
pressure begins to organize south of Maine. Instability appears
to be on the weak side but precipitable water values across the
region should still be in excess of 1.5 inches...and on the order
of 1.75 inches closer to the coast. That should still be
sufficient to produce locally heavy rain rates early in the
period...with the HREF neighborhood probabilities showing the
potential for highest rainfall rates getting shunted eastward
during the afternoon. The UKMET and the CMC both attempt to
develop a decent band of deformation rainfall north of the border
which stays north of the border...while the GFS was tended to
rotate a second round of precipitation across northern New England
later in the day. Kept a Slight Risk area in the area...trimming
some area in deference to a model signal for a somewhat faster
departure while keeping the risk across the north until it becomes
more clear how energy north of the border will evolve.

...Southeast U.S....
Maintained the Marginal Risk area along and immediately south of
an unusually well-defined front for this time of year where
precipitate water values at or above 2 inches are expected to be
in place. Exactly where a morning MCV will be located over
Mississippi or Alabama...and how much interaction between any
mesoscale boundaries...such as sea breezes or outflow from nearby convection...makes it difficult to pinpoint the most likely area
to receive excessive rainfall.

...Western Plains...
Another day of late day and evening convection...with at least
some threat of excessive rainfall...is possible as return flow
draws moisture farther into the western Plains on the west side of
a quasi-stationary front. Thinking is that the convection will
initiate over the Western High Plains with the concern for
excessive rainfall increasing as the convection moves
east/southeastward towards deeper moisture as shown by
precipitable water values of 1.5 inches. There was a fair amount
of spread in the QPF placement over the region...tended to favor
the more southern solutions like the ECMWF/CMC/UKMET (ignoring its
8+ inch convective-feedback bulls eye over northeast Nebraska) due
to the better instability.

Bann


Day 3
Valid 12Z Wed Aug 09 2023 - 12Z Thu Aug 10 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY INTO THE SOUTHEAST U.S....

...19Z Update...

...Mid-Mississippi, Lower Ohio, and Lower Tennessee Valleys...

No major changes were made, but the Slight Risk was expanded in a
number of directions to account for the development of widespread
convection over an area that has been very hard hit in recent days
with convection. Due to the uncertainty with some of the outlier
models depicting heavy rain in areas that have been hard hit, such
as far southern IA/western IL, the Slight Risk area was expanded a
bit to account for that uncertainty. The area of highest risk
within the Slight is around the Tri-Rivers area, which has both
the most favorable antecedent conditions and the greatest
likelihood in the guidance for persistent heavy rains. A Moderate
Risk was considered in coordination with PAH/Paducah, KY office,
but was decided to hold off until there was better certainty in
the guidance on where specifically will be hardest hit. With that
certainty, expect the Slight Risk area will be shrunk, but a
future Moderate Risk remains probable.

...Southeast AZ...

Monsoonal moisture will increase on Day 3/Wed above the potential
from previous days. A Marginal Risk was introduced to account for
the possibility heavy rains move over sensitive areas of AZ. There
is potential the Marginal may need to be expanded to include
portions of the Mogollon Rim in AZ as well with future updates.

Wegman

...Previous Discussion...

...Illinois into the Tennessee Valley and portions of the
Southeast...
Convection is expected to develop during the late
afternoon/evening and grow up-scale as shortwave energy initially
over Kansas/Nebraska heads eastward...leading to height falls
aloft overspreading the Mid-Mississippi Valley by 10/00Z. Gulf
moisture gets pulled northward during the day with both the GFS
and NAM showing precipitable water values exceeding 2 inches from
the southeast corner of Missouri into parts of Arkansas,
Mississippi and Alabama by 09/18Z while the flow aloft ahead of
the strengthening shortwave becomes increasingly difluent. Models
tend to develop convection during the first part of the Day 3
period...but the expectation is for the risk of highest rainfall
rates and accumulations will not occur until the later part of the
period due to the better thermodynamics and the arrival of 70+ kt
speed maximum aloft. Given the time of year, the precipitable
water values are above normal but not terribly so...only 2 to 2.5
standardized anomalies above climatology in Missouri/western
Tennessee by Wednesday evening. However...the moisture flux is
3.5 to 4+ standard anomalies above climatology. The NBM and
Ensemble Bias Corrected QPF focused 2 or 3 inch amounts along an
axis from far southern Illinois into northern Tennessee...and the
07/00Z NAM and 06/12Z UKMET QPF axis was farther north while the
GFS was farther south (supporting the GEFS idea of 3 inch contours
into northern Georgia. Despite the spread...felt confident enough
to begin realigning the axis a bit but left it fairly broad to
deal with the spreads of QPF solutions and related excessive
rainfall threat.

Bann
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 090844
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
443 AM EDT Wed Aug 09 2023

Day 1
Valid 12Z Wed Aug 09 2023 - 12Z Thu Aug 10 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE MID-MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS....

...Southern Missouri/northern Arkansas into western Tennessee,
northern Mississippi/Alabama...
A mid-upper level shortwave will track ESE from the central High
Plains this morning to KY/TN for Thursday morning, supporting the
organization of a surface low near the KS/OK border with attendant
cold/warm fronts. A slow moving warm front currently in place from
near the Red River to LA/MS was associated with precipitable water
values just shy of 2 inches per 00Z RAOBs and recent (07Z) GPS
data. There is good agreement in recent HRRR runs and the 00Z CAMs
that overrunning of the front will lead to elevated thunderstorms
in the vicinity of southern MO/northern AR at the start of the
outlook period, out ahead of ongoing (07Z) thunderstorms across
the southern/central Plains. While individual cells with this
initial round of thunderstorms should move steadily toward the
east and southeast, repeating rounds and training will be possible
with the potential for localized 2-4 inch totals prior to 21Z.

As the anticipated first round of thunderstorms moves east into
the OH/TN Valleys, a second round of storms is expected to develop
near or just prior to 00Z over the Ozarks as the aforementioned
shortwave approaches along with a ~100 kt upper level jet streak.
Increased ascent within the entrance region of the upper level jet
streak should aid in the development of an expanding area of
thunderstorms between 00-03Z, just ahead of an approaching surface
low, with the potential for multiple rounds of storms repeating
over the same region overnight. There is some question as to the
degree of overlap between the two expected rounds of heavy rain
but there remains the potential for upgrading to a Moderate Risk
with the 16Z update if there is better confidence in the two
rounds overlapping with each other or with portions of the region
that are more sensitive given recent heavy rain over the past week
or so. However, current thinking is that the greatest potential
for 3 to 5 inches through 12Z Thursday will set up along the MO/AR
border, extending into western TN...or mainly south of more
sensitive regions located in central/eastern MO and western KY.

...eastern Kansas, central/northern Missouri, southern Iowa into
the Ohio Valley...
Thunderstorms are likely to advance eastward from the central
Plains this morning into the Midwest and OH Valley, out ahead of
an upper level shortwave tracking eastward from the Plains.
Increasing upper level jet divergence within the entrance region
of a 110-130 kt speed max along with diffluent flow aloft should
help to increase convective coverage through the late morning and
afternoon. Elevated low level convergence focused at the nose of a
925-850 mb plume of moisture transport should focus convection
from portions of northern MO, eastward into the OH Valley.
Repeating and short term training of convection may lead to
scattered areas of flash flooding with potential for 3 to 5 inches
locally. Portions of MO into IL and IN have been wetter than
average over the past 1-2 weeks, lowering FFG and increasing
susceptibility to flash flooding.

...Arizona/New Mexico...
Weakly anomalous moisture is expected to be in place across AZ
(standardized precipitable water values of 1 to 1.5) this
afternoon, while higher anomalies (+2 to +3) reside over the
southern CA coast. While cloud cover during daytime heating
remains a bit uncertain, it seems clouds will be more likely over
CA during the early afternoon, associated with remnant moisture
from former Hurricane Eugene in the eastern Pacific. Thunderstorms
are expected to initiate along the Mogollon Rim with daytime
heating (likely 18Z+) followed by additional convection over the
higher terrain of southeastern AZ. Forecasts from a majority of
the 00Z guidance suggest MLCAPE of 500-1500 J/kg during the
afternoon with forecast soundings showing fairly moist low to mid
level profiles. While storm motions don't appear overly slow
overall, there is at least a low-end threat for isolated flash
flooding with the convective peak later in the day.

Otto


Day 2
Valid 12Z Thu Aug 10 2023 - 12Z Fri Aug 11 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE SOUTHERN APPALACHIANS...

...Southern Appalachians/Gulf Coast States...
Convection should be ongoing at the start of the period over the
southern Appalachians, located ahead of an eastward advancing
mid-level shortwave. WSW low level flow of 30-40 kt is forecast by
the 00Z model consensus to be pointed into the southern
Appalachians along with precipitable water values of approximately
1.7 to 2.0 inches just upstream across portions of TN, AL and GA.
Due to the high moisture forecast, sizable CAPE of at least
1000-2000 J/kg is expected to be in place, supporting rainfall
rates of 1-2 inches per hour or less than an hour. A broader
footprint for 1-2 inches appears probable along with localized
potential for small 3 to 4 inch totals embedded within. The
shortwave is expected to reach the Mid-Atlantic coast by 00Z
Friday, likely dissipating the flash flood threat for much of the
southern Appalachians but seasonably high precipitable water
values will remain near a remnant frontal boundary extending from
the Carolinas to the Lower Mississippi Valley. WNW mid-level flow
will be in place (quasi-parallel to the front) with the potential
for convective development within the unstable and moist airmass.
Recent QPF guidance does not paint a solid picture for excessive
rainfall across the remaining Gulf Coast states, but localized QPF
maxima are present in the 00Z model guidance and in the higher
resolution GEM_regional, FV3 and NAM_nest solutions. Latest
thinking is for spotty 2-4 inch rainfall maxima to occur within
the broader Marginal Risk area.

...Mid-Atlantic into the Northeast...
A pair of shortwaves will advance eastward on Thursday, one over
the southern/central Appalachians and the other across southern
Ontario/Quebec. Moisture return ahead of these features and with a
warm front lifting northward is expected to support precipitable
water values climbing into the 1.5 to 2.0 inch range for the NYC
metro and southern/eastern New England. While the degree of
instability is questionable, there appears to be a 2-4 hour window
for training cells over any given area of the northern
Mid-Atlantic into New England from roughly 18-06Z, perhaps
lingering a bit longer for coastal ME. Recent heavy rain may play
a role in flash flooding given increased sensitivity/soil
saturation.

...Upper Midwest...
The Marginal Risk area was expanded slightly to cover the latest
spread across the eastern Dakotas into MM/IA. Increasing low level
ascent across a slow moving warm front in southern MN is expected
to contribute to the development of thunderstorms ahead of an
eastward advancing shortwave from the north-central U.S.
Precipitable water values are forecast to reach into the 1.2 to
1.5 inch range in the pre-convective environment and fairly
strong jet divergence/diffluence will accompany the increasing low
level jet for the evening/early overnight time frame. Instability
appears to be one possible limiting factor with limited CAPE
forecast for northern portions of the Marginal Risk area but this
is where the better forcing for ascent will be located. Better
instability across southern locations into Iowa may support higher
rainfall rates but forcing will be more limited with southward
extent. The end result may be a fairly localized flash flood
threat for the region, focused over urban areas and other
locations with poor drainage.

...Four Corners Region...
The focus for convection Thursday afternoon will shift a bit
further to the north as weakly anomalous moisture spreads into NV,
UT and CO. There will also be the added factor of increased lift
within the right entrance region of a zonally oriented jet streak
aloft to track through UT/CO during peak heating. Once again, the
signal for heavy rain is limited, but models tend to struggle with
QPF in this region of the U.S. given the small scale nature to
convection and local forcing near the terrain.

Otto


Day 3
Valid 12Z Fri Aug 11 2023 - 12Z Sat Aug 12 2023

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR ARIZONA/NEW
MEXICO...THE UPPER MIDWEST AND FROM THE OZARKS TO THE SOUTHEASTERN
U.S. COAST...

...Ozarks, Lower Mississippi Valley into the Southeast...
WNW flow in the mid-levels will be present from the
central/southern Plains to the Southeast coast, located north of
an elongated ridge centered over the west-central Gulf Coast. A
remnant frontal boundary is expected to reside from the Southern
Plains into the Carolinas with high precipitable water values
nearby, locally in excess of 2 inches per the 00Z model consensus.
Given portions of this region have been wet or will be seeing
rainfall leading up to the start of the period on Friday, at least
a localized flash flood threat will exist across this region,
aided by any small scale vorticity maxima approaching from
upstream, on the north side of the mid-level ridge.

...Upper Midwest...
A longwave trough over the north-central U.S. will edge eastward
from Friday into Saturday with an embedded shortwave tracking from
the Dakotas into the Great Lakes region. Weakly anomalous moisture
(+1 to +2 standardized precipitable water anomalies) is expected
to advect northward into the Upper Midwest ahead of an advancing
cold front over the central U.S., and north of what should be a
fairly well-defined warm front extending ESE from the Upper
Mississippi Valley into the Upper Ohio Valley. Daytime heating,
increasing upper level ascent ahead of the shortwave trough and
associated jets combined with increasing 850 mb winds in the 20-35
kt range should yield thunderstorms with areas of heavy rain. West
to east repeating of storms will be possible, although similar to
the previous day across the Upper Mississippi Valley, instability
remains uncertain...especially with northward extent to support
excessive rainfall totals. For now, a somewhat broad Marginal Risk
was drawn to account for localized flash flood potential from
localized 2-3 inch totals, perhaps locally above 3 inches through
Saturday morning.

...Arizona/New Mexico...
The 500 mb pattern from the 00Z models shows good agreement and
continuity from previous runs for a closed low along the central
CA coast and oval-shaped ridge centered over the west-central Gulf
Coast. Southerly flow with embedded shortwave activity is forecast
to move northward from Mexico into eastern AZ/western NM and upper
level jet right entrance region lift may be in play for the region
with jet energy directed from central AZ into CO. Precipitable
water anomalies of +1 to +2 are also forecast to be present along
the AZ/NM border which should help to support rainfall rates of
1-2 in/hr if sufficient instability is able to develop with
daytime heating. A Marginal Risk was introduced to account for
what appears to be the greatest probability of locally excessive
rainfall within the highest QPF maxima depicted by the 00Z
guidance.

Otto
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 100059
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Wed Aug 09 2023

Day 1
Valid 01Z Thu Aug 10 2023 - 12Z Thu Aug 10 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE MID-MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS....

...Mid-Mississippi, Tennessee, and Ohio River Valley Regions...
For the 01z ERO update, the broad Slight Risk was split into two
separate regions (the MS/TN Valley and the OH Valley), with a
break across central portions of KY. This expected bi-modal
distribution of precipitation has become increasingly clear
throughout the afternoon, and there is enough confidence now to
split the risk areas (as separate convective clusters are already
emerging).

Focusing on the MS/TN Valley first, the stage is being set for
impressive upscale growth of convection into a progressive MCS
overnight (tracking from northern AR and southern MO into the
Mid-South). Storms have already initiated across far northeast OK
into southwest MO in association with a potent shortwave just
upstream over eastern KS, and some west-to-east training is
already occurring with fairly slow east-southeastward propagation
of convection initially (with MRMS indicating 1-2"/hr rainfall
rates). Downstream of the current convection, a pool of anomalous
tropospheric moisture (with PWs of 2.0-2.2", over the 90th
percentile) and rebuilding instability (per ML CAPE of 1000-3000
J/kg) via a strengthening low-level jet (20-40 kts of SW flow at
850 mb) is occurring. This environment will continue to support
rainfall rates of 1-2"/hr (and locally higher where convection can
sufficiently train) as the MCS grows upscale. Given the
expectation for a forward propagating MCS, a favored location for
training (and thus flash flooding) would typically be on the
trailing southward flank of the developing cold pool, though this
potential looks fairly low as the low-level inflow is largely
orthogonal to the expected outflow boundary (oriented from NW to
SE). Even still, the 18z HREF indicates a fairly broad area with
potential for localized 3" exceedance through 12z (per 40-km
neighborhood probabilities of 10-30%), focused mainly from
southern MO and northern AR through the Mid-South (northeast AR,
West TN, northern MS, and into northeastern AL). It is also worth
noting that farther east across this Slight risk area (into
northern AL, northwestern GA, and Middle/East TN), a prior MCS
(having traversed the Mid-South during the day) is resulting in
areas of convection that are ongoing this evening (with storm
activity most vigorous over central AL). This activity should
gradually come to an end as it merges and propagates southeastward
over the next several hours, and the aforementioned developing MCS
will then track into some of these same areas toward dawn (having
matured and entering a weakening stage with the veering low-level
jet).

The broad low-level jet across the central CONUS is also driving
anomalous tropospheric moisture farther northeastward into the OH
Valley, where scattered instances of flash flooding also appear to
be likely overnight. The potential for strong to severe storms are
less likely in this region, but tall, skinny CAPE profiles (with
ML CAPE of 500-1000 J/kg) and PW values near or just above the
90th percentile for this time of year will continue to favor some
efficient rainfall from south-central IN/IL into southwestern OH.
A broad area of stratiform rainfall with embedded convection is
ongoing across much of IL/IN/OH, and convection will tend to focus
along the southern periphery of this area. MRMS (and weather
station observations) indicate hourly rain rates of 1-2 inches in
areas of convection, and these rates will continue to be possible
overnight (with localized west-to-east training). A decrease in
available instability will gradually lessen the risk further to
the east toward the central Appalachians overnight, although it
won't completely eliminate it (with the potential for localized
flash flood concerns continuing overnight).

...Arizona/New Mexico...
Weakly anomalous moisture is in place across west-central AZ into
southwestern NM (PWATs of 1.0-1.3"), driving scattered convective
activity with sub-hourly (15-min) rainfall totals of up to 1".
This activity is expected to continue for at least a few more
hours, as instability remains (ML CAPE of 500-1000 J/kg). A
Marginal risk remains in place for this convective activity, as
any flash flooding should remain localized and confined to
particularly sensitive areas (like dry washes and burn scars).

Churchill



Day 2
Valid 12Z Thu Aug 10 2023 - 12Z Fri Aug 11 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST...

...Mid-Atlantic and Northeast...
Model guidance has indicated an increasing heavy rainfall threat
on Thursday from E PA and N NJ into S NY, CT, RI, and MA. This
appears to be associated with a prominent and now better-resolved
mesoscale convective vortex (MCV) in the Midwest. As of 18 UTC on
Wednesday, this MCV was pushing through eastern Missouri, and
models continue the general trajectory through the Upper Ohio
Valley tonight and into the Northeast tomorrow. There is still
quite a bit of variability in the modeled convective patterns and
QPF, and that seems to largely be due to differences in how strong
the MCV is, and how much it will interact with a potentially
coupled jet structure, which could lead to deepening of a low-mid
level cyclone near southern New England.

In general, the model preference was for an intermediate approach
-- not favoring very intense surface lows (for the middle of
summer!) below 990mb as depicted on the 3km NAM, WRF-ARW, and
WRF-ARW2 (with heavier QPF displaced further north and inland),
but also not favoring scenarios showing no MCV continuity or
surface low development (with a much weaker QPF signal overall).
Therefore, the 12Z HRRR and 12Z ECMWF seemed to represent a more
plausible scenario, with enhanced rainfall potential in the
aforementioned area from E PA and N NJ into SNE. An
ingredients-based perspective using those models supports
potential for hourly rain rates in the 1-2 inch range, and
potentially sustained for a couple hours if training can occur.
Therefore, those areas were upgraded to a Slight Risk in
coordination with local WFOs.

Some things to monitor for potential future changes... (1) Whether
the Slight Risk may need to be expanded further north and/or
northeast into portions of New Hampshire and Maine, if the
interaction of the MCV and jet structure leads to a deeper low and
focused heavy rainfall further north; and (2) If the Slight Risk
may need to be extended further back to the west into the central
Appalachians, if focused convection appears more likely earlier in
the day.

...Interior Southeast U.S...
The convective pattern and heavy rainfall threats over the
Southeast on Thursday and Thursday evening will be strongly
influenced by how convection evolves tonight, and any resulting
outflow boundaries. Therefore, there is still some uncertainty. A
notable trend in the ensemble model guidance today was to shift
the stronger QPF signal further south, from near the TN-NC border
region more into N MS, N AL, and N GA. This was also reflected in
our machine learning guidance, based on the 12Z GEFS. Strong
instability and PWs around or in excess of 2 inches would support
potentially intense rainfall rates, and veered low-level inflow
from a westerly direction could favor backbuilding given a likely
upstream CAPE max somewhere near N MS. Given the uncertainty, it's
possible that the risk areas may continue to shift, but in
general, the expectation is for a corridor of enhanced heavy
rainfall, and potentially flash flooding, somewhere in the region.

...Southern Nevada, Southern Utah, Northern Arizona...
Scattered diurnal convection is expected in the Southwest. The 12Z
HREF maintains small probabilities of 1hr rainfall exceeding flash
flood guidance with the most intense rain, and this seems
reasonable. The introduction of higher probabilities is precluded
by a lack of very strong instability or anomalous moisture, so the
broad Marginal Risk was maintained.

...Upper Midwest...
The Marginal Risk was removed for the Upper Midwest region.
Although convection is expected, hi-res models uniformly show it
to be rather progressive and unlikely to last very long at any one
location. Flash flood guidance is also higher across this region
than the remainder of the North-Central U.S., and much of the
region is also in drought. Instantaneous rain rates may be high,
but the probability of exceeding flash flood guidance, while
non-zero, should generally be less than 5 percent in this region
given a lack of persistence of the high rain rates.

Lamers


Day 3
Valid 12Z Fri Aug 11 2023 - 12Z Sat Aug 12 2023

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN THE
SOUTHEAST, GREAT LAKES, DESERT SOUTHWEST, AND CENTRAL SIERRA
NEVADA...

...Greater Southeast Region...
A broad Marginal Risk was maintained over the Southeast region,
generally in the vicinity of where a slow-moving or stalled front
is expected to be located. The front may be positioned relatively
far south and closer to the Gulf Coast in the vicinity of AL and
GA, where consecutive days of convective outflows may reinforce
cooler air in the interior Southeast. However, given that the
frontal position will be crucial to the location of more intense
convection, there is considerable uncertainty more than 48 hours
in the future. The Marginal Risk placement relied on ensemble
probabilities of higher QPF. South of the front, strong
instability is expected, with a continued presence of PWs near or
above 2 inches. This will continue to support localized higher
rain rates potentially reaching 2 inches per hour. As the
mesoscale environment becomes more certain over the next day or
so, it wouldn't be surprising to see a more focused area of higher probabilities, and thus an upgrade to a Slight Risk, somewhere in
this larger region.

...Upper Midwest...
The size of the Marginal Risk was reduced in this region, and
focused more around urbanized areas in the central Great Lakes
region, where flash flood guidance also happens to be a little
lower and the ongoing drought is not so severe. However, as in the
Day 2 period, convection developing along and ahead of an
advancing cold front is expected to be fairly progressive and
therefore any heavier rain rates seem unlikely to be sustained at
any one location for very long. It's possible that this Marginal
Risk could be further shifted, reduced in size, or eliminated if
future model runs continue the trends of showing relatively narrow
convective bands advancing quickly to the east. For the time
being, there appears to be enough of a threat of 1-2 inch per hour
rain rates coinciding with some larger urban areas or regions of
lower FFG to continue the Marginal Risk.

...Eastern Arizona and Western New Mexico...
A northward surge of moisture, increasing precipitable water
values to slightly above average, and the entrance region of an anticyclonically curved, strengthening, upper level jet streak may
combine to lead to an increase in convective coverage near the
AZ-NM border region on Friday. One limiting factor may be
available instability, as most models are indicating less than
1000 j/kg of MUCAPE, which seems to be due to a relatively dry and
deep sub-cloud layer, with marginal surface dewpoints and LCLs
over 700mb. Instability tends to be a factor more highly
correlated to flash flood activity in the Southwest region, so
overall the threat looks to be fairly isolated at this time.
Therefore, the Marginal Risk was maintained.

...Central Sierra Nevada...
A Marginal Risk area was added for the Central Sierra Nevada
region, where there is expected to be a surge of deeper moisture
and a lingering mid-level vorticity center from the now-decayed
tropical cyclone (Eugene) in the East Pacific. The GEFS and ECMWF
ensembles were in fairly good agreement with placing an area of
higher QPF in this region, and the PWs are forecast to be above
the 95th percentile for mid-August in both ensemble systems.
Although instability is not projected to be very strong, models do
show potential for pockets of over 500 j/kg of MUCAPE, which is
not insignificant at higher elevation. The main question will be
whether denser cloud cover associated with the deeper moisture
limits heating and instability more than models are currently
anticipating.

Lamers
$$
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 110728
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 AM EDT Fri Aug 11 2023

Day 1
Valid 12Z Fri Aug 11 2023 - 12Z Sat Aug 12 2023

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN THE
SOUTHEAST, OZARKS, CENTRAL GREAT LAKES, DESERT SOUTHWEST, AND
CENTRAL SIERRA NEVADA...

Zonal flow established across the country has set up a relatively
quiet pattern for a couple days as the areas of deepest moisture
across the southern US are temporarily separated from the greatest
forcing across the northern US. This is not to say there won't be
showers and thunderstorms in various areas, but the lack of
connection between the moisture and the forcing should prevent
them from organizing enough to cause higher-level flash flooding
concerns.

...Michigan, Northern Indiana, Northwest Ohio...

A 15-25 kt LLJ will transport a bit of moisture from the Midwest
that will be lacking a connection with the Gulf into portions of
the Midwest. Convection is likely to break out along this front,
especially during peak heating, and continuing into the evening.
CAMs guidance is in decent agreement that multiple rounds of
showers with some embedded storms will form along the southern
boundary between steadier rain and forcing to the north, and
isolated convection with little forcing to the south. Thus, this
region is likely to be in the middle, and thus subject to multiple
rounds of storms. The southwesterly flow will also transport air
with MUCAPE around 1,500 J/kg, and PWATs to around 1.5 inches.
This is far from impressive for this time of year, but will be
enough moisture availability for the storms. The inherited
Marginal Risk was expanded to the northwest to align with the
latest guidance trends, increasing rainfall amounts into the U.P.
as well.

...Ozarks and Mid-Mississippi Valley into the Southeast...

Unlike further north, this area will have plenty of moisture and
instability, with PWATs exceeding 2 inches in some areas, and
instability exceeding 4,000 J/kg of MUCAPE, which is extremely
unstable. Thus, what will be missing in this area is a strong
enough source of forcing to allow the storms that will form over
this area to organize. Scattered storms are already over portions
of northern AL this morning. They will track southeastward,
following the greatest moisture and instability. However, other
than localized outflow boundaries, the forcing will be limited.
Thus, expect the storms to form and reform as the broader area of
scattered storms moves southeastward, but the storms will be
largely independent of each other. While the plentiful moisture
and instability will support individual storms becoming strong
enough to produce heavy rainfall, the storms will be moving enough
and will be widely scattered enough that the flash flooding threat
will be largely confined to urban and poor drainage areas. High
FFGs common across the Deep South will also limit the flooding
threat.

...Eastern Arizona and Western New Mexico...

A very diffuse monsoonal signature is the best the guidance is
showing as far as convective coverage goes this morning for the
return of storms again this afternoon. With limited forcing and
capping aloft, the convection may not be quite as strong today as
they were yesterday evening. Nonetheless given the sensitivity of
the area to flash flooding, particularly over burn scars, slot
canyons, dry washes, and any urban areas, the Marginal Risk was
maintained with this update.

...Central Sierras into Nevada...

The guidance continues to indicate the potential for isolated
flash flooding from the Central Sierra Nevada eastward into
central NV. No significant changes were noted with this update, as
any storms capable of flash flooding will only be able to form if
there is enough break in the cloud cover to allow instability to
increase.

Wegman



Day 2
Valid 12Z Sat Aug 12 2023 - 12Z Sun Aug 13 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD
AREA FROM THE NORTHEAST, TO THE OHIO VALLEY, THE CENTRAL PLAINS,
AND INTO THE FOUR CORNERS...

Few changes were needed to the Day 2/Saturday ERO. An expansive
front will be draped across much of the country on Saturday. While
the front itself will provide some forcing, the lack of upper
level support for most of the country will once again limit the
potential for flooding. One exception will be across the
Northeast, where an approaching shortwave trough may help to
organize some convection. Moisture will be the primary ingredient
lacking across the area, as PWATs struggle to approach 1.5 inches.
PWAT values around 1.25 inches is enough for showers and storms to
form, but both the lack of advection and that not being a huge
amount of available atmospheric moisture will cut down on the
coverage of storms. Further, there will be quite a bit of cloud
cover over the area, which will limit how much instability can
develop in the afternoon ahead of the shortwave trough. Thus,
despite much of the area being quite a bit wetter than normal with
NASA Sport soil moisture imagery showing almost all of northern NY
and New England having soils at or wetter than normal, think the
flooding threat will be limited due to the fast movement of storms
and lack of coverage.


Widely scattered convection is likely to track along the front
across the middle of the country as well, but with all the forcing
confined to the north, expect the storms to struggle to organize.
Thus, the signal for heavy rainfall in any one area is weak.
Isolated flash flooding will be possible in flood-sensitive areas
should one or more rounds of storms move over the area, but most
areas will see sub-flooding level amounts of rain Saturday.

Scattered showers and storms are also likely to redevelop Saturday
afternoon and evening across the Four Corners, with the tail end
of the front possibly helping focus and organize the convection.
Nonetheless expect limited flash flooding as storms try to focus
along the Mogollon Rim.

..South Dakota and Vicinity...

Much of the guidance shows showers and storms forming along a 20
kt LLJ tracking up the Missouri Valley and into eastern SD
Saturday night. The increased moisture along with upper level
support from a strong shortwave trough diving southeastward out of
the Canadian Prairies should allow for locally heavy rain to
develop over this area. Soils are wetter than normal after recent
rains from a few days ago in this area, and while the area is
notoriously hard to flood, think the antecedent conditions may
allow for isolated instances of flash flooding due to the more
organized nature of the storms over a rather confined area of
eastern SD. A new Marginal Risk was introduced with this forecast
update.

Wegman


Day 3
Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN
KANSAS THROUGH THE LOWER OHIO VALLEY...

The "break" from more widespread flooding concerns from today and
Saturday will end on Day 3/Sunday. The stalled out front draped
across the middle of the country will continue to gather
atmospheric moisture along it on a southwesterly LLJ was it
strengthens to as strong as 45 kt across MO Sunday night. A
strengthening shortwave that starts the day Sunday near the MT/ND
border strengthens as it tracks southeastward toward the Slight
Risk area. KS and MO may start the day with ongoing showers and
storms Sunday morning as moisture increases, but the real threat
for flooding will come Sunday night with the arrival of the
shortwave and PWATs increase to a tropical 2.25 inches in some
areas. The combination of organized storms ahead of the trough and
the abundant moisture will increase the flooding threat. Further,
antecedent conditions from the past several weeks of heavy
rainfall events have left much of the area still at or above
normal for soil conditions.

The inherited Slight risk area was trimmed from the north to favor
areas from Kansas eastward to southwest OH. While areas further
north from eastern ND east across MN and into the U.P. of MI will
also likely get 1-3 inches of rain as a low wraps up over that
area, antecedent soil conditions are much drier in this area,
meaning the soils should be more likely to absorb much of the
needed rainfall, limiting the flooding impacts. It's likely the
axis of heaviest rain may shift north or south in the coming days,
so the Slight was determined the most likely flooding risk at this
point, but upgrades may be needed with better confidence in the
CAMs as the event draws closer.

Wegman
$$
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* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)

FOUS30 KWBC 120838
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
437 AM EDT Sat Aug 12 2023

Day 1
Valid 12Z Sat Aug 12 2023 - 12Z Sun Aug 13 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE CATSKILLS
OF NEW YORK THROUGH CONNECTICUT AS WELL AS FROM SOUTH-CENTRAL
COLORADO EAST THROUGH THE OKLAHOMA PANHANDLE AND SOUTHWEST
KANSAS...

...South-Central Colorado east through the Oklahoma Panhandle and
Southwest Kansas...

An MCS is expected to develop across the Slight Risk area this
afternoon through this evening. This is along a stalled out front
draped west to east from Colorado all the way through the
Northeast. Convection is likely to develop all along the front at
various times throughout the day. Additional rainfall will develop
with the MCS across the Slight Risk area. Easterly mid-level flow
will advect Gulf moisture up into the southeast Colorado High
Plains, meanwhile, MUCAPE values will spike above 3,000 J/kg over
much of KS. A vort max will track northeastward out of the 4
Corners region and into the Panhandles by late this afternoon and
this evening. The combination of all of these ingredients will
lead to shower and thunderstorm development as early as 19Z across
CO. The convection will quickly merge into an MCS by 23Z as the
storms approach the CO/KS/OK junction area. As the storms move
into the moisture and easterly low-level flow, the developing MCS
may allow for cell mergers and backbuilding to locally enhance
rainfall amounts over the area. While this area has been
relatively dry in recent days per NASA Sport imagery, expect the
storms to have ample moisture to produce rainfall rates as high as
2 inches per hour at times. Each individual cell should be moving
along at a fast enough speed to keep the time any one area is
seeing heavy rain low, but the aforementioned cell mergers may
lengthen that time locally. Thus, the Slight Risk was hoisted as
the MCS propagates eastward, generally following the KS/OK border.
By evening, the storms should be fast moving enough that any cell
mergers and/or backbuilding will be lesser. Thus, much of the rest
of southern KS into southwest MO was kept at a Marginal risk for
now.

...Catskills, Lower Hudson Valley into Connecticut...

A fast moving shortwave trough will cross northern NY through New
England this afternoon into tonight. North of the Slight Risk
area, the shortwave should be moving plenty fast enough to keep
associated shower and thunderstorm activity also moving at a rapid
pace. Thus, despite very favorable antecedent conditions in the
form of wet soils across northern NY and northern New England, the
Marginal was kept in place. In the Slight risk area, leftover
energy on the southern periphery of the shortwave will linger back
across the region. With increasing moisture advecting in from the
south with PWATs approaching 1.75 inches by tonight. The influx of
moisture and stalled out upper level energy will support
backbuilding and training convection developing to the lee of the
Catskills and tracking east across the Hudson Valley and into CT.
The storms will be capable of 1 to 2 inch per hour rainfall rates,
and somewhat favorable antecedent conditions could result in more
widely scattered flash flooding across this area. With general
agreement on this synoptic setup across the area, the Slight Risk
was hoisted with this update.

...South Dakota and Vicinity...

Decreasing trends in the guidance as far as rainfall totals
resulted in the Marginal Risk being dropped with this update.
Since the moisture will be cutoff by the storms further to the
south across KS, this will greatly limit storm coverage across the
Dakotas.

...Mogollon Rim...

Renewed convection along the Mogollon Rim associated with the
monsoon is expected to develop across northern AZ into far
west-central NM. Storms in recent days have been successful at
generating flash flooding in limited areas, and a repeat day as
yesterday is likely once again as far as overall storm coverage is
concerned.

...Central Sierra Nevada...

No changes were made to the inherited Marginal Risk area. QPF
values are underwhelming for 24-hour totals in this area, but the
second straight day of convection in this area could support
additional isolated flash flooding this afternoon and evening.

Wegman



Day 2
Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EASTERN KANSAS,
SOUTHERN AND CENTRAL MISSOURI, AND THE LOWER OHIO RIVER VALLEY...

...Eastern Kansas through the Ohio Valley...

Forecasted rainfall across the region remains relatively
unchanged. Convection starting out the day across KS/OK will
slowly advance northeastward at the nose of a southwesterly LLJ.
The LLJ will help raise PWATs as high as 2.25 inches across the
Slight Risk area. The convection will wane during peak heating as
the supporting LLJ weakens, though doesn't entirely dissipate. By
Sunday night, the LLJ will quickly restrengthen. This will support
training convection associated with an MCS that will track
southeastward across MO. Pre-frontal convection forming ahead of
the main line will result in total rainfall in the 2 to 4 inch
range in localized spots. This area is particularly vulnerable to
flash flooding given favorable antecedent soil conditions from St.
Louis southward, which is the area that stands the greatest risk
of flash flooding, both from antecedent conditions and likelihood
of seeing the higher rainfall amounts.

The Slight Risk area was expanded northward to include more of IL
and IN with a little bit of a northward shift in where the
rainfall is likely to develop Sunday night. This makes sense since
the LLJ will be pushing the axis of greatest moisture northward
into the warm front. The area from St. Louis southward remains in
the highest likelihood for a future Moderate Risk upgrade, but
with both inherent uncertainty as to where the axis of heaviest
rain will set up between St. Louis and Cape Girardeau, and also no
significant increase in forecasted rainfall, the upgrade was
postponed with this update.

...Upper Midwest...

A strengthening surface low will develop and track across the
upper Midwest today. The heaviest rainfall is expected north and
east of the low center across central MN and into northwestern WI.
While localized rainfall totals exceeding 2 inches is likely, that
rainfall should be both spread out over a longer duration, and
this portion of MN and WI has been exceptionally dry recently, so
much of the rainfall is likely to be beneficial to the area. Thus,
the Marginal Risk is little changed. Should higher rainfall totals
occur over urban areas such as the Twin Cities, localized Slight
risk impacts are possible. These impacts are not expected to be
widespread enough to warrant a Slight risk upgrade, but the area
will continue to be monitored.

...Central Sierra Nevada...

No changes were made to the Marginal Risk area in the Sierras with
this update. The overall trend in rainfall in this area from day
to day is decreasing, but since the impacts tend to increase with
multiple days of rain, the Marginal Risk remains for this update.

Wegman


Day 3
Valid 12Z Mon Aug 14 2023 - 12Z Tue Aug 15 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT LAKES, AS WELL AS FOR THE CENTRAL AND SOUTHERN
APPALACHIANS THROUGH SOUTHERN PENNSYLVANIA AND NEW JERSEY...

...Great Lakes Region...

A low tracking from the Upper Midwest to western New York on
Monday will produce locally heavy rainfall across the Slight Risk
area as it tracks eastward. The signal for heavy rain in this area
has been consistent for a few days now, though the details are
still unclear. There has been a slow southward trend in the
guidance, which makes sense as the best moisture and forcing trend
southward. Thus, the inherited Slight Risk was moved southward. In
coordination with APX/Gaylord, MI forecast office, the northern
L.P. has been very dry in recent days, the soils are sandy, and
the southward trend should result in less rainfall around the
Mackinac Straits, with more rainfall into southern areas of the
L.P., including metro Detroit. Some of the moisture will be
advected northward from the Ohio Valley ahead of the low, and the
plentiful forcing from the developing shortwave trough should
effectively wring out that moisture in the form of heavy rain.
Across northern portions of the Slight Risk area, the rain should
be relatively light but long in duration, while southern areas
will see much more intermittent rainfall, but with heavier
rainfall rates.

...Appalachians to Southern New Jersey...

An MCS tracking out of the Ohio and Tennessee Valleys will move
northeastward across WV on Monday, as atmospheric moisture
increases to 2.25 inches of PWAT. This will allow for very
efficient rainfall rates, locally exceeding 2 inches per hour
across central WV Monday afternoon. A 40 kt southwesterly LLJ will
supply continued ample moisture to allow storms to backbuild and
train across far southwestern VA and WV. The inherited Slight Risk
is largely unchanged over this region. Further north and east,
similarly anomalous moisture will track northeastward up the coast
with the same LLJ. Expect a notable dry signature along the
mountains of western VA, where the Slight risk area was trimmed.
Think there will be an upslope component enhancing rainfall rates
across WV, which will result in a dry downslope counterpart across
much of VA. A warm front will be pushing north through the day and
especially overnight Monday night. This front will help support
lift of that moisture-laden air mass across southern PA and NJ.
The eastward bend to the Appalachians over central PA should also
help uplift storms across the area. Meanwhile the front will be
supported by wet antecedent conditions from the Philadelphia area
east to the Jersey Shore. There is inherent uncertainty as to
where the warm front will be set up. It's possible heavy rain may
make it as far north as the NYC area, so the Slight Risk may need
to be shifted northward with future updates.

...Arizona/New Mexico Border Region...

Strong easterly flow will advect Gulf moisture up the terrain and
into AZ/NM on Monday. Shortwave disturbances tracking along the
western periphery of a stubborn ridge over TX will provide
additional support for afternoon and evening showers and
thunderstorms across this area. There is considerable likelihood
that portions of the Marginal Risk area may need to be upgraded to
a Slight with future updates. Antecedent conditions will play a
big role in this, as wetter soils will support more widespread
flash flooding by Monday. There is still some uncertainty on both
placement and rainfall amounts, so the Marginal Risk was left with
this update.

Wegman
$$
--- SBBSecho 3.14-Linux
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FOUS30 KWBC 130848
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
447 AM EDT Sun Aug 13 2023

Day 1
Valid 12Z Sun Aug 13 2023 - 12Z Mon Aug 14 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OZARKS OF SOUTHERN MISSOURI...

...Mid-Mississippi Valley and the Ozarks...

A 30 to 40 kt LLJ will advect abundant Gulf moisture northeastward
across portions of the Plains, Ozarks, and into the Lower Ohio
Valley today into tonight. An ongoing MCS moving across OK and KS
this morning will continue slowly moving eastward, and will likely
produce scattered showers and more isolated thunderstorms across
MO through this afternoon. Any rainfall this morning will help
prime the soils for the main event for the area expected tonight.
A front pushing southward across the Plains today into tonight
behind a developing low over the northern Plains and upper Midwest
will contribute additional forcing to the atmosphere as it runs
into the anomalously moist air mass being advected out of the
Gulf. The clash of the front and associated moisture will occur
along the KS/OK border and extend eastward across southern MO and
into the lower Ohio Valley. As the moisture hits both the front
and the topography of the Ozarks, expect numerous strong showers
and thunderstorms to develop and track eastward along the front.
With the moisture being resupplied along the front to the west,
training thunderstorms are likely all along the front, but the
greatest likelihood of training showers and storms will be across
southern MO, where the Ozarks may allow a topographic component to
the lift. Further, the developing low to the north will support
upper level frontogenesis, resulting in a strengthening
southwesterly jet streak. Southern MO will be in the lift-favored
right entrance region of the jet, which will allow the convection
to persist longer over the area.

Southwestern MO eastward through western KY has been very wet over
the past several weeks, getting 200-400% of their normal rainfall
based on AHPS output. Thus, the area is still considered sensitive
to flooding from heavy rainfall. With the abundant moisture
available, thunderstorms will be capable of producing 2 to 3 inch
per hour rainfall rates, easily exceeding FFGs of 1.5 to 2 inches
per hour across southern MO. In coordination with SGF/Springfield,
MO forecast office, a Moderate Risk upgrade was introduced with
this update for much of southwestern and south-central MO.
Collaboration was also conducted with LSX/St. Louis, MO and
PAH/Paducah, KY forecast offices, which were kept in a Slight risk
for this forecast update due to less certainty as to how well the
strongest convection holds up as it moves east into the
mid-Mississippi Valley. CAMs guidance suggests the convection
should reach the MS River, so an expansion of the Moderate Risk
eastward is probable. There is also some inherent uncertainty as
to where the heaviest convection sets up, with some guidance
suggesting it occurs as far north as St. Louis, to as far
southwest as Tulsa and northern AR. Thus, some north-south
shifting of the risk areas are also possible, though the guidance
has been reasonably consistent highlighting the current area over
the past few nights.

00Z HREF 40 km neighborhood probabilities are up to 80% for 3
inches of rain in south-central MO and a 10-15% chance of 8 inches
of rain in southwest MO, with a 10-15% chance of having a 100 year
ARI event in the same area. These heightened probabilities for
heavy rainfall totals contributed to the confidence to upgrade to
a Moderate Risk with this update.

Elsewhere, the surrounding Slight Risk area was adjusted a bit to
account for the latest guidance trends, including expanding the
Slight northwest through Kansas City and St. Joseph, and westward
along the KS/OK border. For northwestern MO, the added convection
will be residual moisture from the storms further south being
forced by the developing low over the Upper Midwest, while the
Slight along the KS/OK border covers likely widely scattered
storms that will congeal into the MCS and flooding rain event
further east.

...Upper Midwest...

As mentioned above, a developing surface low pushing east across
NE/IA will advect moisture northwestward ahead of the low center
into the northern Plains and upper Midwest. The comma-head region
of the circulation north of the low center will feature a
prolonged period of light to moderate rain, with embedded
convective elements drawn north as residual moisture and
instability from the much stronger storms likely to be ongoing
Sunday night further south. While a widespread 1 to 2 inch
rainfall event is expected, with added forcing from a
strengthening and digging shortwave trough, the combination of
separation from the primary moisture source and instability
source, and antecedent very dry conditions over MN/WI should
greatly limit flash flooding potential, despite the expected
widespread 1-2 inch rainfall amounts. The Marginal Risk over this
area was maintained, with the greatest flash flooding potential
over urbanized areas such as the Twin Cities and any other poor
drainage areas.

...Maine...

A negatively tilted trough tracking north of the Canadian border
will support a renewed round of rather fast-moving but still
potentially strong storms across the Pine Tree State this
afternoon and evening. This will follow the convection currently
ongoing over the area. While rainfall rates of 1 to 2 inches per
hour will be possible, this is likely to only exceed FFGs in
isolated instances, so the Marginal Risk inherited over the area
was maintained.

...Central Sierra Nevada...

No changes were made to the inherited Marginal Risk area. Weak
signals for showers and storms continue with the latest set of
guidance. Any storms that form over the slopes of the Sierras
won't take much to develop into localized flash flooding due to
the steep topography of the area. There is considerable
uncertainty as to how widespread that convection will be, so the
Marginal Risk for isolated flash flooding appears reasonable.

Wegman



Day 2
Valid 12Z Mon Aug 14 2023 - 12Z Tue Aug 15 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT LAKES, AS WELL AS FOR THE CENTRAL AND SOUTHERN
APPALACHIANS THROUGH THE NEW YORK CITY METRO AND LONG ISLAND...

...Great Lakes Region...

The low that develops over the northern Plains and upper Midwest
Day 1/Sun will continue slowly tracking eastward as the vigorous
upper level shortwave supporting the low gradually weakens through
Monday night. The low will siphon off a bit of the abundant Gulf
moisture south of it and track it ahead of and north of the low
center as it tracks east. The area of heaviest rain from southern
WI and the Chicagoland area east across Lake Michigan and much of
the southern L.P. of MI is largely from wraparound rain that will
persist to the north of the low center. Instability is likely to
be limited in this area, with the flash flooding threat largely
resulting from the long duration of the light to moderate
rainfall, which is likely to persist in some areas for more than
12 hours. The above isn't to say that there will be no
instability, as in fact some convective elements are likely to be
present as the low advects some instability as well as moisture
from the south into the region. However, extreme rainfall rates
are not expected. The areas most likely to see flash flooding are
the urban centers, which include Milwaukee, Chicago, and Detroit.

...Appalachians to the NYC Metro Region...

An MCS tracking up the TN and OH Valleys will move into the
Appalachians around midday, with additional lines of convection
resulting from daytime heating likely to develop behind the main
MCS and initial line of storms through the central and southern
Appalachians into Monday night. Fast WSW flow should help
individual convective elements to clear the TN and OH Valleys
quickly, as the main MCS tracks into the Mid-Atlantic late Monday
afternoon and evening. It's likely there will be an upslope
component as the 40 kt LLJ runs into the Appalachians, so there's
a notable difference in rainfall totals on the windward/west side
of the mountains across eastern TN/far southwestern VA and much of
WV as compared with areas just east of the mountains, including
most of NC and VA, where relatively little rainfall is expected.
As the terrain will support more rapid-onset flash flooding, the
Slight Risk was maintained from the southern Appalachians of
eastern TN northward through WV and southwest PA. The Slight Risk
area was expanded westward across east-central OH due to much more
favorable antecedent conditions caused by the widespread flash
flooding that occurred yesterday. Despite less upslope forcing,
the low tracking across the area will enhance atmospheric forcing
over that area over to southwest PA/Pittsburgh area.

The Slight Risk area was nudged northward largely out of
central/northeast MD and southern NJ, but now including more of
central and northeastern PA, northern NJ, and the NYC Metro
region. This adjustment was largely based on updated guidance that
shows a weak warm front slowly pushing northward, but with
abundant low-level moisture featuring PWATs exceeding 2.25 inches
running into the front. The warm front being weak will ultimately
play a big role in keeping impacts down across the area, as storms
will likely form further west where there's better forcing, then
race eastward along the front and outrun the forcing, resulting in
a weakening trend with eastward progress. 06Z HRRR guidance shows
the OH Valley MCS that begins the day near Louisville, KY
maintaining strength through central PA Monday evening, then
strengthening overnight as it moves into NY and New England.
Expect strong storms capable of heavy rainfall rates to develop
and race eastward across the Tri-State area around or after
midnight. The storms will be quite capable of rates above 2 inches
per hour, but their fast movement will limit overall rainfall
totals. Nonetheless these rates should easily exceed the 1 to 2
inch per hour FFGs, resulting in widely scattered instances of
flash flooding. There remains considerable uncertainty as to where
these heaviest rainfall amounts will be seen, given changing
guidance as to where the warm front guiding the storms will be set
up, but the likelihood this will include the NYC metro has
increased enough to warrant the Slight Risk upgrade for the area.

...Four Corners south and east...

Mid-level ridging over the Rockies over to Texas will continue to
direct weak upper level disturbances northward along the western
periphery of the ridge along the AZ/NM border and western NM
Monday afternoon. Strong easterly flow will advect somewhat
anomalous moisture into the region. In coordination with
TWC/Tucson, AZ; FGZ/Flagstaff, AZ; and ABQ/Albuquerque, NM
forecast offices, the Marginal was maintained with this update.
Generally, think storm motion will offset rainfall rates to an
extent, which will diminish flash flooding potential. The areas of
greatest risk for flash flooding and highest rainfall totals
including the far eastern Mogollon Rim of AZ and into west-central
NM. The signal has backed off with this set of guidance across the
El Paso metro, so the Marginal Risk area remains for the
Sacramento Mtns north into southwestern CO.

Wegman

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