FOUS30 KWBC 090737
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
336 AM EDT Sun Jul 09 2023

Day 1
Valid 12Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC INTO WESTERN NEW ENGLAND...

...Mid-Atlantic to New England...
Signals for heightened risk for flash flooding with isolated
significant localized impacts possible across the Mid-Atlantic
into Northwestern New England are coming into focus. A potent
shortwave is starting to round the base of the larger scale trough
across the Ohio Valley, resulting in some increased negative tilt
by late afternoon. Strengthening broad southerly flow off the
very moist Northwest Atlantic will increase total moisture across
the Delaware/Hudson Valley into New England with 1.75 increasing
to 2"+ by midday. In response a surface low and associated cold
front (and inverted trof across NY) should support strong moisture
flux convergence along its length by mid-morning (if not already
ongoing by 09.12z) across NY, central PA to Western VA/NC.
Efficient rainfall production should result in localized
rain-rates to 2"/hr and possible instances of flash flooding, deep
layer steering will support northward moving cells supporting
short-term training as the front slowly progresses eastward.

As the afternoon evolves, upper-level divergence increases along
with heating to increase instability across the eastern US while
the surface low strengthens in N VA. This will back the local
flow across the Delaware Valley into the Hudson Valley, increasing
moisture flux and FGEN ascent through E PA, further slowing
forward progress of the convective line while cells still
train/repeat to the north. By evening instances of 3-6" locally
will become increasingly plausible with 00z HREF probability
fields greater than 60% from DC to N VT (with spots near 90-100%
for 3" and 30-60% of 5" across NE PA/N NJ to S VT. 00z Hi-Res
CAMs have trended slightly eastward to align more favorably to the
greatest 14-day precipitation over 300% of climatological normal
axis extending from near Baltimore to Philly to N NJ and W MA,
suggesting increased runoff potential for these. As the surface
wave advances toward the New York Bight by early morning,
increased low level flow, will advect unstable across across S
NY/New England and with continued isentropic ascent through warm
conveyor belt along/north of the surface low, additional efficient thunderstorms a likely to linger overnight, in proximity to SE
NY/SW New England as well as into south facing terrain making a
more likely scenario for flash flooding. As such, the Moderate
Risk of excessive rainfall was shifted eastward slightly to
account for the guidance trends.


...Eastern North Carolina...
At the tail end of the cold front, the Slight Risk was expanded
south and west to account for lower FFG over recently affected
areas of west-central VA. Some consideration was made toward
including the NC Fall Line/Piedmont region including Charlotte as
the southern end of thunderstorm activity may lay out outflow
boundaries for late evening/early overnight redevelopment of
thunderstorms on increased orthogonal ascent of LLJ across the
boundaries. The signal in guidance has increased for this
thunderstorm development, but remain generally too
isolated/scattered with only isolated enhanced rainfall totals to
expand the Slight Risk at this time.



...Southern Plains to Central Gulf Coast...
Like prior days, there is likely to be an ongoing MCS across
Oklahoma at the start of the forecast period from prior night's
upslope flow. Current RADAR and satellite imagery trends denote
some increased in elongation of the wave/convective line from west
to east with weaker isentropic ascent, though the length should
maintain duration for modest rainfall crossing saturated grounds
from multiple prior days' complexes, there remains a slightly
faster trend toward the east. By mid-day, insolation and
convergence in proximity to the MCV across LA/S AR should spur an
additional round of stronger, moderately organized thunderstorms.
00z Hi-res guidance trends depict a more loosely organized
clusters with some weak southwesterly isentropic ascent with cells
moving off/away from the boundary into weaker elevated cells
across LA/MS into SW AL. This is a general reduction of
repeating/training environment expanding the potential for
clusters than one or two broader heavy rainfall
signatures/footprints. As such, the broad Slight has reduced in
confidence for number of potential flooding incidents but remains
generally broad enough to maintain, with a reduction of the
western extent to account for the faster cell motions of the
initial MCS this morning.


Gallina




Day 2
Valid 12Z Sun Jul 09 2023 - 12Z Mon Jul 10 2023

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF
INTERIOR NEW ENGLAND...

...Northeast...
At the start of the day 2 period (Mon 10.12z), the shortwave will
will reaching peak maturity with deep layer DPVA across the
Northeast as it starts to close off across N PA/E NY with surface
low in proximity to E Long Island with a well developed and
focused TROWAL crossing southeast New England with 20-30kts of
warm conveyor ascending across New England into a sharpening
deformation zone from the Catskills across Northern New England.
There are multiple signals for a remaining axis of 500-800 J/kg of
MUCAPE along and southeast of the convergence zone, which will
remain critical for the elevated, slow moving convection along the
TROWAL. Given prior day's heavy rainfall (3-6") and already
elevated wet soil conditions (per AHPS 200-300% above normal
precip anomalies) and potential for slow/stationary cells
resulting in localized additional 2-4"+, the risk of flash
flooding remains highly likely and may need to be further
increased based on how much rain unfolds in the Day 1 period,
particularly across the southeast facing Berkshires and Green Mt
Ranges where upslope flow should enhance ascent/rainfall rates
where FGEN signals overlap the greatest.


...Gulf States into South Carolina...
Deep Layer trough as pressed the cold front and pooled
moisture/instability along the Central Gulf Coast with a stronger
trend toward a further south solution. This places the best
upper-level diffluence and moisture flux convergence overall along
and just offshore of the MS Delta toward western FL Panhandle.
While synoptic guidance continues to hint a stronger moisture flux
convergence within the downstream shear axis across S GA; this
looks like a typical guidance bias, stronger thunderstorms closer
to the AL/GA and FL line are more likely given proximity to better
instability, but should be more scattered in nature and within the
area of higher FFG. As such, kept the Marginal Risk to align with
the best moisture/instability where isolated stripes of enhanced
rainfall could result in isolated flooding concerns.


...Central to Southern Plains...
Stronger closed ridge over the Southwest and reduced activity
overnight late Monday across Central High Plains, should have
allowed for weaker moisture return to build up higher values,
increased instability for potential activity into Tuesday evening.
00z guidance along with prior cycles continues to hint at an
organized cluster or small complex to form at the upwind side of
the cold front in the vicinity of NE NM into the Texas Panhandle/W
OK. Storm mode will likely determine localized very intense
rainfall rates capable of inducing an isolated few instances of
flash flooding, in proximity to saturated grounds across central
Oklahoma, a Marginal Risk is extended from the area in the Lower
MS Valley back across and north of the Red River.

Additionally, northern stream cold front as leading edge of larger
scale height-falls will be pressing southward across the central
Plains and intersecting some of the untapped moisture/instability
that rolled back through eastern Colorado on Sunday and Monday.
Instability of 2000-3000 J/kg and 1.5"+ total PWats pooled along
strengthening (20-25kt) LLJ is likely to converge in early Tuesday
for thunderstorm development across Nebraska, eventually
propagating into KS. As such an extension of the Marginal Risk is
brought northward into southern Nebraska extending toward the Iowa
boarder.


...Upper Midwest...
There is uncertainty to the level of moisture flux return
along/ahead of the cold front dropping out of Canada crossing Lake
Superior to N Nebraska in the day 2 period. Stronger storms are
likely to dot the front, but cell motions should mitigate the
highest totals across portions of Iowa into MN, particularly given
recent dry soil conditions. However, timing of the front with
peak heating and likelihood of Lake Breeze enhanced thunderstorms
off Green Bay/NW Lake Michigan may result in thunderstorm
collision for a few spots of intense short-term rainfall rates
over 2" and spots of 2-3" overall. The area has seen above normal
rainfall over the last few weeks per AHPS and lower FFG values
(mainly along the northern U.P.) may overlap enough for an
isolated potential for localized flooding. So in coordination
with MQT local Forecast Office, a small Marginal Risk of Excessive
Rainfall was added for the core of the U.P. of Michigan for this
cycle.

Gallina

Day 3
Valid 12Z Mon Jul 10 2023 - 12Z Tue Jul 11 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
PORTIONS OF THE MID-MISSOURI BASIN...

...Northern Plains through Upper Great Lakes...
The strong synoptic scale closed low over Canada is expected to
drop south toward the Boundary Waters of Ontario/Minnesota with a
sharpening shortwave digging along the western side across into
the Northern Plains. The associated cold front will have already
started slow slow from the Northern Great Lakes across South
Dakota before angling northwest into northeast MT. Strong upslope
moisture flux will increase across the Northern High Plains, with
a narrow axis of enhanced moisture/instability through the western
Dakotas. Mid-day thunderstorms will start to organize into a
stronger cluster/complex intersecting with 35-40kt SSW LLJ through
the central Plains. MCS with training thunderstorms along the
frontal boundary will increase in coverage/rainfall efficiency
across central SD into SE SD toward 12.03-06z with streaks of 2-4"
becoming likely across the Mid-Missouri Basin. While the area has
been dry, thunderstorms on Day 2 and intense rates continue to
support a Slight Risk across SD into NW IA. A broad Marginal Risk
extends along the track as well as up the frontal zone across N
IL/S WI and Lower MI, where scattered clusters may result in more
isolated heavy rainfall signature (given low level flow will be a
bit more parallel reducing moisture flux convergence.


...Northern New England...
At the start of the forecast period, the increasingly stacked
cyclone will be exiting with the occluded/cold front likely to
have shifted east/northeast of Maine. However, the lingering
deformation zone/TROWAL is likely to have ongoing showers or
embedded smaller thunderstorms across N VT/NH and maybe the top of
the Rooftop of Maine. Additional 1-2" totals are possible but
reducing given limited remaining instability. These totals and
likely ongoing flooding rivers/streams and saturated soils will
still be prone to any rates up to .5-1"/hr, but will be
conservative in higher ERO categories until knowing the greatest
affected areas are known after the prior two days. As such a
Marginal Risk of Excessive Rainfall will be a placeholder until
the ground conditions are better understood.


...Southern Red River through Central Gulf Coast...
The lingering stationary boundary will continue to be the focus of
pooled deeper layer moisture, though being at the tail end of the
large scale flow (exiting deep layer trof), there will be limited
translation of the axis. Anomalous deep layer moisture with
values of 2-2.25" total PWat through the Lower MS Valley and
full-sun should support strongly unstable environment with
2000-3000 J/kg CAPE by midday. Weak upper level diffluence and
weak south-southwesterly flow should allow for localized clusters
of stronger thunderstorms. Deep layer slow steering flow
along/parallel to the boundary should allow for sufficient
organization to maintain the clusters and slow motions may support
very intense but locally focused enhanced rainfall totals to
support isolated instances of flash flooding; enough to maintain
the Marginal Risk of Excessive Rainfall from SE OK to NOLA as well
as the FL Panhandle.


Gallina
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