...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.

This front will serve as a focus for showers and thunderstorms
today as ascent maximizes through low-level convergence, modest
height falls/PVA, and weak RRQ diffluence as a jet streak pivots
southeast during the evening. This ascent will overlap favorable
thermodynamics for heavy rain characterized by MUCAPE of 1000-2000
J/kg and impressive moisture flux as 30kt SW 850mb inflow surges
PWs of 1.25 inches into the boundary. As the front becomes
stretched west to east, this will allow the mean 0-6km flow to
become aligned parallel to it, suggesting an increased training
threat from Wisconsin through the U.P. of MI. At the same time,
Corfidi vectors turning more into the intensifying inflow become
nearly anti-parallel to the mean wind and collapse to just around
5 kts suggesting at least some backbuilding potential into the
higher instability. With rain rates progged by the HREF to exceed
1"/hr at times, this training/backbuilding scenario could produce
areas of 1-3" of rainfall. Despite modest recent rainfall leading
to FFG that is around 2"/3hrs, there are some pockets of more
sensitive soils, especially in the U.P., but HREF exceedance
probabilities peak only around 10-20%. The MRGL risk was
maintained and adjusted cosmetically, as some isolated flash flood
responses are possible during the most prolonged training today.


...Northeastern Washington through Montana...
A shortwave ejecting out of the Pacific Northwest this morning
will amplify into a closed low as it moves into Alberta, Canada,
leaving increasing downstream mid-level divergence, which will
overlap with the favorable LFQ of an upper jet streak to provide
pronounced ascent across the Northern Rockies and Northern High
Plains. This will intersect a cold front digging southward out of
Canada, with increased moist flow advecting from the southeast
along the front. This will push PWs to 1-1.25", more than 1
standard deviation above the climo mean according to NAEFS. The
atmosphere will marginally destabilize ahead of this front, with
MUCAPE possibly exceeding 500 J/kg in places. The overlap of this
deep layer ascent into this narrow ribbon of favorable
thermodynamics should result in scattered thunderstorms, which
will have rainfall rates that may reach 1"/hr at times as progged
by the HREF neighborhood probabilities. Storm motions will
generally be quick to the east, but repeated rounds could produce
some areas of 1-2" of rainfall. While the FFG exceedance
probabilities are modest, parts of MT have had excessive rainfall
noted by AHPS departures that are 150-300% of normal leading to
above normal USGS streamflows, while recent burn scars are also
more vulnerable than depicted by FFG. Any repeating rounds of
heavy rain could produce runoff and isolated instances of flash
flooding.

Weiss




Day 2
Valid 12Z Tue Jul 11 2023 - 12Z Wed Jul 12 2023

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

...1930Z Update...

...Mid-Missouri Basin...

Changes were overall fairly minimal for the Day 2/Tuesday period.
An MCS tracking southeastward along a front will result in
numerous showers and thunderstorms across much of western and
central IA and small portions of the surrounding states developing
around sunset and moving across the Slight Risk region largely
overnight Tuesday night. The abundance of moisture available for
the storms will be primary component driving the threat for flash
flooding, as PWATs may exceed 2 inches, which is over 3 sigma
above normal in southeastern NE. Fast storm motion will be the
primary factor working against flash flooding, along with average
soil moisture conditions in most of the Slight Risk area, and of
course much of IA being farmland that is typically eager for
rainfall. Thus, while this is considered a lower-end Slight risk,
localized rainfall totals to 3 inches are possible. This is most
likely to occur where cell mergers prolong the heavy rainfall
rates over a given area.

There is some hint, particularly in the 12Z HRRR guidance that
some rain may get started as far east as Chicago in the predawn
hours Wednesday morning, however at the moment that Hi-Res model
is the only one hinting as such, so for now the Marginal Risk area
maintained through northern IL will remain as a Marginal. Much of
the rainfall event here will occur into Day 3/Wednesday.

The Marginal Risk along the Gulf Coast remains largely as
inherited as well. The primary concerns are for New Orleans and
Baton Rouge for flash flooding, with much of the rest of LA and
the Gulf Coast not expected to pick up enough rainfall to induce
flash flooding, due to high FFGs.

Afternoon convection occurring into eastern MI around and north of
the Detroit area will occur over an area that has seen around 200%
of climatological normal rainfall over the past 2 weeks. Thus, a
small Marginal Risk was added to account for the isolated flash
flooding potential.

The Marginal risk area across northern New England was trimmed
hesitantly given the ongoing major flooding situation, as the rain
may conclude in this area a bit sooner than previous forecasts.
Always a good thing.

Wegman

...Previous Discussion...

...Northern Plains into the Upper Midwest...
Squeezed mid-level flow across the Northern Rockies will dive into
NW flow over the Upper Midwest as a broad trough persists across
the eastern CONUS. Shortwaves and accompanying vorticity maxima
rotating through the flow will advect eastward and then drop
southeast, joining the LFQ of an upper jet streaking from the
Pacific to produce robust ascent across the area. This synoptic
lift will interact with a wavering frontal boundary draped
northwest to southeast from MT through MO, with increasing
low-level S/SE flow impinging into this boundary. This low-level
inflow will reach 15-25 kts, surging PWs to 1.25-1.75 inches, more
than +1 standard deviation above the climo mean according to
NAEFS, to provide substantial available moisture for heavy
rainfall. Additionally, instability will climb to 1000-2000 J/kg
within a ribbon lifting north along the front, with a steep CAPE
gradient developing from SW to NE during this time. This overlap
of forcing and moisture/instability should result in scattered to
widespread thunderstorms along the front, with the greatest
excessive rain threat likely developing through MCS development
late in the forecast period.

Thunderstorms that develop along the front from eastern MT through
IA will likely have rainfall rates of 1"/hr or slightly higher
within the favorable thermodynamics. The available high-res keeps
activity generally scattered along the front, and storm motions
using the 0-6km mean winds as proxy should be relatively quick,
but along the front to support at least short-term training. This
could result in isolated runoff responses, especially over more
sensitive soils from 14-day rainfall that is 150-300% of normal in
some areas. However, the greater flash flood risk exists from far
eastern SD through northern MO where the guidance suggests an MCS
will develop and potentially train along the instability gradient.
There continues to be spread in the timing and placement of this
MCS, but it is likely one will develop. At this time, the SLGT
risk was tailored to best match the 12-hr HREF and 24-hr GEFS
probabilities for 1 inch and 3 inches, but additional adjustments
are likely.


...Northern New England...
The excessive rain event from D1 will wane during D2, but guidance
indicates the deformation will persist across northern VT/NH/ME
during the first few hours of this forecast period. Additional
scattered showers are possible through the day across
central/northern New England as well before much drier air advects
eastward into New England to shut off the rain. The
ECENS/SREF/GEFS probabilities all suggest at least modest
exceedance for 1" of rainfall, with briefly heavy rates likely
within this pivoting deformation axis thanks to MUCAPE of 500-1000
J/kg. The rainfall in itself on D2 is not likely enough in any
location to produce flash flooding. However, the soils will likely
be extremely saturated from heavy rain and possibly significant
flash flooding during D1, so any additional rainfall could result
in further instances of flash flooding. The MRGL risk was trimmed
to match updated guidance, but at least an isolated threat still
exists before the system exits by late D2.


...Southern Plains through the Gulf Coast...
A stalled cold front aligned along the Gulf Coast will weaken and
dissipate into a trough Tuesday, but still provide a focus for
ascent and thunderstorms during peak heating. A broad trough still
encompassing much of the eastern CONUS will maintain an axis down
to the Gulf of Mexico, producing subtle height falls and steepened
lapse rates to help drive MUCAPE to 2000-3000 J/kg during the
aftn/eve. This impressive instability combined with PWs still
progged to reach around 2 inches will produce a sufficient
environment for heavy rain rates exceeding 1"/hr. The overlap of
highest PWs and greatest MUCAPE does not look ideal on D2 except
near Louisiana and the northern Florida peninsula, so the
excessive rain risk appears modest. However, rain rates of 1+"/hr
within these favorable thermodynamics slowly training along the
front through parallel 0-6km mean winds of just 10 kts, could
produce locally 1-3" of rainfall as progged by HREF, SREF, and
GEFS probabilities. The MRGL area is quite broad as determining a
focus for greatest threat is challenging, but a subtle focus may
exist along the sea breeze/collisions from eastern Louisiana
through the western Florida Panhandle where isolated higher
rainfall amounts are possible.


Weiss


Day 3
Valid 12Z Wed Jul 12 2023 - 12Z Thu Jul 13 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE
MID-MISSISSIPPI VALLEY AND SOUTHERN GREAT LAKES...

...1930Z Update...

No significant changes were needed to the Risk area across the
Midwest for the Day 3/Wednesday period either. Southern portions
of the Slight for MO and southern IL are looking to get a 1-2
punch from 2 separate MCS's. The first MCS will be ongoing from
the overnight period of Day 2/Tuesday night and will move down the
Mississippi River. Meanwhile a stalled out front will keep a
portion of that shower and thunderstorm activity ongoing off to
the east from the Chicago area east through Detroit and into the
Finger Lakes of western NY by Wednesday night.

Meanwhile a second MCS will develop Wednesday evening along the
IA/MO border and intensify as it tracks southeastward across
eastern MO and southern IL overnight Wednesday night. Most
indications are that this second round of storms will be more
intense than the first, and the heavy rain expected during the
overnight period will increase the chances for impacts resulting
from any flash flooding, particularly in the St. Louis area.

Wegman

...Previous Discussion...

...Mid-Mississippi Valley through the Great Lakes...
Elongating ridge over the Desert Southwest and continued expansive
but broad troughing across the east will drive persistent NW
mid-level flow over from the Northern Plains through the Ohio
Valley. Within this regime, subtle shortwave impulses and
associated vorticity maxima will rotate E/SE, combining with the
diffluent LFQ of a zonally oriented jet streak to produce waves of
synoptic ascent. At the same time, a surface stationary boundary
will waver nearly in place, aligned from Nebraska to Indiana, such
that favorable overlap of the synoptic lift will occur with
low-level convergence along the front. This setup will support
scattered to widespread convection on Wednesday.

During the day, return flow around a high pressure over the
Appalachians will surge moisture northward out of the Gulf of
Mexico. 850mb winds will steadily increase to 20-30 kts, possibly
higher Wednesday night, impinging favorably into the front to
produce additional isentropic upglide. This inflow will also
advect robust thermodynamics northward, with PWs rising above 2
inches and MUCAPE possibly exceeding 3000 J/kg. This will support
widespread thunderstorms during the day with intense rainfall
rates above 1"/hr extending from Missouri through Michigan along
the front. These storms should be progressive along the front, but
mean winds aligned to the boundary could support training to
produce flash flooding.

More concerning is the potential for a potent MCS to dive along
the front late D3. Uncertainty is high in the placement of this
feature, but most available guidance suggests the shortwave aloft
will interact favorably with the CAPE gradient and the baroclinic
zone to produce an MCS diving to the southeast in the vicinity of
MO/IL. Some of the guidance is quite aggressive with rainfall as
effective bulk shear of 40-50 kts and Corfidi vectors pointing
from the higher instability pool allows for storms to impressively
organize and backbuild to the west. With rain rates likely 1-2"/hr
or more, this could result in an axis of very heavy rainfall, and GEFS/ECENS/SREF probabilities all support an area of 2"+ rainfall
despite a lot of spread in position. This will need to be
monitored with later forecast packages as it is possible a
targeted MDT risk area may be needed with later updates as the
event comes into focus.

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