FOUS30 KWBC 102016
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Sun Aug 10 2025

Day 2  Valid 12Z Mon Aug 11 2025 - 12Z Tue Aug 12 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN PLAINS THROUGH THE UPPER MIDWEST, THE COASTS OF THE 
CAROLINAS, & THE FLORIDA PANHANDLE...

...Southern Plains & MO-KS-AR-OK...

The South Central U.S. will be sandwiched between two upper level
disturbances; one upper trough approaching from the Southern
Rockies, and mid-upper level trough axis over southern Texas. Atop
the atmosphere, modest diffluent 250mb flow will support divergence
aloft while a nearby surface frontal boundary acts as a trigger 
for thunderstorms. Any residual outflow boundaries from Sunday
night's convection or from Monday afternoon's convection will also
likely play a role in firing off thunderstorms to the south and
east of the surface front. PWATs will generally be >1.8" along the
Red River and points north and east, while areas as far west as 
the TX Panhandle and cap rock are between 1.4-1.6". In terms of 
instability, lesser values (MLCAPE 500-1,000 J/kg) will be 
available in the TX Panhandle, with greater over the MO-KS-AR-OK 
region through Monday night. The 12Z HREF does show spotty areas of
moderate-chance probabilities (40-60%) within the Slight Risk area
for >3" of rainfall, as well as low-chance probabilities (10-30%) 
for >5" in central OK. Given these factors, the Slight Risk was 
focused more here compared to the previous forecast cycle.

...Middle MS Valley...

The Slight Risk was scaled back here given the reduction in
available instability, latest QPF reduction, and lesser footprint
of >3" rainfall probabilities in the 12Z HREF. With soils growing
more saturated and the lingering presence of near 2" PWATs, there
is still a localized flash flood threat, hence the presence of a
rather large Marginal Risk area that stretches from the I-70
corridor from KS/MO on north through the Great Lakes. 

...Florida Panhandle & Big Bend...

In coordination with TAE, introduced a Slight Risk for areas along
and south of I-10 in the FL Panhandle and Big Bend. As a mid-to-
upper level inverted trough axis approaches, it will be accompanied
by a wealth of anomalous PWATs that will approach 2.5" in some
areas. Modeled soundings show warm cloud layers just shy of
16,000ft deep in some cases with little-to-no capping inversions
Monday afternoon. On the eastern flank of the inverted trough,
925-850mb winds will be uniformly out of the SE. ECMWF soundings
over the Big Bend show Corfidi Upshear vectors that are <10 knots 
Monday afternoon, suggesting the potential for back-building and 
training thunderstorms. Given the abundance of tropical moisture 
and MLCAPE up to 1,000 J/kg, it is quite possible to have 
thunderstorms generate 2" of rainfall in as little as 30 minutes. 
The scale of these storms are making QPF forecasts across CAMs 
guidance widely varying on location, but the atmospheric parameters
support an increasingly concerning setup for flash flooding for Monday.

Mullinax

---Previous Discussion---

...Southeast...
Yet another active day across the Southeast is expected Monday as
an inverted low-to-mid level trough drifts northeast along the
coast of the Carolinas. This trough will move within a favorable
thermodynamic environment characterized by still highly anomalous
PWs of 2 to 2.25 inches, although the slow retreat of the mid-level
ridge to the east should help ease some of this moisture the latter
half of D2. Still, the overlap of these elevated PWs with MUCAPE of
1000-2000 J/kg will support convection with heavy rainfall.

In general, convection across the Southeast on Monday should be
scattered in coverage, but some organization, or at least better
clustering, is likely in the vicinity of any shortwaves and the
inverted trough, primarily from the Grand Strand of SC to the
Crystal Coast of NC. Here, repeating rounds of convection with
efficient rain rates exceeding 2"/hr are likely, driven by
impressive low-level confluence. Elsewhere across the Southeast,
heavy rainfall rates are also likely, but with less organization,
but still slow storm motions. The rainfall probabilities for more
than 3" peak along the SC/NC coasts and the SLGT risk was adjusted
cosmetically for the new guidance. Otherwise, a broad MRGL risk
continues for slow moving thunderstorms across areas that have had
recent heavy rainfall, and an expansion was made into the Upstate
of SC where some of the high res CAMs suggest enough upslope to
enhance rainfall locally in that area.

Weiss


Day 3  Valid 12Z Tue Aug 12 2025 - 12Z Wed Aug 13 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER MIDWEST/GREAT LAKES, THE SOUTHWEST, AND GULF COAST/SOUTHEAST...

2030Z Update...

There were some minor adjustments made this forecast cycle to the
Marginal Risk areas based on latest 12Z guidance and WPC QPF. The
Southeast will be closely monitored given the same feature
supporting a Slight Risk on Day 2 in the FL Panhandle and Big Bend
areas will move farther inland on Tuesday. However, there remains a
fair amount of spread regarding how much rainfall occurs and which
areas are favored. Given the lingering uncertainty, opted to
hold on to a Marginal Risk area with no upgrade this forecast cycle.

Mullinax

---Previous Discussion---

...Upper Midwest...
The anomalous mid-level trough that will produce the heavy rainfall
D1 and D2 across the Upper Midwest/Central Plains will weaken
Tuesday as shortwaves embedded within the trough become more
rapidly absorbed into westerlies across the northern tier of the
CONUS. A lingering shortwave trough, likely convectively enhanced,
pivoting across KS/MO will drive sufficient ascent for additional
rainfall along the weakening front, but with lesser moisture and
instability than was available earlier in the week. Still, at least
short term training of intense rainfall rates is likely as mean
layer winds align to the weakening front, which could result in
isolated instances of flash flooding.

...Southeast...
Elongated mid-level ridge extending westward from the Atlantic to
along much of the Gulf Coast will subtly retreat Tuesday as a
shortwave emerging from the Gulf lifts northward. The modest height
falls and PVA accompanying this feature will drive ascent into an
otherwise weakly forced environment, producing a favorable setup
for scattered to numerous thunderstorms from the central Gulf Coast
northeast along the periphery of the ridge into Georgia.
Thunderstorms that develop will contain heavy rainfall rates of
1-2"/hr in response to favorable thermodynamics characterized by
PWs of 2-2.25 inches overlapping MUCAPE as high as 2000 J/kg. The
general SW return flow will push storms along steadily to the N/NE,
but with repeating rounds possible and some organization beneath
the shortwave, a few instances of flash flooding could result as
rainfall reaches 2-3" in some areas.

...Southwest...
Mid-level ridging will expand but shift subtly westward on Tuesday
while weak impulses rotate around its periphery. This will help to
intensify instability across the region, and progs indicate a
larger plume of 1000 J/kg MUCAPE spreading across the southern
portions of AZ and NM, especially during peak aftn heating. This
will combine with PWs that will be above 1" in many areas south of
the Mogollon Rim, leading to a favorable environment for
convection. Where any of these impulses rotate aloft, enhanced
ascent will yield scattered to widespread thunderstorms with rain
rates approaching 1"/hr at times, and these storms will move
slowly/chaotically beneath the ridge. This could result in
isolated instances of flash flooding, most likely where any storms
track across more vulnerable terrain features or burn scars.

Weiss
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