FOUS30 KWBC 100835
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
435 AM 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, AND THE COASTS OF THE CAROLINAS...

...Central Plains through the Great Lakes...
The anomalous trough drifting slowly eastward from the High Plains
will weaken slightly as heights rise across the Central Plains
Monday. This will have limited overall affect on the downstream
rainfall, however, as spokes of vorticity embedded within the flow
and the accompanying surface front remain in place to drive ascent,
with only slow eastward advance expected through D2.

Despite this eastward translation, pinched 850-500mb flow between 
this trough and a ridge to the east will maintain efficient moist 
advection out of the Gulf, albeit with PW anomalies lower than 
forecast for the weekend (PWs around 1.75 inches or 0.5 to 1.0
standard deviations above the climo mean). This suggests that 
another day of training convection is likely along the front from 
Oklahoma through Michigan, but with somewhat reduced intensity from
prior days. Another challenges for D2 is the uncertainty in frontal
position and convective timing, as lingering boundaries from
Sunday's convection (with additional uncertainty in placement) will
play an important role into the training potential and heavy rain
axes on Monday. This suggests that despite a still favorable
synoptic pattern and elevated thermodynamics, the SLGT risk remains
the highest needed category for the D2 ERO.

On Monday, rainfall rates will likely still peak above 1"/hr at 
times, which through training could produce 1-3 of rainfall 
(ECENS/GEFS probabilities for 3+ inches below 10%), and where this 
rain overlaps with heavy rain from the weekend (most likely on the 
W/SW edge of the SLGT risk area), scattered instances of flash 
flooding are possible. Otherwise, any cells training along the 
front could produce isolated responses and a broad MRGL risk draped
along the length of the front into the southern High Plains continues.

...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...

...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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