FOUS30 KWBC 090755
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Sat Aug 9 2025

Day 2  Valid 12Z Sun Aug 10 2025 - 12Z Mon Aug 11 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM EASTERN
KANSAS NORTHEAST THROUGH SOUTHERN WISCONSIN...

...Central Plains through the Upper Midwest...
An extremely active pattern continues across the Plains and Upper
Midwest as the synoptic regime from D1 persists through the weekend.

An amplifying mid-level trough centered from ND through NM will
deepen to produce 500mb heights that may locally fall below the
10th percentile according to NAEFS. ALthough the primary vorticity
maxima in the core of the trough spins northeast into Ontario,
lingering impulses shedding back into the Four Corners maintain the
trough axis back to the west. This will persist mid-level SW flow
angling across the Central Plains and into the Upper Midwest, with
weak impulses rotating through the flow and persistent upper
diffluence within the RRQ of a poleward arcing jet streak
downstream of the primary trough axis providing deep layer ascent.
This pattern matches the conceptual model for a Maddox Frontal
rainfall event, and another day of heavy rainfall is likely Sunday
in portions of the region.

As the synoptic pattern stalls, a surface stationary front will
oscillate from WI through OK, providing a focus for convective
generation along the low-level convergent boundary. Additionally,
any weak impulse transiting the front could result in weak low-
pressure development along the boundary. This makes for a
challenging QPF and ERO forecast since the environment will be
extremely favorable for training heavy rainfall, but the exact
placement remains very much uncertain.

As far as the environment, thermodynamics are expected to be 
exceptional for the area supporting intense rainfall rates. PWs
above 2", or well above the 90th percentile, are progged to surge
northward on return flow ahead of the front, combining with MUCAPE
of 2000 J/kg to fuel widespread convective development as the 
850mb inflow/LLJ propagates northeast. Mean 0-6km winds will again 
be parallel to the boundary, although with wind speeds are expected
to be generally more progressive than on Saturday, reaching 15-20 
kts. However, bulk shear progged to reach 40 kts will support 
organized cells, and Corfidi vectors collapsed to just 5 kts and 
aligned against the mean wind indicate a high threat for 
backbuilding cells leading to longer duration of heavy rainfall and
slow net cell motions. With rainfall rates of 2-3"/hr expected, 
this could result in axes of 3-5 inches of rain with locally higher
amounts. This will likely result in scattered to numerous flash 
flood instances, with locally significant impacts possible should 
heavy rain train across urban areas.

There remains uncertainty into the placement of the heaviest
rainfall, and the inherited MDT risk was adjusted cosmetically for
the recent guidance to align both with the highest REFS/SREF
exceedance probabilities while also accounting for the expected
heavy rainfall footprint from D1. Additionally, some adjustments
were made to better reflect the CSU first guess field which pulled
the higher risk areas a bit SW from previous runs.

...Southwest...
A MRGL risk remains for portions of the Southwest on Sunday as
monsoonal moisture begins to return to the area leading to a more
active convective pattern. A mid-level trough extending down from
the Northern High Plains will continue to amplify Sunday, producing
lowered heights across the Four Corners and Desert Southwest,
through which weak spokes of vorticity will rotate. These lowered 
heights and locally enhanced ascent will impinge into an 
increasingly favorable thermodynamic environment to support 
convection with heavy rainfall. Low-to-mid level flow will 
gradually back more to the SW, allowing for a better draw northward
of PWs exceeding 1", which may spread as far north as the Mogollon
Rim and combine with MUCAPE of 500-1000 J/kg across the southern 
half of AZ and NM. This will support scattered to widespread 
showers and thunderstorms which will move slowly eastward through 
the day. With rainfall rates as much as 1"/hr at times, this could 
result in at least isolated instances of flash flooding, especially
across more vulnerable soils or terrain, and the MRGL risk was 
maintained and adjusted for new guidance.

...Southeast...
Another very active day across portions of the Southeast is
expected as tropical moisture (PWs above 2.25 inches) persists in
response to a wave of low pressure drifting across the eastern
Gulf. The mid-level pattern remains quite diffuse into Sunday, with
a col sitting over much of the Southeast in response to broad
ridging along the Atlantic Seaboard. This suggests that features
will move little during the period, and guidance indicates a weak
shortwave drifting along the GA/SC coast once again on Sunday.

850mb inflow from the southeast will maintain impressive moist
advection onshore, and this wind, while modest at 15-20 kts, will
generally exceed the mean 0-6km winds which will be just 5-10 kts.
This will provide a two-pronged effect on the heavy rainfall threat
in that the 850mb wind exceeding the mean wind will drive large
scale enhanced ascent, while the weak mean winds will additionally
support slow movement of convection that develops. With rainfall
rates likely peaking at 2-3"/hr (30% chance for 2"/hr from the
HREF), maximum rainfall of 3-5" with locally higher amounts is 
again possible on Sunday as reflected by SREF/GEFS/HREF/REFS
probabilities. Some of this may fall atop soils that are sensitive
from prior heavy rainfall on Friday and Saturday, leading to a
locally higher flash flood risk. The latest UFVS CSU First guess
field has a SLGT risk over coastal SC, which overlaps almost
identically with the SLGT risk on D1 and the highest REFS/HREF/GEFS
probabilities into D2 (when available). For this reason a targeted
SLGT risk was again added, primarily for the SC coastal plain.

Weiss


Day 3  Valid 12Z Mon Aug 11 2025 - 12Z Tue Aug 12 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHERN
MISSOURI THROUGH NORTHERN ILLINOIS AND SOUTHERN WISCONSIN...

...Central Plains through the Great Lakes...
The amplifying trough across the High Plains and the accompanying
cold front producing the heavy rainfall across the Plains/Upper
Midwest Saturday and Sunday will gradually pivot eastward on D3
/Monday/. 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. This suggests that another day of
training convection is likely along the front from Oklahoma 
through Michigan, but with somewhat reduced intensity. 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 western 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...
Rinse and repeat for the Southeast, especially in the vicinity of
South Carolina, Georgia, and the Florida Panhandle, as onshore 
return flow around an elongated high over the Atlantic spreads 
tropical moisture onshore. PWs are again progged to be 2-2.25
inches with overlapping MUCAPE of 1000-2000 J/kg. Within these
impressive thermodynamics, forcing for ascent will be provided both
by land/sea convergence axes but also downstream of two 
shortwaves: one moving near the SC coast and a second rotating 
northward from the Gulf into the FL Panhandle. Both of these mid-
level features may reflect to the surface as inverted troughs, 
providing additional convergence for slow moving thunderstorms 
containing rainfall rates of 1-3"/hr. There is quite a bit of 
spread among the global ensembles by D3 as to how far inland 
convection will track, and as to whether there will be any focused 
areas of heavier rainfall. For these reasons a broad MRGL risk 
remains across the area for at least isolated flash flood 
potential, especially since soils will be primed from heavy 
rainfall anticipated on D1 and D2.

Weiss
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