563
FOUS30 KWBC 100056
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Sat Aug 9 2025

Day 1
Valid 01Z Sun Aug 10 2025 - 12Z Sun Aug 10 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
NEBRASKA THROUGH PORTIONS OF IOWA...

01Z Update: Current trends have allowed for a shift in the previous
MDT risk with a greater emphasis encompassing the NE/KS line up
through IA with the highest threat likely extending from southeast
NE extending northeast through southwest and east-central IA.
Stationary boundary remains situated across KS with a surface
reflection settled over northeast KS providing a consistent theta_E
flux in-of southeast NE and neighboring areas as noted via the
latest 00z surface analysis. MUCAPE between 3000-5000 J/kg will be
situated within a warm sector environment situated to the eastern
flank of the surface low, continuing until encroached upon by an
upscale convective complex currently maturing over the CO Front
Range up into southwest NE. Latest hi-res guidance indicates a
favorable upscale growth regime along the KS/NE line with discrete
cell development downstream of the main cluster as LLJ genesis
provides a favorable low-level shear mechanism for cells to develop
within proxy of the warm front located over southeast NE down into
northeast KS. Cell mergers eventually will occur with the
advancement of the complex overnight leading to a congealing heavy
convective core focused over that area of southeast NE and the KS
border. Latest iterations of the HRRR have been consistent in a
maxima of 2-4", locally upwards of 5" located across the area of
southeast NE into far southwest IA with 1-3" plausible over
southern NE up into the Missouri river area of far eastern
NE/western IA. A MDT risk not extends back over those
aforementioned areas where 2-4" is increasingly favorable with an
expansion to the northeast across IA where frontal positioning will
enact as a focal point for convective redevelopment overnight as
the LLJ cranks and noses northeastward out of the Missouri Valley.
Models have been wavering on specifics of where the heaviest will
occur in IA, but considering some of the antecedent wet soils in
place and the threat for heavy rainfall remaining elevated, the MDT
was sufficient given the above and incredibly favorable environment
in place.

Another area of interest will reside over southern WI as LLJ
initiation will likely interact with a residual outflow boundary
currently bisecting the southern portion of the state. Hi-res
guidance, including the the latest WoFS have been insistent on a
period of convective development along the confines of the
boundary as it remains parked over the same areas. The combination
of convergent low-level flow and mean flow relatively parallel to
the boundary in question, this would entice a period of slow-
moving, repeating cells over the area in an area from Milwaukee
over towards Madison down towards the IL border. This is where the
signal is greatest in the CAMs and was the greatest shift in probs
for >2" in the 18z HREF suite. The MDT was maintained across
southwest WI for the potential with the SLGT expanded eastward into
the Milwaukee metro, including the lake shores along I-94.

Minimal changes to the MRGL risk area over the Southwest U.S. and
the SLGT/MRGL across the Southeast as the pattern remains favorable
for isolated to scattered instances of flash flooding given the
persistent signal. MRGL over the Southwest U.S. was scaled back in
size to reflect the latest trends with a majority of the heavy rain
likely over southeast AZ terrain.

Kleebauer


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...

...2030z update...

The moderate and slight risk areas over the Upper Midwest/Middle
Mississippi Valley were expanded to the southeast and trimmed over
southern Wisconsin with this afternoon`s update. 12z HREF
signaled a southern and eastern shift in QPF max for the 24hr
period ending at 12z on Monday. Deterministic and ensemble 12z
guidance were closer to what was issued overnight, which allowed
for a less drastic change to the outlook areas with this
afternoon`s update.

In the Central High Plains, a slight risk was introduced to account
for antecedent rainfall from this evening`s storms. A deepening
trough axis and a over the Rockies should support scattered to
isolated storms. Modest instability and notable moisture flux could
produce efficient rainfall rates, but weak low level jet and bulk
shear won`t support long lived storms.

Wetter QPF trends over the Georgia/Carolina coast prompted a small
expansion of the slight risk over those areas. Coastal areas
between Georgetown and Wilmington could receive 3-5 inches with
isolated higher amounts possible. If trends continue then an
upgrade is likely along the coast in particular.

Kebede


...Previous Discussion...


...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...

...2030z update...

The Central Plains to Upper Midwest slight risk area remains mostly
unchanged with this afternoon update. 12z guidance keeps axis of
highest qpf mainly within the inherited outlook areas so not much
of an update necessary here. In the Southeast, a new slight risk
was added from near Georgetown to around Wilmington due to the
persistence of rainfall over the next few days with over 5 inches
possible through Tuesday morning.

Kebede

...Previous Discussion...

...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


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt