060
AWUS01 KWNH 212234
FFGMPD
WIZ000-ILZ000-IAZ000-220400-

Mesoscale Precipitation Discussion 1108
NWS Weather Prediction Center College Park MD
634 PM EDT Fri Aug 21 2026

Areas affected...Southern WI...Northern IL

Concerning...Heavy rainfall...Flash flooding possible

Valid 212230Z - 220400Z

SUMMARY...Potent thunderstorms ahead of an approaching cold front
will be capable of producing up to 2"/hr rainfall rates and
localized max rainfall totals of 2-3" through this evening. Some
training thunderstorm activity is possible, potentially leading to
areas of flash flooding this evening. Urban locations and poor
drainage spots are most at-risk.

DISCUSSION...22Z surface analysis showed a cold front racing south
and east through the heart of the Midwest being the culprit for
initiating scattered thunderstorm development this afternoon. Some
storms have formed out ahead of the front in far southern WI and
northern IL as well. RAP mesoanalysis indicates the air-mass ahead
of the front is highly unstable with MLCAPE approaching 3,000 J/kg
over Iowa`s nose. PWs are above 1.5" from the IA/IL border on east
across northern IL. The 12Z DVN sounding showed a sounding
favorable for discrete cells with effective layer shear between
35-40 kts and effective SRH >100 m2/s2. The sounding also showed
the warm cloud layer was generally 10,000ft deep.

Between the shear, more shallow warm cloud layer, and drier
mid-level RH values, the soundings suggests the environment
overall should limit the areal extent of the flash flood
potential. Not only that, but mean cloud layer winds are 26 kts,
so most storms will be moving along. That said, storms firing
ahead of the cold front have moistened some of the soils in
southern WI and northern IL to the point where additional storms
that have produced between 1-1.5"/hr rainfall rates. 1-hr FFGs in
some of the more urban areas of southern WI and northern IL are as
low as 1.5". Latest 18Z HREF does show some low-to-moderate chance
probabilities (20-40%) for 3-hr QPF surpassing 3-hr FFGs near the
Chicago/Milwaukee metros.

With these storms also encroaching upon the more populous areas of
southern WI and northern IL, excessive rainfall rates approaching
2"/hr could lead to flash flooding in poor drainage areas within
more heavily urbanized environments. Given those factors,
progressive but potentially multiple rounds of storms could lead
to some cases of flash flooding this evening.

Mullinax

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...ARX...DVN...GRB...LOT...MKX...

ATTN...RFC...MSR...NWC...

LAT...LON   43968929 43898887 43808864 43608831 43368800
            42808769 42358764 41978761 41628771 41298823
            41188900 41318984 41499039 41619076 41879125
            42269135 42609111 42899066 43419019 43678993
            43898968