536
AWUS01 KWNH 182150
FFGMPD
MIZ000-INZ000-WIZ000-ILZ000-MNZ000-IAZ000-190300-

Mesoscale Precipitation Discussion 0956
NWS Weather Prediction Center College Park MD
548 PM EDT Mon Aug 18 2025

Areas affected...eastern IA, southern WI, and northern IL into
surrounding portions of MN/IN

Concerning...Heavy rainfall...Flash flooding likely

Valid 182145Z - 190300Z

Summary...Thunderstorms with hourly totals of up to 1-2" are
expected to train/repeat with short-term totals of 2-5". Scattered
to numerous instances of flash flooding are likely (and locally
significant/life threatening flash flood concerns are particularly
elevated in the vicinity of the Chicago metro).

Discussion...Convection is proliferating once again today across
portions of the Upper Midwest, focused primarily in the vicinity
of a weak surface low pressure and an associated warm front. The
mesoscale environment very supportive of heavy rainfall with
plentiful instability (1500-5500 J/kg of SBCAPE), highly anomalous
tropospheric moisture (PWs 1.7-2.0", between the 90th percentile
and max moving average, per DVN sounding climatology), and
effective bulk shear of 20-40 kts (with the best shear farther
north along the warm front, and generally below 20 kts farther
south into the warm sector (southeast IA into north-central IL). A
shortwave and associated vorticity maxima aloft (located nearly
directly above the weak surface low @ 500 mb) will provide
additional forcing for lift, along with a bit of additional
divergence from an associated right-entrance region of a jet
streak (though the best upper-level divergence is displaced to the
northeast into northern WI/MI, where instability is much more
limited). As the low-level jet strengthens into the evening hours,
this should help to sustain updrafts and organize convection
(despite only conditionally unstable mid-level lapse rates) with a
series of linear segments of thunderstorms resulting in localized
west-to-east training (despite relatively strong steering flow
with 850-300 mb winds of 20 kts). With MRMS already indicating
localized hourly totals as high as 1-2" with 3-hr Flash Flood
Guidance (FFG) generally ranging from 1.0-2.0" across the MPD
area.

As the strongest convection (with hourly estimates as high as 2")
approaches the greater Chicago metro area, the risk for instances
of life threatening flash flooding are elevated late this
afternoon and evening. Nearly all of the latest hi-res CAM data
supports these significant flash flooding concerns, as the 18z
NAM-nest and hourly runs of both the HRRR and experimental RRFS
since then depict localized totals of 3-5" in the vicinity of the
Chicago metro (with the NAM-nest and some RRFS runs indicating the
potential for extreme localized totals of 5"+, which is also
reflected in the 18z HREF neighborhood probabilities of 5"
exceedance ranging from 10-20%). While southeast WI into northeast
IL is certainly the greatest area of concern for prolong/repeated
heavy rainfall, areas farther south and west are still likely to
flood from expected 2-4" localized totals (given aforementioned
FFGs of only 1.0-2.0" for much of the area). As a result,
scattered to numerous instances of flash flooding are likely.

Churchill

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

ATTN...WFO...ARX...DMX...DVN...ILX...IWX...LOT...MKX...

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

LAT...LON   44239208 44019148 43569066 43198951 43098863
            43098782 42518752 42038714 41728636 41038677
            40638724 40488814 40548896 40839010 40749126
            40979179 41359214 42859191 43049262 43369245
            43619248 43779265 43999272 44149246