Heavy rainfall poses a flooding threat overnight across portions of northern Illinois and southern Wisconsin

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Mesoscale Precipitation Discussion 0850
NWS Weather Prediction Center College Park MD
1046 PM EDT Thu Sep 12 2019

Areas affected...Northern Illinois...Southern Wisconsin

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

Valid 130246Z - 130746Z

Summary...A line of thunderstorms will be capable of producing
hourly totals up to 2" across southern Wisconsin and northern
Illinois this evening. This intense rainfall over areas with
reduced flash flood guidance and near urban centers may lead to
flash flooding.

Discussion...GOES IR imagery this evening showed convective cloud
tops associated with a squall line still cooling, particularly
over far eastern Iowa, southwest Wisconsin, and northwest
Illinois. Radar estimates from KARX/KMKX/KDVN showed the highest
rates to be around 2-2.5" with a more broad area of 1-2"/hr
amounts. A few nearby observations showed in the last hour
1.5-2.0" have fallen across portions of southern Wisconsin.

The best instability resides across northern Illinois into far
southern Wisconsin where upwards of 2000 J/kg exists. MUCAPE of
around 1500-2000 J/kg extends further north to around the I-90
corridor. Ahead of the line, moisture remains anomalously high
with PWs of 1.7 to 1.9" given dewpoints in the lower 70s (northern
IL) to upper 60s (southern WI).

Over the next several hours, a wedge of favorable environmental
parameters will exist across northern IL and southern WI as the
flow becomes increasingly unidirectional along with an increase in
the low level jet through 07-08Z should favor more efficient rain
producing thunderstorms.

Guidance through 08Z shows widespread areal totals of 1-2" with
local amounts up to 3-4"  across far northern IL into southern WI.
This is likely to fall over areas that have seen quite a bit of
rain in recent days (7-day departures are 200-400 percent of
normal) and as such, the latest FFG is reduced down to 1-1.5" for
1-hr. This seems attainable given what's happened upstream,
current radar trends and supported by the latest HREF
probabilities which are very high for 1" in 1 hour and moderate
for 2" in 1 hour through 06Z. As a result, some instances of flash
flooding will be possible through the early overnight hours.


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