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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.