Storms later this evening could not only produce severe weather, but also heavy rainfall that could cause flash flooding

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Mesoscale Precipitation Discussion 0460
NWS Weather Prediction Center College Park MD
529 PM EDT Thu Jul 09 2020

Areas affected...Southern WI....northern IL...far eastern IA

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

Valid 092130Z - 100230Z

Summary...Storms training along a boundary in a high moisture
environment could produce hourly rainfall rates near 2.00 inches,
which poses a flash flood threat into the evening hours.

Discussion...Storms continue to develop/propagate along a boundary
across southern WI into far northwest IL ahead of a strong short
wave located over central IA. Ahead of the storms, the most recent
RAP analysis showed a reservoir of 2500/3500 J/KG of MLCAPE in
place, and the combination of synoptic scale ascent associated
with the short wave and instability should be sufficient to
support continued development across southern WI/northern IL into
the evening hours.

A 20 knot low level southerly flow replenishes to 1.50/1.75 inch
precipitable air into the pre convective environment, which should
be enough to support hourly rainfall rates between 1.50/2.00
inches. The most recent KARX hourly rainfall estimates have been
as high as 2.00 inches, though mesonet observations has been
closer to 1.50 inches over far southwest WI.

Within the southerly flow ahead of the short wave, short term
training and cell mergers are expected, mainly across southern WI
through about 10/02z or so. Where the mergers and training occur,
hourly rainfall rates could remain near 1.50 inches (which is
depicted by the most recent HRRR runs). The HRRR runs also
indicated the potential for local 3.00 inch rainfall amounts just
north of the WI/IL line. These amounts are plausible were training
and cell mergers continue. The National Water Model continues to
show streamflow increasing to well above normal within the
convection.

Further south across northern IL, the mid level flow may veer
enough with time to allow the storms to become more progressive,
as they tap the deeper instability still in place and become more
outflow driven. However, local 2.00 inch rainfall amounts are
possible here as well.

The potential for flash flooding could continue past 10/02z across
portions of the area based on the most recent HRRR output, but it
should be noted that much of the high resolution guidance lags
reality by a couple of hours. Based on the above, flash flooding
is considered possible this evening.

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