Mesoscale Discussion – possible heavy rains and local flash flooding across northern Illinois/southern Wisconsin this Monday evening

As the warm front slowly makes its way north through northern Illinois, strongest thunderstorms developing along and ahead of the frontal boundary could not only trigger severe storms, but also localized flood-producing downpours of 1 to 2-inches in less than an hour.

The National Weather Prediction Center has issued a technical Mesoscale Discussion on the potential heavy rain conditions over our area…

Mesoscale Precipitation Discussion 0702
NWS Weather Prediction Center College Park MD

Areas affected...Northwest IL & Southwest WI

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

Valid 6:15PM - 9:15PM CDT...

Summary...Showers and thunderstorms are broadening in coverage
across northwest IL and southwest WI.  Hourly rain totals to 2"
with local amounts to 5" are possible through 9PM CDT.

Discussion...The combination of a deep layer cyclone in IA and a
shortwave moving through southwest Ontario have led to a very
divergent environment aloft over this region.  Thunderstorms in
southern WI have shown rotation while convective coverage
increases region-wide.  Precipitable water values are 1.5-1.75"
per GPS data.  Inflow is cyclonic out of the south to southeast
near and north of an occluded front at 15-25 kts, importing ML
CAPE values of 500-1500 J/kg into the area.  Effective bulk shear
of 25-40 kts exists here, which appears to have led to the meso in
southern WI and short bands across eastern IA and northern IL
which have occasionally shown some backbuilding.

The mesoscale guidance appears underdone on areal coverage of
convection at this time.  Despite that, more than a few pieces of
mesoscale guidance have a signal for 4-7" of rain near southwest
WI during the next several hours.  Available moisture and
instability should allow for hourly totals to 2".  There is a risk
of the two bands across IA/northern IL and WI merging over time,
which would enhance rain totals from west to east.