Mesoscale Precipitation Discussion 0702
NWS Weather Prediction Center College Park MD
715 PM EDT Mon Aug 20 2018

Areas affected...Northwest IL & Southwest WI

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

Valid 202314Z - 210314Z

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 03z.

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.  Flash flood
guidance is modest in this area, and could be overwhelmed with 1-2
hours of training, a mesocyclone or two, or cell mergers. 
Considering the available instability, ML CAPE values could become
exhausted should coverage increase further during the next four
hours, which defined the time horizon of this MPD.

Roth

ATTN...WFO...ARX...DVN...LOT...MKX...

ATTN...RFC...NCRFC...

LAT...LON   43418876 43298794 42948771 42328767 41788745
            41478741 41078808 41488938 41879044 41579193
            42619131 43299009