Mesoscale Precipitation Discussion 0717
NWS Weather Prediction Center College Park MD
751 PM EDT Fri Aug 24 2018

Areas affected...Western and Central WI

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

Valid 242350Z - 250250Z

Summary...Convection training in deepening moisture ahead of a
short wave could produce localized flash flooding into the evening
hours.

Discussion...Regional radars across southeast MN into central WI
showed convection forming in a nose of 500-1000 J/KG of MUCAPE
extending from southwest MN into central WI. The instability is
more surface based over southeast MN, but elevated over central
WI, where the airmass is stable in the lowest layer (as evidenced
by the CIN on the most recent RAP stability analysis.

The convection sits in an axis of 1.50 inch precipitable water,
and soundings from the HRRR showed a moist column through about
500 mb. This suggests that warm rain processes are occurring above
the stable layer near the surface. The convection across west
central WI has been tracking northeast, along with the mean
850-300 mb wind. As propagation vectors become better aligned with
the mean wind, the potential for training increases. This will
pose as the main flash flood threat for the evening hours.

The main forecast issue concerning more than an isolated flash
flood threat is the presence of stability to support the
convection into the evening hours. The convection has remained
intact as it has become elevated, aided by synoptic scale lift
ahead of a potent short wave over MN. The convection has already
produced hourly rainfall rates near 2.00 inches over southern St.
Croix and northern Pierce counties in west central WI. As the
convection moves east, individual cells will move northeast,
creating the flash flood threat.

Much of the high resolution guidance has either underplayed the
current convection (the most recent HRRR) or is lagging the
activity by a couple of hours (12z WRF ARW). The 21z GEM regional
appears to have the best handle on the activity, suggesting that
hourly rainfall rates in excess of 1.50 inches are possible
through at least 01z. Since there is some question about the
convective coverage (due to the lack of deeper instability), flash
flooding is only considered possible.

Hayes

ATTN...WFO...ARX...DLH...GRB...MPX...

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

LAT...LON   45909017 45688975 45068932 44458964 44309050
            44449226 45199243 45759143