Mesoscale Precipitation Discussion 0769
NWS Weather Prediction Center College Park MD
1206 AM EDT Wed Sep 05 2018

Areas affected...Southeast MN into North-Central WI and the U.P.
of MI

Concerning...Heavy rainfall...Flash flooding likely

Valid 050405Z - 050905Z

Summary...Flash flooding ongoing will likely become more
widespread and significant over the next several hours. Bands of
heavy rain from thunderstorms will persist and likely increase in
coverage while training to the northeast through parts of central
Wisconsin and the Upper Peninsula of Michigan. Upstream rainfall
of 3-6 inches is indicative of the potential through the threat
area, and flash flooding is likely overnight.

Discussion...Satellite and radar imagery confirm clusters of heavy
showers and thunderstorms oriented NE to SW from N Central
Wisconsin through SE Minnesota, northern Iowa, and into Nebraska
and Kansas. The heaviest rainfall is lifting into the threat area,
and the latest radar estimates indicate rainfall rates of 1-2
inches per hour as seen from the KARX WSR-88D. Recently, a
rainfall observation of 6 inches was reported near Fall Creek
which is evidence of the potential through the overnight period.

This cluster of convection is likely to persist and intensify
during the next few hours in a very dynamic environment. Strong
300mb jet level diffluence is evident on GOES-E WV imagery, which
combined with PVA ahead of a shortwave lifting across the area and
convergence ahead of a 700mb trough axis will drive strong omega
through much of the column. At the same time, the latest RAP
soundings depict 850mb winds increasing to 30-40 kts, driving
strong moist air advection with PWATs rising above 2 inches in the
pre convective environment. This WAA will combine with increasing
FGEN between 850-700mb and the right rear quadrant of the upper
jet impinging on the area to drive strong lift, aided additionally
by MLCape over 1000 J/kg.

Although 0-6km storm motions are forecast to increase to near 40
kts, forward propagating corfidi vectors are aligned nearly
parallel to the mean storm motion, suggesting that training of
thunderstorms is likely. Latest high-res guidance is in good
agreement that widespread 3-5" of rainfall will occur, in a few
places potentially in as little as 2 hours, which far exceeds the
local 3-hr FFG guidance of 1.5 to 2.5 inches. Locally, more than
6" of rainfall is possible, and flash flooding may become
significant if these higher rainfall totals are reached.

Weiss

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

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

LAT...LON   47028825 46908766 46708738 46518721 46468665
            46158667 46048683 46048681 45768732 45438812
            45008894 44648961 44299043 43939119 43699167
            43449238 43579323 43999327 44629299 45309235
            45829141 46099089 46548983 46848895