Mesoscale Precipitation Discussion 0861
NWS Weather Prediction Center College Park MD
304 AM EDT Tue Sep 18 2018

Areas affected...southern MN/WI into northern IA/IL

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

Valid 180703Z - 181300Z

Summary...An increasing threat for training rainfall and flash
flooding will develop through 12Z across southern MN/WI into
northern IA/IL. Rain rates of 1-2 in/hr are expected to lead to
2-4 inches through 13Z for a few locations.

Discussion...Regional radar across the upper Midwest at 0630Z
showed a narrow, discontinuous axis of elevated convection
extending from near MVE to MSP into western WI between EAU and
LSE. Trends show the coverage of storms becoming more widespread
over the past 30-60 minutes, located at the nose of a 25-35 kt low
level jet overrunning a warm/stationary front extending west-east
across northern Nebraska into the upper Midwest. Where convection
was ongoing, MUCAPE was estimated to be between 1000 and 2000 J/kg
per the SPC mesoanalysis and precipitable water values were
increasing via GPS measurements of 1.5 to 1.7 over northwestern IA
as of 06Z.

As a low-amplitude shortwave over WY/SD at 06Z continues toward
the ENE this morning, moisture will continue to increase across
the upper Midwest as low level flow veers from SSW to SW to WSW.
This orientation of the low level flow will become increasingly
close to the RAP forecast LFC-EL mean wind from the WSW at ~30 kt.
Convective coverage is forecast to increase through the morning
ahead of the shortwave with an ongoing small cluster of convection
propagating through SD and as low-mid level wind vectors become
more favorably aligned for training, the threat for flash flooding
will increase temporarily across MN/WI. Propagation toward the
southeast may occur later in the morning as forecast by most
hi-res model guidance, but only a select few models are even close
to resembling the ongoing precipitation pattern across the
north-central U.S. so overall confidence in the future evolution
is a bit lower than average. Regardless, a notable threat for
flash flooding will increase given radar trends and the fact that
much of the area Flash Flood Guidance values are only 2 to 2.5
inches in 3 hours.

Otto

ATTN...WFO...ARX...DMX...DVN...FSD...GRB...MKX...MPX...

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

LAT...LON   45319516 45249368 44879139 44388995 43588917
            43038912 42438996 42319108 42739373 43039497
            43749606 44959618