Mesoscale Precipitation Discussion 0867
NWS Weather Prediction Center College Park MD
907 PM EDT Wed Sep 19 2018

Areas affected...NRN IA...SRN MN...ERN SD...NE NEBRASKA

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

Valid 200106Z - 200506Z

Summary...Increasing ascent in a narrow zone of favorable
instability will overlap with wet antecedent conditions to promote
likely incidences of flash flooding.

Discussion...Areas of convection had been active throughout the
day, but were showing a tendency toward greater intensity and
coverage this evening as synoptic forcing increased over the
Plains and Upper Midwest. At 01Z thunderstorms producing
instantaneous rain rates of 1 to 2 inches per hour were scattered
throughout parts of NE/SD/IA, and bordering on MN. Over the long
haul, models suggest an active night within this narrow corridor
bounded by dry mid level air in the warm sector over southern Iowa
and stable air well into the cool sector over central Minnesota
and northeast South Dakota. In the near term, observational trends
suggested there will be ample opportunity for cell mergers and
upscale growth through 05Z, with multiple clusters of storms
training west to east or WNW to ESE along the instability gradient.

The 12Z NSSL WRF was doing a good job capturing most of the
activity, including thunderstorms in eastern Iowa where the
environment was less supportive, but lift will be increasing
overnight. Throughout much of this zone, Flash Flood Guidance
values had dipped to around 1.0 to 1.5 inches in 1 to 3 hours,
owing to 1 to 4 inches of rain that fell in the past 36 hours. The
overlap of new rainfall with wet antecedent conditions makes a few
instances of new flash flooding appear likely overnight. Even in
drier parts of eastern Nebraska the initial cells had shown some
supercell tendency, with slow deviant motion, and heftier rain
rates estimated above 2.0 inch/hr via dual-pol, although perhaps
with some hail contamination.

Overall, the synoptic environment will just become more supportive
in response to a tightening upper jet circulation and approaching
height falls, leading to a focused heavy rain event, and one that
may last many hours, with the heaviest rates occurring earlier in
the event.

Burke

ATTN...WFO...ARX...DMX...DVN...FSD...LBF...MKX...MPX...OAX...

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

LAT...LON   44089606 43079007 42439024 42449400 42349663
            41769893 42829952 43919757