Day 1 Severe Weather Outlook

Valid 211300Z – 221200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS
AFTERNOON/EVENING FROM KS TO IA…AND ACROSS PARTS OF MN AND
NORTHWEST WI…

…SUMMARY…
Large hail, damaging winds, and perhaps a couple of tornadoes are
possible this afternoon/evening from Kansas to Iowa, and across
parts of Minnesota and northwest Wisconsin.

…MN/northwest WI this afternoon/evening…
A midlevel shortwave trough over the northern High Plains this
morning will progress to northern MN by early tonight, as an
associated surface cyclone develops east-northeastward from ND to
western ON, and a trailing cold front moves eastward across MN late
this afternoon into tonight. Widespread overnight convection has
produced a cold pool across northwest MN, and convection to the
south near the SD/MN border is expected to weaken this morning.

Additional storm development is possible in advance of the cold
front and southeast of the remnant cold pool this afternoon as
modest surface warming occurs in cloud breaks, with MLCAPE up to
2000-3000 J/kg in the warm sector. Modest forcing for ascent
suggests storm coverage may remain rather isolated, while effective
shear greater than 40 kt and storm motions off the front also
support semi-discrete modes. The main uncertainty will be the
degree of cloud breaks/surface heating and storm coverage. Will
maintain the Slight risk area for the conditional threat of damaging
winds/large hail. A tornado or two will also be possible in the
moist low-level environment with 0-1 km SRH of 100-150 m2/s2, though
low-level flow will tend to veer and reduce hodograph curvature in
closer proximity to the cold front this afternoon.

…KS to IA through tonight…
A trailing shortwave trough over southern NV/UT will move eastward
to the central High Plains by Sunday morning. In the wake of the
separate shortwave trough over the northern Plains, a surface cold
front will move southeastward across NE/KS, though the front should
slow by late afternoon/evening in response to lee cyclogenesis
across southeast CO. A narrow corridor of stronger surface heating
is expected across KS along the front and to the northwest of the
thicker cloud band with the subtropical moisture plume from OK into
MO, and storm development will become more probable after about 22z.
Boundary-layer dewpoints at or above 70 F, steep midlevel lapse
rates, and strong deep-layer shear with long hodographs will favor
supercells initially, capable of producing isolated very large hail,
damaging winds, and perhaps a tornado or two. Storms should grow
upscale into more cluster/linear modes overnight, with a gradual
decrease in the hail/wind threat.

..Thompson/Leitman.. 09/21/2019

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