SPC AC 161557
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1057 AM CDT Thu Sep 16 2021
Valid 161630Z - 171200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF MN
SOUTHWARD INTO EASTERN SD AND NORTHERN NE...
Severe storms will be possible late this afternoon and evening
particularly across Minnesota into southeast South Dakota and parts
Relatively fast zonal flow extends from the northern Rockies across
the northern Plains today, with multiple fast-moving shortwave
troughs embedded in this flow. A surface low over SD will migrate
eastward into MN later today, with an associated cold front
providing the focus for scattered thunderstorms. Activity will
begin by mid/late afternoon over northern and central MN, where
strong heating is occurring. Forecast soundings show relatively
steep mid-level lapse rates and MLCAPE values of 1500-2000 J/kg in
this area. Vertical shear profiles are also quite strong,
supportive of supercells capable of large hail, damaging winds, and
a few tornadoes. Initial storms may be discrete for a few hours
before linear forcing mechanism and merging cold pools result in a
squall line. Storms are expected to weaken this evening after dark
as instability diminishes over northeast MN.
Full sunshine is present today over most of SD/NE, leading to rapid
heating and destabilization. A cold front will sag into this region
this evening, providing the focus for scattered thunderstorms.
Model guidance suggests that most of the storms will be behind the
surface front and somewhat elevated. Nevertheless, hail and gusty
winds may occur in the stronger cells for a few hours.
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