438
ACUS01 KWNS 210542
SWODY1
SPC AC 210540

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1240 AM CDT Thu Aug 21 2025

Valid 211200Z - 221200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
NORTHERN PLAINS INTO MINNESOTA...

...SUMMARY...
Isolated severe thunderstorms are possible across portions of the
northern Plains into Minnesota later this afternoon and evening.
Severe gusts and hail are the primary concern.

...Northern Plains/Minnesota...

Northern Plains upper ridge will flatten considerably during the
day1 period as heights fall ahead of a digging upper trough. Early
this morning, water-vapor imagery depicts the short wave along the
BC/AB border. This feature is forecast to advance into eastern SK by
22/00z before it advances to the SK/MB border by the end of the
period. While the primary large-scale forcing will be noted north of
the international border, high-level diffluent flow will be noted
across the eastern Dakotas/Minnesota - coincident with the front
that will be draped across this region at peak heating. Latest model
guidance suggests a plume of steep low-level lapse rates will extend
across the central Plains into south central SD where surface
temperatures are expected to soar through the mid 90s. The majority
of CAM guidance struggles to develop more than isolated convection
along the surface front - possibly due to the aforementioned weak
large-scale ascent. In addition, LLJ does not strengthen appreciably
ahead of the boundary, and this also supports isolated coverage.
Notably, during the overnight hours, robust convection may develop
within a weak warm advection zone across eastern SD into southwest
MN - possibly in response to a bit more forcing along the southern
fringe of the digging upper trough after 06z. Wind/hail will be the
primary threats with both the late-afternoon and overnight
convection. Isolated storm coverage favors MRGL Risk at this time.

Upstream, surface front will trail across the southern Black Hills
into central WY by peak heating. Northeast upslope flow should
encourage convection to develop across the higher terrain by late
afternoon. While forecast buoyancy is not expected to be
particularly strong, adequate deep-layer shear will prove supportive
of organized updrafts that could generate hail/wind.

..Darrow/Lyons.. 08/21/2025

$$