941
FXUS63 KUNR 021705
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1105 AM MDT Tue Sep 2 2025

.KEY MESSAGES...

- Cold front moves through tonight with a chance of showers and
  thunderstorms
- Dry cold front Thursday brings gusty northwest winds (gusts
  40mph+) with elevated fire danger

&&

.DISCUSSION...(Today Through Monday)
Issued at 214 AM MDT Tue Sep 2 2025

08z surface analysis had weak high moving into the central Plains
and a trough slipping into northern MT/ND. Mostly clear skies
over the CWA except for some patchy fog that is trying to poke
into south-central SD from NE. Water vapour loop had upper ridge
over the four corners region with a mean upper trough from north-
central Canada into the northeast CONUS. Embedded upper low over
northern SK/MB is the main short term forecast concern.

Today through Wednesday, upper low moves southeast into the
Midwest. It will push a pre-frontal trough into the CWA this
afternoon with weak buoyancy ahead of it, albeit with decent
MLCIN. Might be a stray shra/TS very late this afternoon, but main
PoP chances will be tonight as cold front moves through in
response to primary synoptic forcing ahead of upper low. Post
frontal environment will have increasing effective shear, but
weak MUCAPE. Chance PoPs still look reasonable with embedded TS.
Lingering PoPs into Wednesday, but drier air will filter in
bringing an end to any precipitation. Temperatures will be ~5F
above normal today and 5-15F below normal Wednesday, except for
the far southwest where cold air advection is weakest.

Thursday, relatively fast north/northwest flow aloft between
upper ridge to our west and upper trough over eastern North
America. Embedded shortwave trough will push a dry cold front
through the CWA. Tight pressure gradient combined with moderate
isallobaric forcing/25-40kt 800mb winds should spawn gusty
northwest winds. Wind headlines possible. NBM has 40-90% chance
of >40mph gusts over western SD with 30-50% chance of >55mph gusts
over the usual squeeze areas of northwest SD. That may be
overdone given LREF probabilities. It is like the atmosphere
knows the first week of meteorological Fall has arrived. Timing of
front will modulate temperatures, so confidence not particularly
high.

Cooler temperatures Friday are replaced by a warming trend into
early next week per upper/thermal ridge nosing into the northern
Plains. Temperatures will remain seasonal with minimal chances for
precipitation given lack of forcing/moisture per preponderance of
ensemble guidance.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1104 AM MDT Tue Sep 2 2025

Mainly VFR conditions are expected through the period. A cold front
moves through tonight, bringing some smoke southward from Montana
and Canada. Isolated to scattered showers and thunderstorms are
possible after 22Z.

&&

.FIRE WEATHER...
Issued At 214 AM MDT Tue Sep 2 2025

A dry cold front will move through Thursday. Minimum relative
humidities will dip into the 20s with gusty northwest winds
potentially exceeding 40mph (40-90% chance western SD plains).
Elevated fire danger is expected, especially for the fine fuels of
the grasslands.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...Smith
FIRE WEATHER...Helgeson