508
FXUS63 KUNR 092342
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
542 PM MDT Tue Sep 9 2025

.KEY MESSAGES...

- Warm temperatures through at least Thursday.

- Precipitation chances increase Friday into Saturday with
  temperatures cooling to near normal readings.

&&

.DISCUSSION...(This Evening Through Tuesday)
Issued at 117 PM MDT Tue Sep 9 2025

Upper level analysis has a ridge axis along the eastern Rockies
with a trof extending through the Mississippi River Valley on the
east and a deep trof and closed low over the Pacific Northwest
extending down the California coast. Water vapor imagery is
showing a few impulses rounding the ridge and pushing toward the
area from the west. At the surface, an area of low pressure is
located over northeastern South Dakota with a frontal boundary
extending westward from the low across northwestern South Dakota
into northeastern Montana.

For later this afternoon and tonight, the upper ridge will continue
to build and move eastward into the western High Plains as the
frontal boundary sags southwestward through the forecast area. Weak
impulses in the flow combined with frontal boundary, and steep
low/mid level lapse rates around 8 deg C/km and destabilization via
daytime heating will result in a few isolated high-based storms over
the Black Hills and will allow a few showers and storms (also high-
based) to move into the southwestern portion of the forecast area
from the higher terrain in Wyoming. The activity will mainly wane
overnight, with just some lingering ACCUS into the morning.

For Wednesday, the upper ridge axis begins to slide further eastward
into the eastern Dakotas as the upper low over the Pacific Northwest
deepens and slides into northern California. At the surface, a low
develops over southeast Montana and a lee trough extended down the
eastern slopes of the Bighorns. This will increase the pressure
gradient over the western South Dakota Plains, resulting in gusty
southeast winds during the afternoon hours. Right now, ensembles are
giving a 15-30% chance for winds to reach advisory criteria, with
most ensembles indicating speeds will likely remain in the 15 to 25
mph range, therefore will forgo any wind highlights at this time.
Again, CAMS are showing some convection over the higher terrain of
Wyoming moving into the forecast area late in the afternoon/early in
the evening. The majority of the convection should remain over
Montana and western North Dakota, closer to the surface low, but
isolated showers and storms can`t be ruled out.

The upper ridge moves well east of the forecast area by Thursday and
Friday as the upper low moves toward the Great Basin area and the
surface low lifts into western North Dakota. Southwest flow aloft
will allow for moisture to advect into the area, resulting in a
little more cloud cover (Pt Cldy) skies. Isolated showers and storms
will still be possible, with weak impulses moving through the flow.
Temperatures remain warm, with highs remaining above average.

The progression of the upper low from the Great Basin into the
Northern/Western Plains continues to slow with each model run, with
most of the ensembles indicating the impacts will begin in the
Friday night/Saturday time frame now. Moisture in the southwest flow
will continue to advect into the area, resulting in PWATs greater
than 150% of average. Moisture combined with the terrain and
frontal lift will result in some showers and storms for the
weekend. Temperatures will be cooler, but likely near to slightly
below average. Overall, there is still quite a bit of uncertainty
on the evolution of this low, as some models are keeping it
further north, which would result in less precipitation across the
area. Will continue to monitor the latest trends and adjust the
forecast accordingly.

Unsettled weather lingers into Sunday, before weak ridging ahead of
the next western trof brings drier conditions to the area early next
week.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Wednesday Evening)
Issued At 540 PM MDT Tue Sep 9 2025

VFR conditions will persist through the TAF period. Isolated
thunderstorms are expected to continue through 06Z across portions
of eastern WY. Winds will shift more southeast by 04Z along a
stationary boundary. The pressure gradient will increase tomorrow
morning resulting in gusts up to 25 kts at all terminals.

AEH

&&

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

$$

DISCUSSION/KEY MESSAGES...Smith
AVIATION...Herdliska