604
FXUS63 KUNR 261237
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
637 AM MDT Sat Sep 26 2026

.KEY MESSAGES...

- Near-normal to slightly above-normal temperatures through the
  weekend with highs mainly in the 70s.

- Another lobe of tropical moisture is expected early next week,
  supporting more chances for rain late Sunday into Monday.

- Slightly cooler next week.

&&

.UPDATE...
Issued at 1028 PM MDT Fri Sep 25 2026

Based on current trends in sfc obs and consensus between NBM and
HRRR guidance, somewhat generously added patchy fog to the
forecast across the South Dakota plains north and east of Rapid
City extending out toward Winner and the Missouri River corridor.
Overall coverage may be a bit less than last night. Areas of low
stratus clouds are also expected and are already observed on
satellite and sfc stations in some of the same areas. Confidence
on patchy fog and/or low stratus reaching as far west as the Rapid
City area is very low at this time, with best potential definitely
north/east.

&&

.DISCUSSION...(This Afternoon Through Thursday)
Issued at 1144 AM MDT Fri Sep 25 2026

Upper trough will remain in place through early next week over
the western CONUS, supporting mainly WSW flow aloft. Pattern
change expected toward next weekend as a strong NE Pac upper
trough dives into the Pac NW and supports a couple stronger upper
troughs and associated sfc low`s across southern Canada and the
northern CONUS. Lingering monsoon wave will phase with a stronger
upper trough moving across southern Canada today into Sat. Enough
diurnal instability will be in place to support isolated to
scattered showers/storms in the SW FA. However, the better chances
for rain will be to the east of the FA where deeper moisture is
in place. Associated cool front will move through Sat with breezy
NW winds developing over western SD for a bit. Drier air will move
into the region, supporting dry weather through most of Sunday
before the next upper trough and batch of tropical moisture
advects into the region. Upper trough will drop into the SW CONUS
Sunday and direct tropical moisture NE across the Rockies. The
path of this moisture and the location of best forcing is still
not settled in forecast models and will hinge on the upper closed
low`s track, which are difficult to predict for forecast models.
Hence, rain chances are likely to change in the next few model
cycles. The best window for rain will be Sunday night into Monday,
with things drying out after. Slightly cooler, but seasonal the
rest of the week. Models are still indicating a stronger cold
front next weekend, which could bring a widespread frost as the
upper trough over the NE Pac shifts east and forces cold air south
out of Canada. However, the degree of CAA with this system varies
substantially among the various forecast models and ensembles.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Sunday Morning)
Issued At 633 AM MDT Sat Sep 26 2026

VFR conditions are mainly expected for northeast Wyoming and far
western South Dakota through the TAF period.

That being said, low clouds and fog developed in NW SD, including
the Rapid City area and areas along the northeast Black Hills.
This fog will slowly diminish through the morning hours and should
be gone by midday.

Winds will shift to northwest through the morning as a cool front
slides through. This front will push clouds out of the area and
bring increased wind speeds that will last into the afternoon.

&&

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

$$

UPDATE...KSmith
DISCUSSION...JC
AVIATION...GS