848
FXUS63 KUNR 260428
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1028 PM MDT Fri Sep 25 2026

.KEY MESSAGES...

- Near average temperatures through the weekend with highs mainly
  in the 70s.

- Another lobe of tropical moisture expected early next week,
  supporting more chances for rain Sunday and Monday, starting
  especially Sunday evening.

- 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 06Z TAFS Through 06Z Saturday Night)
Issued At 945 PM MDT Fri Sep 25 2026

VFR conditions are expected for northeast Wyoming and far western
South Dakota through the TAF period. Low clouds will stick around
across the south-central South Dakota plains, expanding west into
parts of northwest and southwest South Dakota during the
overnight hours tonight. Patchy fog and low cigs may lead to
IFR/LIFR conditions in this same region through Saturday morning.
At this time, the fog and low stratus are not expected to reach
KRAP, but once again it may be close. Conditions will improve west
to east, with most areas returning to VFR by the end of the TAF
period. Winds will shift to northwest Saturday morning.

&&

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

&&

$$

UPDATE...KSmith
DISCUSSION...JC
AVIATION...Dye