173
FXUS63 KUNR 201747
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1147 AM MDT Sun Sep 20 2026

.KEY MESSAGES...

- Showers will gradually decrease in coverage overnight.

- Areas of fog will continue through Monday morning. Reduced visibilty
  expected around the Black Hills and across the western South
  Dakota plains for the Monday morning commute.

- More clouds than sun to start the week, followed by warmer and
  drier conditions later in the week.

&&

.DISCUSSION...(This Afternoon Through Saturday)
Issued at 1144 AM MDT Sun Sep 20 2026

Water vapor and upper level analysis depicts a low centered over
Wyoming, a ridge extending up the West Coast, and another low just
off-shore of California. At the surface, the low is over the western
Nebraska Panhandle with a weak trough extending northwestward along
the Bighorns. Latest radar imagery has a large swath of rain over
the western and central South Dakota plains, rotating counter-
clockwise around the upper low.

For the rest of today, the main band of rain over the South Dakota
plains will continue to lift northward, being replaced by a few
smaller, less organized showers for the afternoon and evening hours.
Rain showers across northeastern Wyoming will be less, as drier gets
pulled up from southeastern Wyoming. High temperatures will remain
cool with the damp conditions and abundant cloud cover, with no
deviation from blended guidance which is close to current temps.

The upper low opens up and drifts east overnight, which will in-turn
ends the rain showers from west to east. Lingering moisture in the
air, combined with surface pressure rises and light winds, will
result in areas of fog and low clouds into Monday morning.
Visibility will gradually improve late Monday morning, with the
lower ceilings and clouds sticking around throughout the day.

More clouds and some fog for MOnday night/Tuesday across the South
Dakota Plains and the southern Black Hills - with the Northern Black
Hills and portions of northeastern Wyoming becoming sunny. Clouds
will gradually decrease across the entire area through the day, with
increasing south-southeasterly winds. The strongest winds, 15 to 25
mph, will likely be across northwestern South Dakota where the
pressure gradient looks to be the tightest.

A weak shortwave trough moves through Wednesday and Wednesday night,
which could result in some showers across the area. The wave moves
through fairly quickly, thus, some areas should remain dry.

A drier, warmer pattern then sets up for the end of the week into
the weekend, with highs about 5 to 10 degrees above average.


&&

.AVIATION...(For the 18Z TAFS Through 18Z Monday)
Issued At 1126 AM MDT Sun Sep 20 2026

An upper level low continues to spin over the SD/WY border,
supporting widespread BR/DZ and showers today. Start time is not
certain yet, currently targeting early afternoon, and will last
into the evening. Ceilings will remain IFR through the forecast
period. Some transient bands of slightly higher/lower visibility
will be possible tied to any stronger areas of showers.

&&

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

$$

DISCUSSION...MLS
AVIATION...Schweigert