787
FXUS63 KUNR 212321
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
521 PM MDT Fri Aug 21 2026

.KEY MESSAGES...

- Upper ridge will dominate the weather through early next week.

- Isolated severe thunderstorms may occur late this
  afternoon/evening in/near the Black Hills to the SD/NE border.

- Isolated severe thunderstorms may occur over northeastern WY
  late Saturday afternoon and evening.

- Temperatures will average about 4-8F above normal.

&&

.DISCUSSION...(This Afternoon Through Thursday)
Issued at 1149 AM MDT Fri Aug 21 2026

Current upper-air analysis and water vapor imagery depict large-
scale ridging continuing to strengthen over the southwest. Satellite
imagery indicates partly cloudy skies behind the thunderstorms and
showers that moved through the region this morning. Current radar
scans show a few weak returns associated with showers/virga over
western SD. Surface observations show temperatures mainly in the
80s, with dewpoints in the 40s across northeast WY transitioning to
the low 60s across south-central SD. Winds are generally easterly
across western SD and southerly across northeast WY at 10 to 17kts,
gusting to 25kts. Campbell Co. in particular may see gusty
northeasterly winds up to 25kts, behind a weak frontal passage, and
minRH values in the upper teens this afternoon. Due to these
conditions, elevated fire weather concerns will continue to persist
through this afternoon for Campbell Co.

There is still uncertainty around today`s chances for thunderstorms
across southwestern SD, both in timing and severity, with
disagreement remaining among the CAMs. However, MLCAPE values of
~750-1250 J/kg, dewpoints in the 50s to low 60s in the SD plains,
and 0-6km bulk wind shear values around 55kts indicate that the
environment in southwestern SD could support thunderstorm
development and a supercell or two. The timing of the shortwave
today, the cap currently in place, and the strong vertical moisture
gradient between the SD plains and the Black Hills will all play a
large role in storm development this afternoon/evening and into the
overnight hours.

Due to these factors, there is a slight risk for severe
thunderstorms southeast of Rapid City down to the SD/NE border, with
the primary threats being large hail (up to 2") and damaging winds
(60-70+ mph). As a result, 15 percent POPs were added to the
southern Black Hills and into southwestern SD for this afternoon
into the evening.

Multiple disturbances riding along the ridge will keep scattered
thunderstorms chances in play for Saturday and Sunday, with
northeast WY favored for better coverage Saturday. As is typical for
thunderstorms in August, chances for precipitation will be hit-and-
miss. Cloud cover and storm placement will also drive day-to-day
temperatures, though highs are expected to remain above average
across most of the region through the weekend and into next week as
the ridge remains in place.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 521 PM MDT Fri Aug 21 2026

With today`s isolated thunderstorms pushing across the border into
Nebraska and mostly clear skies in their wake, attention turns to
the next potential round Saturday afternoon. CAMs support the best
chance of development holding off until late afternoon and mostly
focusing across northeast Wyoming, with otherwise VFR conditions
tonight and Saturday across the region and breezy southeast winds
Saturday as well. Any storm may contain brief vsby drops to MVFR
as well as hail and strong gusty/erratic winds to at least 40kts
late Saturday afternoon/evening. Have gone ahead and introduced a
PROB30 at KGCC starting at 22z/4pm.

&&

.FIRE WEATHER...
Issued At 1149 AM MDT Fri Aug 21 2026

Elevated fire weather concerns will continue through this
afternoon for Campbell Co., where gusty northeasterly winds up to
25kts will combine with minimum relative humidity values in the
upper teens. Thunderstorm chances increase across northeast
Wyoming late Saturday afternoon, increasing the dry lightning
risk, however, relative humidity values will increase to the
upper 30s to low 40s.

&&

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

&&

$$

DISCUSSION...Schultz
AVIATION...KSmith
FIRE WEATHER...Schultz