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FXUS63 KUNR 211121
AFDUNR

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

.KEY MESSAGES...

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

- Thunderstorm chances will be very limited through Saturday
  morning.

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

&&

.UPDATE...
Issued at 1035 PM MDT Thu Aug 20 2026

Latest guidance continues to hint that enough moisture and
instability may be available for a thunderstorm or two to fire
over the central or southern Black Hills Friday afternoon
(potential initiation perhaps trending later...now favored after 3
PM), tracking southeast across the adjacent plains into the
evening as far east as perhaps Mellette or Todd counties. This
scenario remains washed out in NBM guidance, so have updated
PoPs/QPF/Wx grids accordingly. West of the Black Hills, and north
of I-90 in South Dakota, confidence remains moderate to high in
dry conditions holding with the ridge of high pressure in place.
Expect another hot day with highs in the 90s.

&&

.DISCUSSION...(This Evening Through Thursday)
Issued at 1217 PM MDT Thu Aug 20 2026

Current upper-air analysis and water vapor imagery depict continued
large-scale ridging over the southwest, with drier air pushing into
northeast WY and far western SD. Satellite imagery indicates mostly
clear skies with only a few mid-to-high clouds. Surface observations
show temperatures in the mid 80s to low 90s, with dewpoints in the
30s across northeast WY transitioning to the low 60s across south-
central SD. Winds are generally northwesterly to northerly at 5 to
10kts, gusting to 20kts across much of the region. Portions of
northwest WY and southwestern SD may see wind gusts up to 23kts,
where minRH values could be as low as the mid teens. Due to these
conditions, elevated fire weather concerns will be possible across
portions of northeast WY and southwestern SD.

Today will mark the beginning of above average temperatures for the
region, generally ranging 4-8F above average, with drier conditions
as the ridge continues to build off to the west. The chances for
thunderstorms are unlikely today due to rising heights and lower
dewpoints, with a slightly better chance for isolated initiation
Friday afternoon over the Black Hills, where MLCAPE will be nearing
1000 J/kg and shear will increase throughout the afternoon. As a
result, 15 percent POPs were added to the southern Black Hills and
into southwestern SD for Friday afternoon into the evening. The CAMs
show some disagreement on Friday`s thunderstorm chances across south-
central SD, so POPs were lowered to below 15 percent. This will have
to be reevaluated as newer runs come in. Friday will also see
elevated fire weather concerns for northeast WY and southwestern SD
due to wind gusts up to 25kts and minRH values in the mid teens.

Saturday and Sunday will see increased chances for scattered
thunderstorms as multiple disturbances move along the ridge and over
the region. Chances for precipitation will be hit-and-miss, similar
to last week, where some may see decent amounts while many will get
nothing. Temperatures will also vary depending on cloud cover and
the location of the thunderstorms, however, highs are still expected
to remain above average for most locations through the weekend and
into next week as the ridge remains in place.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Saturday Morning)
Issued At 517 AM MDT Fri Aug 21 2026

Isolated showers with embedded thunder stretch from NE WY into the
central Black Hills. A stronger storm developed near KIEN and is
pushing into Nebraska. Additional isolated storms are possible
this afternoon after 22z, mainly south of KRAP; have kept thunder
mention out of KRAP forecast given low probability. Hail,
lightning, and gusty winds are the main threats with any
convection along with transient MVFR/IFR flight conditions.

Outside of convection, VFR flight conditions will prevail through
the forecast period.

&&

.FIRE WEATHER...
Issued At 1217 PM MDT Thu Aug 20 2026

Fire weather conditions will be marginal today across northeast
Wyoming and southwestern South Dakota. Northwesterly to northerly
winds of 5 to 10kts, gusting up to 20 to 23kts, will combine with
minimum relative humidity values as low as the mid teens. Friday
will see similar conditions, though winds gusts could be up to
26kts. Thunderstorm chances remain low through Saturday morning,
limiting dry lightning risk, however there is still a chance for
thunderstorms Friday afternoon into evening for southwestern South
Dakota.

&&

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

$$

UPDATE...KSmith
DISCUSSION...Schultz
AVIATION...SE
FIRE WEATHER...Schultz