283
FXUS63 KUNR 211117
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
517 AM MDT Thu Aug 21 2025

.KEY MESSAGES...

- A cold front will move through the area today.

- Scattered showers and storms through Friday night, drier this
  weekend and next week.

- Mild temperatures Friday through the weekend -- and through much
  of next week!

&&

.DISCUSSION...(Today Through Wednesday)
Issued at 150 AM MDT Thu Aug 21 2025

A positively tilted ridge extends from the Great Basin northeast
to the Great Lakes while a shortwave trough extends through the
northern Rockies. At the surface, a cold front is just northwest
of the forecast area, extending through western North Dakota, into
southeastern Montana and northwestern Wyoming. At 09Z there are a
few prefrontal showers and storms across northwestern South
Dakota, with additional showers and storms along the cold front.
Temperatures are still fairly mild, with readings mainly in the
70s across the area.

For today, the front will move slowly through the area, reaching
south central South Dakota by this evening, and exiting the
forecast area Thursday night. Isolated showers/storms are possible
along the front this morning as it makes it way through the area.
However, large scale ascent is fairly week, thus coverage will be
limited. Have added some low pops to account for this. For this
afternoon, there is a chance for a few stronger to severe storms
across two different areas. First, along the front in south
central South Dakota where daytime heating will bring highs in the
90s. Steep low-level lapse rates and MUCAPE of 1000-1500 J/kg
could result in a stronger storm, but shear is fairly weak and
most of the CAMS struggle to produce much of anything. The second
area with the potential for a stronger/severe storm or two is
across northeastern Wyoming and the Black Hills. Shear in this
area is much more favorable (35-40 kts), which could result in
organized updrafts that could bring a potential for hail and
strong winds. For this area, would expect to see some storms
moving off the higher terrain and into this area by late afternoon
with an eastward progression across the southern half of the
forecast area during the evening. After midnight tonight, could
see some showers and storms move through northwestern South
Dakota, associated with the main upper trough. These showers and
thunderstorms will linger across the area Friday morning.

For Friday, the upper trough continues to push through the area in
the post-frontal regime. Upper level divergence and steep mid
level lapse rates will combine with the baroclinic zone over the
southern half of the forecast, resulting in some afternoon/early
evening convection. Deep layer shear will once again be marginal
(30-40 kts), thus would expect scattered showers and storms with
isolated severe storms.

The upper high over the desert southwest will rebuild this weekend
as the upper low moves into the Great Lakes region. This will
result in northwest flow across the forecast area. Embedded waves
in the passing flow may result in some isolated showers/storms,
but overall it will be more dry then wet. Temperatures will be
seasonal, with highs mainly in the 70s to low 80s.

The overall model consensus for next week indicates northwest flow
will continue. This will result in a much drier airmass across
the entire region and cooler, but seasonal, temperatures for
almost the entire week.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Friday Morning)
Issued At 511 AM MDT Thu Aug 21 2025

Isolated showers and storms are expected along the cold front this
morning as it moves slowly southeastward through the forecast
area. Scattered storms are expected to move into northeastern
Wyoming late this afternoon and progress eastward through western
South Dakota this evening. Isolated severe storms are possible
with hail and strong winds. Additional showers and storms are
expected through Friday morning, especially across northwestern
South Dakota.

&&

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

$$

DISCUSSION/KEY MESSAGES...Smith
AVIATION...Smith