674
FXUS63 KUNR 262317
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
517 PM MDT Sat Jul 26 2025

.KEY MESSAGES...

- Scattered storms through this evening, some of which will be
  strong to severe.

- Hot temperatures through Monday, especially closer to central
  South Dakota where heat indices will exceed 100F.

- Cooler temperatures and chances for storms for most of the
  upcoming week.

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 149 PM MDT Sat Jul 26 2025

Upper level analysis has a broad ridge situated over the Western
High Plains with an open trough digging down the California Coast
and a short-wave trough over western Canada. These features are
creating a southwest flow pattern across the forecast area. Water
vapor imagery indicates a fairly potent shortwave moving through
Wyoming, entering the southwestern portion of the forecast area as
of 20Z. At the surface, an area of weak low pressure is centered
over the eastern CWA, with a trough/dryline extending southward
through western Nebraska. Additionally, a cold front extends
across the northern half of Montana through northern portions of
North Dakota. Temperatures at 20Z range from around 80 in the
higher Black Hills into the upper 90s and around 100 across the
South Dakota plains. Heat indices have already hit 100-104F
across the advisory area. Additionally, decided to add Bennett and
Todd Counties to the advisory for today, as indices are very
similar to the rest of the area with temps already in the upper
90/around 100F. Dewpoints in Bennett are a little lower as the
dryline splits the county, but conditions in Tuthill will be
similar to Parmelee.

For later this afternoon and tonight, large scale ascent and subtle
mid-level cooling ahead of the approaching shortwave will combine
with strong heating, moist upslope winds, SB CAPE of 2500+ J/kg, and
deep layer shear (0-6 km) of 30-40 kts to support organized
convection. A few CU have begun to developed over the Black Hills
with some convection already firing over the higher terrain in
northeastern Wyoming. Storms will continue to develop and will
likely strengthen this afternoon as the the shortwave propagates
northeastward. Additionally, a few discrete cells will likely
form along the trough in south central South Dakota as towering CU
is just now forming in that area. Scattered storms this afternoon
will likely have mainly a wind threat with LCLs of 7k to 9k ft.
As the storms expand into more favorable conditions of greater
CAPE and shear this evening, it looks like the wind threat will
increase over northwestern South Dakota as the storms organize
into a MCS. By 06Z, the storms should be out of the area, leaving
behind clear skies.

For Sunday, the upper ridge builds over the forecast area, centering
the thermal ridge over the front range of the Rockies. The moisture
axis shifts a little eastward into central South Dakota, providing
some brief relief of the muggy conditions. However, dewpoint
temperatures in the lower/middle 60s combined with high temperatures
in the low 100s, will create heat indices of 100-104F over
Mellette, Tripp, and maybe Todd Counties. Therefore, have opted to
issue a heat advisory for Sunday for those three counties.

Hot temperatures will continue for the southern half of the forecast
area on Monday, as the cold front over Montana and North Dakota
slowly drops southward Monday and Tuesday. Expect chances for
storms, some strong to severe Monday afternoon and evening along and
behind the front.

The upper ridge over the central CONUS will build before becoming
centered over the Western High Plains by the end of the week.
Ridge riding shortwaves from a trough over the west coast will
lead to continued daily chances for storms. Although the timing
and strength of these waves remains uncertain at this time, CIPS
experimental and CSU machine learning probabilities indicate
chances for strong to severe storms will continue through the
week.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Sunday Evening)
Issued At 512 PM MDT Sat Jul 26 2025

Scattered thunderstorms along the NE border will bring gusty winds
along with local MVFR cigs/vsbys through 05Z. Small hail and brief
heavy rain is also possible with the stronger storms. Isolated to
scattered thunderstorms are also possible through 04z across
northeast WY, with locally heavy rain and isold MVFR cigs/vsbys
with some of the stronger storms also. Aft 06z VFR conds are
expected for the remainder of the terminal period.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Heat Advisory until 7 PM MDT /8 PM CDT/ this evening for SDZ014-
     032-043-044-046-047-049-077.
     Heat Advisory from 1 PM to 8 PM CDT Sunday for SDZ046-047-049.
WY...None.
&&

$$

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