440
FXUS63 KUNR 311102
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
502 AM MDT Fri Jul 31 2026

.KEY MESSAGES...

- Isolated thunderstorms possible this afternoon and early evening
  across northeast Wyoming and far western South Dakota.

- Very hot weather returns for the weekend with the hottest day
  Sunday.

- Elevated (Saturday) to critical (Sunday) fire weather conditions
  expected this weekend.

&&

.UPDATE...
Issued at 1033 PM MDT Thu Jul 30 2026

Introduced light PoPs Friday afternoon and evening across portions
of northeast Wyoming and the Black Hills, as the 00z HREF suite of
CAMs is finally coming into better agreement on potential
placement and timing of isolated storms compared to significant
disagreements noted in previous runs. Coverage does not look to
reach scattered at this time, and most of the region will remain
dry. Also evaluated fire weather potential and heat for this
weekend...Saturday continues to look near-critical for fire
weather with portions of our central plains east of Rapid City
getting the closest to Red Flag criteria. Overlap of critical RHs
and winds expands for Sunday, which continues to look like the
more concerning day (coupled with the hottest temperatures as
well). As for the heat...latest forecast continues to advertise
100-107 degF temps by Sunday per the NBM (and GFS/ECMWF ensemble
meteograms don`t currently support making any deviations from the
blend), but heat index values are holding just slightly cooler,
which reduces our confidence in the need for any watches at this
early juncture or eventual warnings/advisories. It`s certainly
possible that a Heat Advisory may eventually be needed for
portions of the CWA Sunday, but we`ll continue to monitor the
latest guidance and trends. Dry conditions remain expected for the
weekend.

&&

.DISCUSSION...(This Afternoon Through Wednesday)
Issued at 1105 AM MDT Thu Jul 30 2026

16z surface analysis had weak low over the NE panhandle with a
boundary into central SD per 0.5km 1-minute visible satellite
loop. Cold front was located from western ND into western WY.
Water vapour loop had shortwave trough over northern WY with 80kt
jet streak. These features starting to force CU/TCU/TS along the
surface boundary with evidence of decent lift ahead of the
shortwave over northeastern WY/eastern MT. Concern this
afternoon/evening is severe weather.

This afternoon/evening, shortwave trough moves east as
convergence along the surface boundaries increase. Mid-level flow
increases effective shear to 25-35kts with locally higher amounts
over central SD. Strong buoyancy will develop this afternoon,
especially east of the Black Hills. Initial storm mode may be
cellular with large hail potential, but the trend will be toward
severe clusters/linear severe convection as storm cold pools
congeal. Primary threat will be damaging winds gusts (70+ mph).
Looking for a fairly early start this afternoon with CAMs focusing
on the central/southern Black Hills and central SD this afternoon
and then Lemmon to Winner by evening as the main shortwave wraps
through the CWA. PWATs will be 150-200% of normal east of the
Black Hills, so locally heavy rain and potential flash flooding
may occur. Activity wanes overnight. Temperatures will be near
guidance.

A weak shortwave moves through Friday, but drier air will be
filtering into the CWA, so PoPs should be limited to our higher
terrain areas.

Hot to very hot weather returns for the weekend with the
preponderance of ensemble guidance giving highs near 110F for some
folks Sunday. Much drier air in place, so thunderstorm chances
will be unmentionable. Cooler weather returns Monday with actual
"cool" air in place for the middle of next week per flattening
upper ridge and west/northwest flow aloft for the northern Plains.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Saturday Morning)
Issued At 500 AM MDT Fri Jul 31 2026

VFR conditions are expected through the TAF period. Some
potential for isolated thunderstorms exists during the afternoon
and early evening, generally over NE WY into far western SD.

&&

.FIRE WEATHER...
Issued At 1105 AM MDT Thu Jul 30 2026

Strong/severe thunderstorms are expected this afternoon/evening
with strong gusty, erratic winds as the primary threat. Elevated
(Saturday) to critical (Sunday) fire weather conditions are
expected this weekend. Minimum relative humidity will drop into
the upper single digits/teens Saturday in the west and Sunday over
much of the area. Winds Saturday will be marginally gusty for
critical Fire Weather, but should be more robust on Sunday per
very deep boundary layer mixing.

&&

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

$$

UPDATE...KSmith
DISCUSSION...Helgeson
AVIATION...13
FIRE WEATHER...Helgeson