485
FXUS63 KUNR 301110
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
510 AM MDT Sun Aug 30 2026

.KEY MESSAGES...

- Isolated to scattered thunderstorms are expected this afternoon
  and evening from northeast Wyoming, across the Black Hills, into
  the northwestern South Dakota plains, especially north of
  Interstate 90; a few could produce damaging winds of 60-75 MPH,
  some hail, and frequent lightning.

- Very warm weather continues throughout the next 7 days.

- Near-critical to locally critical fire weather conditions are
  expected this afternoon and evening from the Weston County,
  Wyoming area down across the southwestern South Dakota plains
  due to wind gusts of 25 to 35 MPH and relative humidity values
  falling to 15 to 20 percent.

&&

.UPDATE...
Issued at 1001 PM MDT Sat Aug 29 2026

Little change to the overall forecast. Some minor tweaks to PoPs
for tomorrow. Stronger wave moves through during the day will
spark a good chance for isolated strong to severe storms for the
northern half of the forecast area. Based off the most recent
CAMs, timing for storms looks between 20Z and 02Z Sunday afternoon
and evening. Main threats for storms Sunday will be hail and
strong winds. For the near term, general storms linger in
northwestern SD and will gradually move out over the next few
hours.

&&

.DISCUSSION...(This Afternoon Through Friday)
Issued at 1105 AM MDT Sat Aug 29 2026

Upper level analysis depicts upper trof making its way onshore
into the PNW with broad ridging over the central US. The Northern
Plains sits under southwest flow with water vapor imaging showing
a few weak waves and convection over the Rockies. High based
storms capable of damaging wind gusts and hail are possible over
the area this afternoon as modest CAPE develops over the region.
This morning`s CAMs show some activity developing later this
afternoon and evening.

Stronger wave/upper low moves in for Sunday from the PNW and
grazes the CWA. Modest CAPE will once again fuel high based
convection over the CWA Sunday afternoon. Again, the main hazards
with any storms that develop will be damaging wind gusts and hail.
Low forecast confidence in storms developing as optimal forcing
and moisture will be located to the north and west of the CWA.

Broad ridging will continue over the central US with trof over
the western US. The CWA will sit under southwest flow, keeping
conditions warm (highs in the 80s to 90s) with near daily chances
for afternoon storms to close out the month. There are some fire
weather concerns for the end of this weekend into early next week,
mainly over northeastern WY into far southwestern SD. More
details in the fire weather discussion.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Monday Morning)
Issued At 510 AM MDT Sun Aug 30 2026

Isolated to scattered thunderstorms remain expected this
afternoon and evening from northeast Wyoming, across the Black
Hills, into the northwestern South Dakota plains...especially
north of Interstate 90...starting as early as 19-20z/1-2pm. Some
storms will be capable of producing gusty, erratic, damaging
winds of 60-75 MPH, localized hail, frequent lightning, and brief
vsby drops to MVFR if not IFR. Based on the 00z HREF suite of
CAMs and more recent runs of the NAMNest and HRRR, have updated
PROB30 timing for KGCC (slightly higher confidence), and have
opted to introduce a PROB30 for KRAP (slightly lower confidence
but the terminal could be impacted by cells firing off the Black
Hills or cells tracking ENE in from the west). Breezy northwest
winds are also expected across the region. Outside of
thunderstorms, VFR conditions will dominate.

&&

.FIRE WEATHER...
Issued At 1105 AM MDT Sat Aug 29 2026

Near-critical to locally critical fire weather conditions may
form over Weston County into southwestern SD on Sunday afternoon.
Breezy westerly winds will develop by the afternoon with sustained
winds of 20 to 25 mph, gusting to 35 to 40 mph. Meanwhile
afternoon RHs will range from 15 to 20%. At this time, will hold
off on issuing any fire weather headlines due to low confidence in
coverage and duration of any critical fire weather conditions.

Fire weather concerns will continue into early next week as hot
and mostly dry conditions continue across the area. Main areas of
concern will be over northeastern WY into southwestern SD where
afternoon humidities will drop into the teens to low 20s.

&&

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

&&

$$

UPDATE...Dye
DISCUSSION...Wong
AVIATION...KSmith
FIRE WEATHER...Wong