545
FXUS63 KUNR 272318
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
518 PM MDT Mon Jul 27 2026

.KEY MESSAGES...

- Marginal to slight (1-2 out of 5) risk of severe thunderstorms
  this afternoon and evening primarily for damaging winds, but
  large hail may also occur.
- Hot weather will continue through the weekend with daily
  chances for active thunderstorms as ridge-riders rotate through
  the northern Plains

&&

.DISCUSSION...(This Afternoon Through Sunday)
Issued at 1048 AM MDT Mon Jul 27 2026

16z surface analysis had a low over northeastern WY with a couple
of troughs; one south into CO and another east into eastern SD.
Sustained east/southeast flow over western SD bringing increasing
moisture to the area. Water vapour loop has upper ridge centered
over NM with ridge axis into the northern Plains. Shortwave
evident over northwest WY, ready to overtop the ridge and cross
the CWA during peak insolation this afternoon/evening. That will
be the main short term concern.

This afternoon/tonight, 1-2KJ/kg SBCAPE develops along the
aforementioned boundaries with similar amounts of DCAPE. Effective
shear forecast to be 30-45kts as shortwave crosses. CAMs show
convective initiation by 20z, focused on the higher terrain
initially and then the surface boundaries as SBCIN erodes.
Clusters of storms will move east, potentially with congealing
cold pools, increasing the damaging wind threat. CAMs show a
couple clusters of enhanced wind gusts. Thunderstorms end
overnight. Temperatures will be near guidance, but below heat
headline criteria.

Tuesday looks less active right now given shortwave ridging and
lack of a distinct shortwave moving through the area, but would
still expect some convection tied to the terrain. Wednesday,
ensemble guidance shows another distinct wave moving across the
area during peak heating resulting in an elevated risk of severe
weather. Temperatures remain hot, but below heat headlines
criteria.

The rest of the forecast will be dominated by the upper ridge over
the central CONUS and hot weather. There will be waves kicking off a
few storms from time-to-time. If you believe the long term ensemble
guidance, someone of a cooldown may develop next week, but
confidence low.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Tuesday Evening)
Issued At 518 PM MDT Mon Jul 27 2026

Strong thunderstorms will continue eastward out of the forecast
area by mid evening. Isolated storms may develop during the
overnight hours, with gusty winds. The next round of storms will
be Tuesday afternoon and evening across NE WY and far Wrn SD.
These may again have gusty winds. Otherwise, VFR conditions are
expected through the forecast period.

&&

.FIRE WEATHER...
Issued At 1048 AM MDT Mon Jul 27 2026

Elevated fire weather conditions are expected much of this week
due to hot temperatures as well as minimum relative humidity in
the teens and 20s at times. The joint probability of MinRH <15%
and winds >25mph low at this point. The cores of thunderstorms
that occur this week should contain wetting rain, but lightning
strikes outside of the cores as well as strong, erratic winds may
create opportunities for new fires.

&&

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

$$

DISCUSSION...Helgeson
AVIATION...13
FIRE WEATHER...Helgeson