895
FXUS63 KUNR 220415
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1015 PM MDT Mon Jul 21 2025

.KEY MESSAGES...

- Slight risk (level 2 of 5) for severe thunderstorms this
  afternoon and evening over northwestern SD and far northeastern
  WY, with a surrounding marginal risk (level 1 of 5)

- Heat indices exceeding 100 degrees over south central SD this
  afternoon and again tomorrow

- Daily chances for thunderstorms continue through much of the
  period, with some strong to severe storms possible

&&

.DISCUSSION...(This Evening Through Monday)
Issued at 244 PM MDT Mon Jul 21 2025

Widespread cumulus/cumulonimbus development is evident across
central and eastern WY at this hour ahead of a wave lifting toward
the region in southwest flow aloft. Convection across northeastern
WY has shown recent signs of intensifying. Farther east, over the
Black Hills, orphaned anvils are evident following earlier attempts
at convective initiation. Residual stratocumulus over northwestern
SD after this morning`s persistent low cloud cover provides evidence
of the juicy boundary layer, with dew points over the SD plains
generally in the 60s to mid 70s.

Environment this afternoon per SPC mesoanalysis is characterized by
MLCAPE values ranging from around 4000 J/kg across south central SD
to about 2000 J/kg across our far northwest, with values rapidly
diminishing to the southwest owing to drier boundary layers. As
implied by midlevel lapse rates of 8-8.5 C/km, much of the area
remains under a stout cap, which will require the arrival of large-
scale ascent from the west to erode. As this ascent arrives, further
destabilization is certainly possible given vertical theta-E
differences of 20-25 C. Deep-layer shear is maximized over
northwestern SD and far northeastern WY, generally consistent with
the outline of the slight risk area in the midday update, with
effective bulk shear magnitudes of 35 to 45 kt. Much of this shear
is maximized in the 1-3 km layer, as reflected by 0-3 km bulk shear
magnitudes around 30-35 kt over northwestern SD. Combined with CAPE
in the hail growth zone of 800-1200 J/kg and DCAPE exceeding 1000
J/kg, the thermodynamic and kinematic environments are capable of
both very large hail and damaging winds. Later in the
afternoon/evening, the predominant hazard will likely be tied to
convective mode. Most likely scenario is hybrid multicell/supercell
cluster(s), but if upscale growth prevails, could see a transition
to primarily wind as a hazard.

Another wave crosses the region tomorrow, with a generally similar
environment in place. Main difference is slightly less steep lapse
rates over the area. However, the environment upstream looks more
than suitable for the development of supercells, with near-60 degree
dewpoints extending west to southeastern MT contributing to 1000-
2000 J/kg of SBCAPE and deep-layer bulk shear magnitudes of 40-50
kt. As the wave approaches late afternoon into the evening, suspect
that at least one or two storms--likely supercells--will cross the
region from west to east, bringing potential for significant hail
and wind. Outside of the storms, an expanding low-level thermal
ridge combined with slightly rising midlevel heights will bring a
hotter day. With dew points remaining abnormally high, suspect heat
indices will clearly break into at least advisory criteria across
south central SD.

Active southwest to zonal flow aloft combined with above normal
precipitable water will continue to support daily chances for
showers and thunderstorms through at least Friday.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Tuesday Night)
Issued At 1010 PM MDT Mon Jul 21 2025

Scattered TSRA/SHRA with associated MVFR conditions will continue across
northwestern and portions of western SD plains through ~09z.
Outside of the storms, some MVFR/IFR conditions associated with
low stratus/fog can`t be ruled out, especially over northwestern
SD into northeastern WY. Otherwise, VFR conditions should prevail.

&&

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

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...Wong