413
FXUS63 KUNR 172322
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
522 PM MDT Thu Jul 17 2025

.KEY MESSAGES...

- Daily chances for at least isolated thunderstorms, some of
  which could be strong to severe, are expected over the next
  several days.

- More seasonable temperatures are anticipated through at least
  the middle of next week, with above normal temperatures likely
  at times.

&&

.DISCUSSION...(This Evening Through Thursday)
Issued at 149 PM MDT Thu Jul 17 2025

Satellite imagery reveals gradual erosion of low stratus and
stratocumulus in place over the region since yesterday, though dense
stratus remains in place over much of southwestern SD (which is fog
in some portions of the Black Hills). Water vapor imagery shows
subsidence over much of the area, shifting eastward with time as a
shortwave ridge axis approaches the Black Hills. However, a compact,
robust vorticity maximum is evident over southern AB/northern MT,
which is leading to some thunderstorm development near the CN/US
border at this hour. Latest surface analysis depicts a trof axis
extending northward thru AB ahead of the wave, with a
stationary/warm front from central MT southward toward the Bighorns.
Pressure falls owing to a deepening trof upstream are supporting
breezy southerly winds over much of the region, but cloud cover has
kept warming at bay for the most part, with afternoon
temperatures in the 60s and 70s--and even the 50s again over the
Black Hills.

Upper-level disturbance/vorticity maximum approaches the region
later this afternoon into the evening. Corridor of 500-2000 J/kg of
SBCAPE extending from the Front Range northward through the Black
Hills region and into central/eastern MT combined with 0-6 km bulk
shear magnitudes of 45 to 60 kt will support strong to severe
thunderstorm development, with discrete supercells likely the
primary mode. Poor diurnal timing of the wave given diminishing
surface-based buoyancy post-sunset may limit overall severe
potential across our area, but any storms arriving this evening
through the overnight could pose a threat for at least large hail,
and maybe even damaging winds. Elsewhere, capping will likely hold
given residual cloud cover over much of the region that limited
destabilization this morning/afternoon.

Midlevel wave drags a front through the region tonight into
tomorrow, which will likely approach south central SD by around
midday/early afternoon and may get hung up around higher terrain of
the Black Hills. Steepening midlevel lapse rates will support SBCAPE
values around or over 2000 J/kg ahead of the front. Weaker but still
sufficient deep-layer bulk shear magnitudes (30-35 kt) will support
organized thunderstorms if initiation can occur. However, a stout--
and strengthening--cap may limit convective initiation outside of
the immediate frontal zone in the afternoon/evening and deeper
ascent associated with the right entrance region of a jet streak
arriving later in the evening/overnight.

Beyond tomorrow, predominantly zonal flow aloft is expected through
the bulk of the forecast period, which will occasionally bring
convectively induced potential vorticity anomalies across the area
while promoting low-level thermal ridge expansion and allowing for
gradual boundary layer moisture return. The result looks to be more
seasonable to above normal temperatures and daily chances for at
least isolated thunderstorms for the foreseeable future.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 519 PM MDT Thu Jul 17 2025

VFR conds expected. Isolated TSRA are possible tonight across the
northern third with the best chance of strong/isolated severe
thunderstorms (local IFR conditions) near the SD/ND border area.
More chances for storms can be expected Friday late in the day.

&&

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

$$

DISCUSSION/KEY MESSAGES...Sherburn
AVIATION...JC