052
FXUS63 KUNR 181949
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
149 PM MDT Mon Aug 18 2025

.KEY MESSAGES...

- Isolated to scattered thunderstorms, some strong to severe,
  remain possible this afternoon and evening.

- Drier and warmer conditions expected Tuesday and Wednesday.

- More seasonable temperatures and unsettled conditions return
  late in the week.

&&

.DISCUSSION...(This Evening Through Monday)
Issued at 149 PM MDT Mon Aug 18 2025

Water vapor imagery this afternoon shows a weak wave shifting over
the central/eastern Dakotas, with trailing enhanced midlevel/upper-
level moisture extending over the Black Hills/northeastern WY.
Deepening cumulus/cumulonimbus field is evident over the eastern and
northern Black Hills, with a single thunderstorm ongoing between
Lead/Deadwood and I-90. Additional cumulus fields are evident near
the Cheyenne River in eastern Meade/Pennington counties and closer
to residual outflow over Perkins/Ziebach counties, the latter of
which is becoming increasingly agitated/congested at this hour.
MLCAPE values of 1-2 kJ/kg combined with effective shear magnitudes
around 30 kt may be sufficient for multicell or transient supercell
organization and maintenance through the remainder of the afternoon
into the evening. However, lack of a distinct forcing mechanism
outside of the Black Hills and the lingering outflow boundary
mentioned above may limit longevity of this activity given a fairly
warm/dry layer near the LCL/LFC in forecast soundings. East of the
Black Hills, assuming convection can mature and maintain itself
outside of these zones of forcing, a corridor of hail growth zone
CAPE exceeding 1000 J/kg and DCAPE over 1200 J/kg could support both
a severe hail and wind threat, particularly with longer-lived
rotating updrafts. Convection should wind down as it shifts
eastward/southeastward near and after sunset.

Midlevel ridging strengthens over the Four Corners tonight into
tomorrow, with an associated ridge axis pivoting into the northern
High Plains. Meanwhile, low-level thermal ridge expands from the
Southwest northward through the Great Basin into the northern
Rockies. Our area remains on the eastern periphery of the thermal
ridge, but subsidence/clear skies associated with the rising heights
and the arrival of a drier air mass should promote near-optimal
diurnal heating. Relatively weak background flow will emphasize the
importance of localized flow perturbations and associated
convergence, such as over the Black Hills/Pine Ridge and areas of
differential land use/cover, including the Badlands/Sandhills, where
one or two storms cannot be ruled out. Otherwise, subsidence should
preclude the erosion of a strengthening capping inversion. Highs
likely in the 90s (probabilities 80%+ per NBM) for most locations
outside of the Black Hills. Low relative humidity values (generally
30% or less in the hottest areas) should keep apparent temperatures
below advisory criteria.

Ridge axis remains over the region but gradually tilts southeastward
through early Thursday as a deep trough is carved out over the
Pacific Northwest/northern Rockies. Attendant low-level thermal
ridge expands over the northern Plains on Wednesday, bringing what
should be the warmest, driest day of the week for most of the area
before being shunted southward by a trof/weak cold front late
Thursday. Increased forcing for ascent via the front and mid/upper-
level Q-vector convergence combined with improved deep moisture
(PWATs 125-150% of normal per EPS/GEFS/GEPS ensemble means) should
bring a return of showers/storms.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Tuesday)
Issued At 1120 AM MDT Mon Aug 18 2025

VFR conditions are expected through much of the period. The
exception will be in any SHRA/TSRA this afternoon and evening,
which could bring localized MVFR/IFR conditions along with hail
and strong, erratic wind gusts.

&&

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

$$

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