755
FXUS63 KUNR 081920
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
120 PM MDT Tue Jul 8 2025

.KEY MESSAGES...

- Isolated to scattered thunderstorms continue over the next few
  days, some of which could be strong to severe, especially
  Wednesday afternoon and evening across northwestern South Dakota.

- Hotter temperatures persist through tomorrow, with milder
  conditions expected by Friday as a series of disturbances
  crosses the region.

&&

.DISCUSSION...(This Evening Through Tuesday)
Issued at 118 PM MDT Tue Jul 8 2025

Water vapor imagery this afternoon primarily reveals a building
ridge over the western half of the CONUS. However, a pair of
embedded shortwave disturbances/vorticity maxima can be identified
near the Great Basin and off the northwestern CA coast. These will
become impactful for weather close to home over the next 24 to 48
hours. Another, more subtle disturbance over the north central CONUS-
-along with diurnal terrain-driven convergence--may be contributing
to convective initiation over the Black Hills at this hour. Latest
SPC mesoanalysis reveals MLCAPE of 1500-2500 J/kg paired with
effective bulk shear magnitudes of 20-25 kt, with the caveat that
the mesoanalysis is likely not representing local effects on shear
owing to the Black Hills. Would not be surprised to see further
intensification of these storms with an outside chance at marginally
severe hail if a persistent rotating updraft can emerge.

Tomorrow, midlevel ridging continues to shift eastward, with the
ridge axis extending from the Four Corners northeastward thru the
northern Rockies and into south central CN. Atop the ridge, the
shortwave currently over the Great Basin region will rapidly
approach the area early tomorrow, crossing the region from the
afternoon into the evening after being convectively enhanced today
into tonight. The warming/expanding low-level thermal ridge will
lead to decreasing boundary layer moisture/instability across our
west/southwest by early afternoon, but farther north and east,
SBCAPE of 1500-2500 J/kg paired with deep-layer bulk shear
magnitudes of 25 to 35 kt will be sufficient to support robust
multicellular or supercellular updrafts. Weak low-level shear but
relatively strong midlevel shear coupled with ample DCAPE, CAPE
within the hail growth zone, and midlevel lapse rates (8+ C/km)
should support the potential for both large/significant hail and
damaging winds, with the predominant hazard being dependent upon
mode.

In the wake of the wave tomorrow afternoon/evening, shortwave
ridging may briefly overspread the region, leading to a brief
reprieve of convection. Thereafter, a more robust
shortwave/vorticity maximum approaches the region, which will bring
another round of thunderstorms to the area. The timing of the wave
continues to look a bit early for peak heating, at least across our
west. Farther east, the environment may have sufficient time to
destabilize, with LREF mean SBCAPE reaching 1000-2000 J/kg from west
to east by 18z. Regardless of buoyancy, deep-layer shear vector
magnitudes on the large scale remain paltry (LREF mean 0-6 km
bulk shear magnitudes of 15-25 kt areawide). As such, convection
may be largely disorganized, with marginal hail and strong wind
gusts the primary concerns. However, any areas of locally enhanced
shear, particularly in the vicinity of any lingering boundaries
from convection Wednesday afternoon through the overnight, could
promote more robust, organized convection with increased severe
potential.

A more amplified, broader trof crosses the region Thursday night
into Friday, bringing milder temperatures and breezy northwest
winds. Lingering low-level to midlevel moisture may support
additional shower activity, but decreasing MUCAPE (<500 J/kg) should
limit much in the way of thunderstorm activity. Deep ridging returns
on Saturday, bringing warmer and drier conditions to start the
weekend.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1102 AM MDT Tue Jul 8 2025

VFR conditions are expected through the TAF period. A storm or
two is possible over the Black Hills this afternoon.

&&

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

$$

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