436
FXUS63 KUNR 011759
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1159 AM MDT Tue Jul 1 2025

.KEY MESSAGES...

- Hot with isolated storms today through Thursday

- More widespread storms possible from west to east late Thursday
  through the holiday weekend

&&

.DISCUSSION...(Today Through Monday)
Issued at 200 AM MDT Tue Jul 1 2025

It`s a quiet night regionwide. Water vapor imagery shows a corridor
of deep subsidence over much of northeastern WY/western SD gradually
shifting eastward with time. A weak midlevel/upper-level disturbance
is evident upstream across southern MT/northwestern WY, with more
robust but nebulous deep moisture farther upstream across the
Pacific Northwest. Latest surface analysis shows high pressure
extending from the central Rockies/Plains northward thru the
Dakotas. To the west of the surface ridge axis, southerly return
flow has emerged this evening. Temperatures at this hour range from
the upper 40s in portions of the Black Hills to the lower to mid 60s
where some mixing persists. There`s not a cloud in the sky over the
area, with the nearest in the vicinity of the Bighorns.

Deep ridging expands eastward into the central CONUS today. As it
does so, a subtle shortwave trof atop the ridge is to poised to
cross the region around midday. Timing remains not quite ideal for
potential afternoon convective initiation, which would otherwise be
supported by increasing surface dew points/low-level theta-E
advection and steepening midlevel lapse rates. However, this wave,
or localized forcing owing to diurnal convergence tied to terrain
features, may be sufficient to overcome modest capping. Should the
cap break, 00z HREF mean SBCAPE/deep-layer shear vector magnitude
combination of 1-1.5 kJ/kg and 30-35 kt, respectively, could support
at least transient supercellular mode to convection, particularly
given 0-3 km storm-relative helicity values of 100-150 m2/s2.
Limited low-level shear but relatively strong midlevel shear would
likely support hail as the primary threat with any storms that
develop. HREF mean/median and especially 75th percentile dew
points indicate potential for moisture pooling and locally
enhanced buoyancy across the northern foothills/eastern Bearlodge
Mountains, which is a climatologically favored corridor for such
local enhancements. Convection initiating in that vicinity would
be especially capable of rapid intensification supportive of
severe hazards. CAMs remain somewhat bearish on the potential for
convection, possibly owing to subtle height rises and associated
subsidence in the wake of the passing trof. Nonetheless, have
opted for slight chance PoPs over all of western SD and portions
of northeastern WY during the afternoon. Continued low-level
theta-E advection and forcing associated with a nocturnal low-
level jet may support additional thunderstorm development (or
maintenance and expansion of any ongoing activity) into central SD
overnight. Mean MUCAPE of 500-1000 J/kg and strengthening deep-
layer shear may support some strong updrafts capable of small hail
with this overnight convection.

Low-level thermal ridging continues to build eastward Wednesday and
Thursday. On Wednesday, upslope easterly flow in response to
pressure falls over the Rockies may limit max temperatures somewhat
compared to what would be expected with such high 850 mb temps. NBM
probabilities of highs reaching triple digits on Wednesday have
accordingly dropped somewhat compared to this time yesterday.
Nonetheless, a toasty day is still in store, with highs likely to
certainly exceeding 90 F for most. As the low-level thermal ridge
expands eastward, the midlevel ridge axis also shifts overhead
Wednesday afternoon, which could again limit convective initiation.
CI will be further complicated by the best corridor of surface dew
points being shunted northeastward as the low-level thermal ridge
expands eastward, thus decreasing buoyancy in locations that would
otherwise benefit from easterly upslope flow in the pursuit of
overcoming the cap. Altogether, coverage again looks spotty at best,
but mean SBCAPE values of 1-1.5+ kJ/kg coupled with increasing deep-
layer shear would suggest that if the cap breaks, severe weather
could occur. As has been the case, Thursday looks to be the warmest
day of the week for the area, with triple digit highs probable to
likely (60%+ chances per NBM) across west central SD. However,
decreasing heights as the ridge axis shifts eastward and attendant
strengthening Q-vector convergence from west to east late Thursday
afternoon through the overnight will also lead to increasing storm
chances. Although widespread thunderstorms will be increasingly
likely late Thursday, particularly over western portions of the
area, severe potential will decrease owing to less favorable
CAPE/shear. Notable Q-vector convergence maximum overspreads the
remainder of the area on Independence Day, which will likely bring
more numerous/widespread general thunderstorms. Severe potential
again looks limited given weak shear and modest CAPE.

Active zonal flow pattern persists thru the weekend, which should
promote periods of unsettled conditions.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Wednesday)
Issued At 1158 AM MDT Tue Jul 1 2025

VFR conditions are generally expected through the period. The
primary exception will be within any very isolated thunderstorms
beginning around 21z, where brief, localized MVFR conditions may
occur.

&&

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

$$

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