292
FXUS63 KUNR 261710
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1110 AM MDT Thu Jun 26 2025

.KEY MESSAGES...

- Less active weather expected today, though there is a chance for
  a few afternoon/evening storms over and near the Black Hills.

- There is a slight risk (level 2 of 5) for severe thunderstorms Friday
  afternoon, mainly east of the Black Hills. Main hazards will be
  large hail (quarter size or larger) and damaging wind gusts.

- Very warm to hot on Friday and Saturday, with highs climbing
  into the 90s for most.

&&

.DISCUSSION...(Today Through Wednesday)
Issued at 142 AM MDT Thu Jun 26 2025

Water vapor imagery tonight shows an upper-level wave shifting from
the Rockies onto the High Plains. Ahead of this wave, a corridor of
enhanced deep moisture extends from near the west coast of Mexico
northward through the north central U.S., supporting strengthening
convection across the central Plains. Earlier thunderstorm activity
over western SD is largely weakening at this hour as it continues to
shift eastward, though lingering showers with occasional lightning
strikes persist over central SD. In the wake of the shower/storm
activity, nighttime microphysics RGB imagery and obs/webcams show
developing low stratus/fog over the western/central Dakotas into far
eastern WY. This activity will likely expand through the overnight
and early morning hours, with some areas of dense fog not out of the
question. Will continue to monitor and evaluate the need for any fog-
related headlines.

Midlevel shortwave trof shifts east of the area early today, with
shortwave ridging arriving in its stead. Height rises thru 18-21z
should limit early afternoon convection. However, terrain-driven
convergence in a general absence of large-scale drivers of flow
along with CAPE ~1000 J/kg may support some spotty storms
predominantly over the Black Hills. Aside from isolated storms, a
warm and mostly dry day is in store. High temperatures may be
trending a smidge milder, with some locations across our eastern
tier and over the Black Hills likely (EPS/GEPS probabilities 70% or
greater) remaining in the 70s. Later in the afternoon/evening,
height falls will likely support additional convection over higher
terrain features to our west in MT/WY, as suggested by most members
of the 00z HREF. A subset of members carry some thunderstorms
associated with this activity into portions of northeastern WY and
northwestern SD late tomorrow evening through the overnight,
potentially aided by increasing 850 mb flow and attendant
frontogenesis/theta-E advection. Increasing MUCAPE (HREF mean
approaching 1000 J/kg) and marginal deep-layer shear (25-30 kt)
could support storm organization and potentially a sneaky,
localized, conditional severe threat.

The next midlevel shortwave and attendant surface trof cross the
region Friday. Timing of the surface trof will be critical to
potential of thunderstorms, particularly any severe chances, across
our area. For now, given a rather early trof passage over
northeastern WY/far western SD, the best convective environment
featuring SBCAPE in excess of 2 kJ/kg and deep-layer shear of 30-40
kt looks to set up east of the Black Hills Friday afternoon. This
placement is also consistent with the primary zone of integrated
vapor transport per the latest GFS/ECWMF deterministic models
coupled with the potential for a pseudo-dryline mixing eastward off
of higher terrain toward the Badlands. Fat CAPE profiles, relatively
weak low-level shear (but decent low-level hodograph curvature), and
deep, well-mixed boundary layers should favor large hail
(potentially significant) and damaging winds as the primary hazards.
Outside of the storms, and particularly west of the aforementioned
pseudo-dryline, hot and dry conditions will be the story, with highs
climbing into the 90s.

Hotter temperatures shift eastward toward central SD on Saturday as
a lobe of the low-level thermal ridge pivots eastward and a weak
baroclinic zone sinks into the area. Zonal midlevel flow carries
another shortwave/corridor of Q-vector convergence toward the region
very late Saturday through Sunday, which may support multiple rounds
of convection atop the lingering baroclinic zone. Behind this
disturbance, relatively high confidence in a southeast-to-northwest
oriented ridge building early next week, which will bring warmer
temperatures but may favor at least some isolated/scattered
thunderstorms through at least the first half of the week.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 1109 AM MDT Thu Jun 26 2025

Isolated showers and storms are possible after 18-19z this
afternoon, mainly around the Black Hills where transient MVFR
conditions will be possible in any heavier showers.

Otherwise, VFR conditions are expected through the forecast
period.

&&

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

$$

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