028
FXUS63 KUNR 212344
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
544 PM MDT Thu Aug 21 2025

.KEY MESSAGES...

- Isolated to scattered thunderstorms are expected late this
  afternoon through tonight, some of which could be strong to
  severe.

- Additional rounds of showers and thunderstorms are likely
  tomorrow into tomorrow night, especially across our southwestern
  half.

- Mild and mostly dry conditions should prevail this weekend
  through much of next week.

&&

.DISCUSSION...(This Evening Through Thursday)
Issued at 233 PM MDT Thu Aug 21 2025

Water vapor imagery this afternoon shows a large upper low spinning
over AB/SK, with a footprint extending into the northern High
Plains. This is contributing to largely zonal/west-southwest flow
aloft across our region. At the interface of the cyclonic flow
associated with the upper low and anticyclonic flow with ridging to
our south, deep Pacific moisture is streaming from off the CA coast
through the Great Basin and toward the Dakotas. Through the aid of
daytime heating, convection is becoming more widespread over the
central Rockies into the Bighorn Basin/Mountains, some of which will
make a beeline toward our area later this afternoon/evening. Closer
to home, additional areas of relatively shallow cumulus with some
embedded showery convection are evident over the southern Black
Hills and in the vicinity of the cold front pushing into south
central SD. Ahead of the front, a hot and very dry airmass remains
in place, with milder temperatures and steady north/northeast flow
behind the front.

Latest SPC mesoanalysis fields depict a strongly sheared but
relatively weakly buoyant convective environment across the area,
with effective bulk shear magnitudes of 40 to 50 kt combined with
MLCAPE values of approximately 500-1000 J/kg. However, the
CAPE/shear space varies considerably across the region depending on
location relative to the front and mid/upper-level speed maxima.
HREF mean/median fields suggest that effective bulk shear may
further increase this evening into tonight as stronger 500-300 mb
flow overspreads the region. Divergence associated with the right
entrance region of a 300-mb jet combined with low-to-midlevel
frontogenesis should provide further support for maintenance of
developing convection over northern WY despite modest buoyancy. With
large DCAPE (1200-1400 J/kg over much of northeastern WY) and 0-3 km
bulk shear magnitudes exceeding 30 kt, could very well see an
evolution toward a severe/near-severe wind producing MCS late
afternoon/early evening east of the Bighorns. Despite nocturnal and
postfrontal low-level stabilization after sunset, CAM consensus
carries convection west-to-east across much of the area, again aided
by low-level frontogenesis and upper-level Q-vector convergence. Low-
level baroclinic zone sinks farther southward/southwestward
tomorrow, leading to further reductions in CAPE (~800 J/kg or
lower). Though the upper jet will sink southward toward the area,
the core of the streak will shift eastward away from the region,
which will bring increased forcing for ascent but decreasing deep-
layer bulk shear magnitudes. Altogether, this should lead to
continued shower/storm activity--especially closer to the baroclinic
zone--but potentially a reduced potential for strong/severe storms.

As the upper trof shifts eastward tomorrow into the weekend,
northwest flow aloft will develop over the region and persist
through much of next week. Ensemble consensus suggests that below
normal 850-mb temperatures are likely to definite tomorrow night
through at least the middle of next week. At the same time, while
PWATs will surge to 150-175% of normal over much of the region
tonight through tomorrow, below normal PWATs are anticipated for
much of the area (aside from our far southwest) during that period.
As such, a mild and mostly dry period is likely from Saturday
through much of next week.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 544 PM MDT Thu Aug 21 2025

VFR conditions will prevail in most locations through much of the
period. However, periods of sub-VFR vis conditions are probable
in some spots with SHRA and TSRA on station. More widespread
SHRA/TSRA are expected to develop soon over higher terrain in
northern WY and move eastward through the evening/overnight hours,
possibly bringing localized MVFR/IFR conditions and gusty,
erratic winds. SHRA/TSRA should become more predominate in
tomorrow morning in northeast WY and then a couple of hours later
in RAP. By afternoon, precip should move south and east of the TAF
sites. Winds will be northerly through the period at around 10kts
or less.

&&

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

$$

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