960
FXUS63 KUNR 161949
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
149 PM MDT Sat Aug 16 2025

.KEY MESSAGES...

- Daily risk for thunderstorms through Monday night.

- Turning hot and dry Tuesday through remainder of week, with
  hottest day being Wednesday.

- Cooler and perhaps a bit more unsettled heading into next
  weekend.

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 145 PM MDT Sat Aug 16 2025

Current satellite imagery shows 500hpa ridge axis located
through central Dakotas, with exiting MCV pushing into Great Lakes
region. Next 500hpa shortwave to impact the region is currently
sliding up the backside of the ridge through south central WY, and
will impact the Black Hills region about the time of peak heating
this afternoon. Regional radar analysis is already showing some
showers in the CYS CWA pushing northward, and regional webcams
across SW SD showing virga/sprinkles associated with ACCAS
activity. There is plenty of low level moisture around as sfc
dwpts across western SD are in the mid 60s to even lower 70s, with
mid 50s to near 60 dwpts found across nern WY.

First concern is evening convection, where it will fire, how long
it will last and resultant weather. As aforementioned 500hpa
trof/shortwave swings into the region, ensembles show mean SB CAPE
values from 1500-2500 J/Kg with shear magnitudes from 25-35 kts.
PWATS values push 150-200% of normal as well. There is also some
decent 700hpa theta-e forcing ahead of the approaching 500hpa
system which is reflected in the omega fields. Will most likely
see convection fire along or just to the west of the Black Hills,
then move east through the evening/overnight hours congealing into
another MCS as it moves east. As has been the norm lately, severe
wind gusts will be the most likely flavor of severe weather
although an occasional hail stone cant be ruled out. With such
high PWATS and sfc dwpts, locally heavy rain is also possible
through the evening and overnight.

Wash, rinse and repeat best describes Sunday although PWAT values
are somewhat lower and the 500hpa wave train is a bit less
defined. Marginal instability does exist with steep lapse rates (8
degrees C or higher) are still in place. The biggest question for
Sunday will be if moderate upslope flow combined with terrain
effects will be enough to overcome expected cap (700hpa temps +12
or higher) that will be in place. This will be the same question
for Monday with southwest flow aloft bringing in weak 500hpa
shortwaves over marginally unstable conditions at the surface.
Steep lapse rates will remain in place, but the strength of the
cap will be the deciding factor.

From Tuesday through the remainder of the week, 500hpa ridge
pushes back to the west with warming 700hpa and 850hpa temps. This
should lead to warm and dry conditions across the area with
Wednesday right now looking to be the warmest day. The ridge
begins to collapse with region falling under slight NW flow aloft
by 00z Friday, which should lead to slightly cooler and perhaps
more unsettled conditions toward next weekend.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1048 AM MDT Sat Aug 16 2025

Main aviation concern will be the development of thunderstorms that
will primarily impact the KRAP terminal between 20-01z. Added a
TEMPO group for TSRA with MVFR VSBY for KRAP between 20-23z.
Additional hazards near thunderstorms will be gusty and erratic
winds. Outside of thunderstorm impacts to the KRAP terminal, VFR
conditions are expected under a veil of high clouds. Winds across
KRAP and KGCC will be light out of south before turning to the
north at KRAP by 01z and becoming light and variable at KGCC by
03z.

&&

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

$$

DISCUSSION/KEY MESSAGES...Hintz
AVIATION...Woodward