932
FXUS63 KUNR 131713
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1113 AM MDT Thu Aug 13 2026

.KEY MESSAGES...

- Unsettled and cooler through the weekend, with daily chances for
  thunderstorms.

- Patchy wildfire smoke will linger through Thursday. There could
  be some improvement toward the end of the week.

&&

.UPDATE...
Issued at 944 PM MDT Wed Aug 12 2026

03z surface analysis had a snaking frontal boundary from western
MT into eastern WY and then eastern CO. Multiple outflow
boundaries from earlier convection seen per radar/wind shifts.
Primary batch of shra/TS east of the Black Hills seen via
regional radar loop, which is ahead of a shortwave over northern
WY/southeastern MT. Widening our lens, southwest flow aloft exists
over the northern Plains.

Main themes of current forecast in good shape. A series of
disturbances will rotate through the northern Plains tapping into
sufficient moisture for daily chances for convection. In the near
term, mean MUCAPE per SPC HREF <1KJ/kg Thursday afternoon with
highest values over northeastern WY. 0-6km bulk shear suggests a
few stronger storms may occur where solar insolation breaks out,
but CAMs aren`t particularly excited given paucity of stronger
updraft tracks. PWATs at least 150%, so welcome rainfall will
grace some folks. The most likely outcome will be a relatively
rare "general" thunder risk in August over the CWA Thursday.
Temperatures will be a few degrees below guidance Thursday given
expected morning stratus on the western SD plains.

&&

.DISCUSSION...(This Evening Through Wednesday)
Issued at 1200 PM MDT Wed Aug 12 2026

Water vapor imagery shows broad upper ridging across the south
central CONUS. Upper flow over the CWA is nearly zonal, but active
WSW flow over the western CONUS is quickly approaching, along with
monsoonal moisture. A strong shortwave is over NW UT/eastern
ID/western WY pushing eastward toward the CWA. Forcing ahead of the
wave is bringing showers and thunderstorms to much of WY, including
NE WY into the Black Hills and far SW SD. Deep SE low level flow is
keeping moisture-rich air across the CWA even into NE WY, with
dewpoints in the upper 50s to upper 60s areawide. MLCAPE of 1000-
1500 J/kg is expected as the cap quickly erodes this afternoon. 0-
6km shear will increase this afternoon to 40-60kt, supporting strong
to severe storms, including isolated supercells. Large hail and
damaging winds will be the main threats. Heavy rainfall will also be
supported by PWATs of 150-175% of normal. CAMs show the current
precip continuing northeastward, then storms developing afterward by
mid afternoon across NE WY and the Black Hills area...then pushing
eastward across the western SD plains. Generally quiet weather is
expected overnight tonight but could still see some linger showers
through early Thursday morning. Upper flow becomes more
southwesterly on Thursday as another strong wave crosses the region.
This will likely bring another round of high-coverage showers and
thunderstorms. Instability will be slightly weaker tomorrow and
shear will be much weaker, so organized severe weather is not
expected. However, with PWATS nearing 200% of normal, hoping to see
more much-needed precipitation though models are differing on QPF
amounts and placement. Moist monsoonal flow will keep daily chances
for storms across the region through the end of this and the
weekend, along with near to slightly below average temps. Can`t rule
out strong/severe storms over the weekend.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 1111 AM MDT Thu Aug 13 2026

Local MVFR/IFR ceilings may linger around the Black Hills
throughout the afternoon and into the evening due to moist
easterly flow against the Black Hills. Scattered showers will be
possible across western SD for the remainder of today into
tonight, with thunderstorms possible this afternoon and through
the overnight hours for northeast WY. MVFR to IFR conditions are
expected tonight across the area as moist southeast upslope flow
continues. Fog may develop overnight across much of western SD
before burning off by the end of the TAF period.

&&

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

$$

UPDATE...Helgeson
DISCUSSION...13
AVIATION...Schultz