921
FXUS63 KUNR 141713
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1113 AM MDT Sat Jun 14 2025

.KEY MESSAGES...

-Daily chances for thunderstorms, some possibly severe, through
 Monday

-Fog again in the Plains and foothills Sunday morning

-Wet and cooler mid-week, followed by mostly dry and warmer
 weather

&&

.UPDATE...
Issued at 954 AM MDT Sat Jun 14 2025

The synoptic-scale pattern has changed little from yesterday. Moist
southeasterly lower tropospheric flow coupled with the diurnal
minimum once again resulted in fog and low stratus across the
Plains, up to the foothills this morning. This is eroding a
little quicker than was the case yesterday.

Early thoughts on convective potential later today based on
observational data and new models allows us to offer a little more
clarity now, and we will continue to refine the forecast throughout
the day. Current thinking is that the capping EML is a negative for
convective initiation over the Black Hills, though the chances are
higher than yesterday given higher magnitude of surface diabatic
heating and a more pronounced upper level perturbation seen in 6.3-
micron water vapor imagery upstream. This wave has minimal effect at
lifting the capping EML though, per model forecast soundings, but
could be just enough. If convection does develop, hodographs look to
favor splits, with initial cloud-bearing wind carrying cells
eastward at around 15-20 mph, but left splits/anticyclonic mesos
deviating north-northeastward and right movers/cyclonic meso slowing
slightly and deviating south-southeastward. Some terrain-driven
variance in the wind field could shift this storm-scale behavior
some.

The primary impetus for convection is expected to be the Bighorns
and Laramie mountains again, where elevated heating will be the
strongest. As convection moves eastward it will almost immediately
enter a more unstable regime causing convective intensity to
increase by mid-afternoon. Although midlevel flow is still quite
weak, lower tropospheric directional shear is enough for supercells.
Similar motion and behavior to yesterday is expected with cellular
convection tending to split and move fairly slow with primarily a
large (potentially very large, 2+") hail threat, while any cold
pool-driven convection propagates eastward and transitions to
primarily a wind threat.

Forecast challenge for today will be to refine footprint of
convective coverage. The best chance for the foothills and Plains to
receive precipitation today will be if cold pool driven upscale
growth of clusters or an MCS happens, and probably fairly late in
the day. We will likely need to refine timing and precipitation
probabilities as we see this start to evolve.

&&

.DISCUSSION...(Today Through Friday)
Issued at 315 AM MDT Sat Jun 14 2025

Upper air analysis shows weak ridging across the central/northern
Plains. SW flow persists across the central/northern Rockies ahead
of a stalled Pac NW upper low. This active flow continues across the
western CONUS as several shortwaves cross the northern Plains. KUDX
radar shows a weakening MCS across NW SD continuing eastward. Also,
lingering showers across south central SD over Todd/Tripp counties
will soon be departing the CWA. Low clouds/areas of fog are again
expected early this morning but shouldn`t be as widespread as
yesterday. Warm/moist airmass will persist across the area, with
dewpoints in the low to mid 60s. Highs will reach the upper 70s over
NW SD to near 90 near the Neb border. The next wave will cross the
CWA this afternoon and evening. Convective initiation will take
place along the Bighorns/Laramie mountains and move into NE WY in
the afternoon. CAMs show storms then enter an axis of stronger
instability and minimal CIN centered across the Black Hills. Shear
profiles will be strong enough for supercell development, with large
hail and damaging winds possible. Then as storms move east off the
Black Hills after ~02z through the western SD plains, the threat
transitions more to damaging winds. The 06z HRRR is also suggesting
a secondary line of storms later in the evening coming off the
Bighorns and pushing across NE WY. These storms then reach the
Black Hills area closer to 05/06z before weakening. Initially over
NE WY, these storms could produce both hail and wind, but appear
to be more wind based by late evening as it nears the Black Hills.
Confidence is low with this exact solution & intensity of storms,
but a couple of other CAMs do suggest some secondary precip over
NE WY (though much weaker). With little change to the pattern
through Sunday and Monday, both days should be a general repeat of
today. Storms develop across NE WY in the afternoon, cross the
Black Hills region around 00z, then push east, becoming more wind
based through the evening. An exception on Monday is a cold front
crossing the CWA during the day will limit the best
instability/storms to SW/SCentral SD. Models continue to show a
strong wave crossing the region on Tuesday. Cooler temps in the
upper 60s/70s are expected with more widespread showers/isolated
storms Tue/early Wed. Severe weather is not expected. Generally
drier weather is then expected for the second half of next week,
with above average temps.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1113 AM MDT Sat Jun 14 2025

Winds will generally be southeasterly with some variance in and
near convection that develops later today. We refined the window
for convection slightly in the TAFs for GCC via a TEMPO, holding
PROB30 for RAP where uncertainty is highest. Amendments may be
needed. If a storm impacts either airport with expect brief
visibility reductions and strong wind gusts. Fog is again possible
at RAP and all across the Plains Sunday morning. Low probability
of IFR or lower, so MVFR for now. Greater chance of convection
just after the TAF period late Sunday for RAP.

&&

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

$$

UPDATE...Bowers
DISCUSSION/KEY MESSAGES...13
AVIATION...Bowers