496
FXUS63 KUNR 161715
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1115 AM MDT Wed Apr 16 2025

.KEY MESSAGES...

- Warm with elevated fire weather conditions today due to very
  low relative humidity
- Thursday/Friday, accumulating snow for northeastern WY/Black
  Hills
- Unsettled weather Sunday through the first half of next week

&&

.DISCUSSION...(Today Through Tuesday)
Issued at 217 AM MDT Wed Apr 16 2025

08z surface analysis had trough from southern SK to eastern CO
and a cool front entering northern MT. Water vapour loop had upper
low/trough spinning over northern AB/SK. Ahead of upper trough,
flattening upper ridge promoting CI spilling into the northern
plains. Effect of upper trough the main concern this forecast.

Today, surface trough slips eastward as it becomes
southwest/northeast oriented, bisecting the CWA this afternoon.
Temperatures will be near guidance with readings well into the
70s/lower 80s ahead of the trough (HREF 70%+ chance >80F)
along/south of I-90 east of the Black Hills.

Tonight through Friday, upper trough expands into the Rockies and
slowly moves east. Cold front slips through the CWA as it becomes
parallel to upper flow and stalls southeast of the CWA Thursday.
Post frontal lift per modest 800-600mb frontogenesis ahead of
upper trough will create a band of precipitation tonight, but
especially for Thursday. Low level cold air advection enhanced by
precipitation will support mostly snow Thursday over the Black
Hills/northeastern WY and rain east of the Black Hills. QPF will
be enhanced by northeasterly boundary layer flow as it interacts
with the terrain, but it may entrain drier air from ND limiting
QPF amounts east of the Black Hills. Not particularly confident in
QPF amounts given ensemble guidance uncertainty, so snow amounts
tricky given warm ground and good chunk of QPF falling during the
day Thursday. HREF depicts 60-90% chance >0.25" QPF from the
northern Black Hills into northeastern WY and 60-90% chance of
>0.50" QPF for a small part of the northern Black Hills. Have
adjusted QPF to match with a few inches of snow possible over
northeastern WY and several for the Black Hills. Will hold off on
headlines given low confidence, and especially since WSSI shows
almost no minor impacts over the CWA. Have shaded temperatures
colder for Thursday/Friday given cloud cover and precipitation
hopes. Finally, gusty winds expected with 50% chance of >45mph
wind gusts over southern SD Thursday.

Saturday looks seasonal and dry. Progressive flow continues for
Sunday through early next week with unsettled weather foreseen with
near seasonal temperatures.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 1114 AM MDT Wed Apr 16 2025

VFR conditions are expected through this evening. A cold front
will move through after midnight, bringing some MVFR/IFR/LIFR
conditions and rain/snow to northeastern WY and northwestern SD
after 08Z. Additionally, northwest winds will be 15 to 25 mph
with gusts up to 40 mph with the front.

&&

.FIRE WEATHER...
Issued At 217 AM MDT Wed Apr 16 2025

Elevated fire weather conditions today due to warm temperatures
and very dry air in place. Minimum relative humidities from 10-20%
expected over the southeast two thirds of the CWA with 15-25%
over the northwest third. Winds look to be on the lighter side
with joint probabilities of Red Flag Warning criteria conditions
very low over most of the area except for far southern Campbell
County with a 20-40% chance. Will forgo headlines given
probabilities. Vastly improved fire weather conditions develop
tonight as a storm system brings very good chances for wetting QPF
Thursday over much of the area with accumulating snow for
northeastern WY/parts of the Black Hills.

&&

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

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...SE
FIRE WEATHER...Helgeson