944
FXUS63 KUNR 101127
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
427 AM MST Fri Jan 10 2025

.KEY MESSAGES...

- Snow this weekend with areas of hazardous travel due to
  snow/blowing snow
- Strongest, gustiest winds Saturday
- Heavy upslope snow for the favored areas of the northern Black
  Hills this weekend
- Heavy snow possible over northeastern Wyoming

&&

.DISCUSSION...(Today Through Thursday)
Issued at 237 AM MST Fri Jan 10 2025

09z surface analysis had weak ridge over the northern Plains.
Water vapour loop had upper ridge spilling from the northern
Rockies into the northern Plains ahead of upper trough coming
onshore via BC. This will be the main weather maker this weekend.

Today, upper/surface ridge shift eastward into the Midwest as
upper trough slides into the northern Rockies. Reasonably pleasant
January day expected.

Tonight through the weekend, upper trough moves into the Plains
by 00z Sunday and then the upper Midwest by 00z Monday as it
merges with an upper low sinking south out of central Canada.
Surface low develops over central Canada, which will move across
the Dakotas and then the upper Midwest. Low will pull a warm front
through the CWA tonight/Saturday morning with a trailing cold
front moving through Saturday, and a secondary/stronger cold front
late Saturday night/early Sunday morning. Precipitation will
breakout along warm front tonight with first impulse of synoptic
lift/modest low level frontogenesis. Some rain will occur behind
warm front and ahead of the cold front, especially east of the
Black Hills per forecast soundings. A brief period of freezing
rain may occur ahead of the warm front over central SD, but ice
accumulations should be light per forecast soundings/NBM guidance.
Secondary impulse with secondary cold front brings additional
chances for snow into Sunday. Cyclonic flow atop northwest low
level flow will promote upslope snow machine per Froude >1 and
0-2km mean RH>90% over a prolonged period for the northern Black
Hills. 70-100% chance of >0.75" QPF for the northern Black
Hills/foothills, so confidence is high enough to issue winter
storm warning to account. 30-50% chance >0.40" QPF for the
northeastern WY plains adjacent to MT, so will continue the Winter
Storm Watch there. Headlines elsewhere more nebulous given sharp
decrease in QPF east of the Black Hills, but two things for later
shifts to watch. 1) Blowing snow potential given tight pressure
gradient/cold air advection and >50% chance >40mph gusts per HREF.
May need wind headlines if blowing snow remains minimal. 2) High
resolution guidance suggests the Pine Ridge may see enhanced
downstream convergence snowfall leading to headlines. Temperatures
will be tricky with multiple frontal passages, so haven`t strayed
too far from ensemble depictions.

Upslope snow winds down Monday. Northwest flow aloft expected
next week with low level downslope flow promoting seasonal
temperatures, especially for the favored banana belt areas on the
eastern slopes of the Black Hills. Drier pattern expected by
Tuesday, but can`t rule out passing shortwaves creating puffs of
precipitation.

&&

.AVIATION...(For the 12Z TAFS Through 12Z Saturday Morning)
Issued At 422 AM MST Fri Jan 10 2025

VFR conditions are generally expected through today. A cold front
approaching the region late tonight will bring increasing chances
for rain/snow and associated MVFR/IFR conditions over
northeastern WY into western SD.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning from 5 AM Saturday to 5 PM MST Monday for
     SDZ024-025.
WY...Winter Storm Warning from 5 AM Saturday to 5 PM MST Monday for
     WYZ057.
     Winter Storm Watch from late tonight through Monday afternoon
     for WYZ060.
     Winter Storm Watch from late tonight through late Sunday night
     for WYZ054-056.
&&

$$

DISCUSSION/KEY MESSAGES...Helgeson
AVIATION...Wong