820
FXUS63 KUNR 022256
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
356 PM MST Thu Jan 2 2025

.KEY MESSAGES...

- Seasonally cold through Friday with weak disturbances shaking
  out some light snow or flurries
- Upper trough brings accumulating snow over most of the area
  late Friday night through Sunday afternoon
- Arctic air for most of the area for the weekend through Tuesday

&&

.DISCUSSION...(This Evening Through Thursday)
Issued at 212 PM MST Thu Jan 2 2025

20z surface analysis had 1041mb arctic high over central AB,
extending into IA. On the back side, stationary frontal boundary
extended from western MT through WY into eastern CO. Water vapour
loop depicted northwest flow aloft with 150kt jet stream ripping
across the northern Plains, as well as an upper trough spinning
toward the west coast. Weak upslope flow over the eastern half of
CWA sustaining stratus and a few flurries. Main forecast concern
revolves around trough approaching the west coast.

Tonight through Friday afternoon, upper trough plows into the
northwest CONUS. Southwest flow ahead of upper trough will promote
transient upper ridge over the CWA and then southwest flow
aloft/warm air advection over stationary frontal boundary.
Continued chances for very light snow/flurries/patchy fog given
increasing easterly component to boundary layer flow. Temperatures
will be near guidance.

Friday night through Sunday, upper trough moves through the
Rockies into the plains, reaching the CWA Sunday morning as main
energy spins up vigorous upper low over the southern plains.
Stationary frontal boundary becomes inverted trough as main
surface cyclogenesis spins up over OK. 1042mb arctic high squeezes
into the northern plains with tightening pressure gradient
Saturday leading to gusty southeast winds. 100-150% PWATs pool
above boundary as isentropic lift/low-level frontogenesis
increases across southeast MT into northwest/central SD. Should
see a band of light snow develop Friday night/Saturday,
intensifying Saturday night as main QG-forcing clips the CWA.
Potential for banded snow, with highest accumulations from K2WX-
KICR (>80% chance 4"+ snow). Lower amounts over southwest
SD/east-central WY (>80% chance 1"+ snow). Confidence is high
that most of CWA will have an inch of snow on the ground by Sunday
night, but confidence in where higher amounts low-moderate given
main upper/surface low so far south. Will have to rely on robust
isentropic lift/banded snow and higher SLRs to pump up amounts.
Winds behind inverted trough not too strong, although patchy
blowing snow will occur given fluffy snow. Thoughts portrayed by
pWSSI with only minor impacts for this storm foreseen.
Temperatures will be near guidance.

Monday through Thursday, confidence is high that northwest flow
will bring several weak/moisture-starved disturbances. Low
chances for snow with best chance over the northern Black Hills
given lower tropospheric flow orientation. Chilly air
Monday/Tuesday looks to be replaced by warmer air by mid-week, but
suspect snow-covered areas on the plains will loathe to release
arctic air quickly.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Friday Evening)
Issued At 349 PM MST Thu Jan 2 2025

MVFR/IFR to localized LIFR conditions in stratus and BR/FG will
continue across portions of the western SD plains, foothills, and
northern Black Hills/Bearlodge Mountains through much of the
period. Light snow and fog may develop early Friday morning
(after ~12z), mainly across the western SD plains and the Black
Hills and persist through the end of the period.

&&

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

$$

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