Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
224 PM EST Sat Dec 28 2024

Valid 00Z Sun Dec 29 2024 - 00Z Wed Jan 01 2025

...The West...
Days 1-3...

Successive atmospheric river events continue for the West Coast
and northern Intermountain West but will start to wind down in a
couple days. Strong 150kt upper jet has come ashore the OR coast
this morning and will continue to focus modest/heavy precipitation
over the Northwest into the northern Great Basin/Rockies tonight
into Sunday downstream of the upper trough axis. The base of the
mid/upper trough will reach the coast tomorrow morning and progress
inland, allowing colder air to move into the region as the
heaviest precipitation moves through the Sierra and across
Idaho/western MT. Sunday night into Monday, the jet will continue
through the Great Basin to the central Rockies, promoting broad
lift on the poleward exit region from ID to WY. Combined with
favorable upslope into the terrain, heavy mountain snow is likely
for the central ID ranges into the Tetons/Yellowstone and SW MT. By
Monday late morning through the afternoon, height falls will exit
the Rockies though lingering Pacific energy will be poised to come
into the West Coast albeit with much less QPF. By day 3, another
short wave moving through WA into western MT will wring out some
more snow for the region, focused around Glacier NP beneath the
track of the vort.

For the 3-day period, WPC probabilities for at least 12 inches of
snow are high (>70%) across the Cascades above about 4000-5000ft
(north to south) and above about 4000-6000ft toward the Divide. On
the southern side, with higher snow levels from UT to CO, WPC
probabilities for at least 12 inches of snow days 1-3 are high
above about 8000-9000ft.


...North-Central Plains...
Days 2-3...

Strong height falls moving out of the Rockies early Monday will
support an expanding area of precipitation across the
northern/central Plains where marginal temperatures will support
snow (or rain to snow) across the area from around the Black Hills
ESE across SD into Nebraska. Though the QPF area-wide may be on the
order of a few tenths of an inch (thus, 1-2" snow), mid-level
forcing on the north side of the 850/sfc low could support a narrow
axis of heavier amounts per some of the guidance (closer to 0.5"
liquid or ~4-5" snow). In addition, heavier rates even during the
afternoon may allow for more accumulation than advertised as the
system quickly passes through. WPC probabilities for at least 4
inches of snow are highest over the Black Hills but do extend
across SD a bit.


Fracasso