FOUS11 KWBC 290840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Sun Dec 29 2024

Valid 12Z Sun Dec 29 2024 - 12Z Wed Jan 01 2025

...The West...
Days 1-2...

Strong trough with the base over far northern CA reaches the
Pacific Northwest this morning, sweeping east to the Great Basin by
this evening on a powerful 150kt zonal jet. The atmospheric river
going on ahead of the trough will be brought into focus up through
the OR Cascades today with height falls allowing snow levels to
drop to mountain passes such as Santiam by midday. Onshore flow and
cellular activity continues over the Cascades through tonight. Day
1 snow probs for >8" are 60-90% along the OR Cascades and 50-80%
for the WA Cascades, including around Snoqualmie Pass. The base of
the trough sends a strong plume of moisture down the northern/
central Sierra this afternoon with snow levels generally above 7000ft.

This afternoon through tonight, 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 again expected for the
central ID ranges into the Tetons/Yellowstone and SW MT where Day 1
snow probs for >12" are 50-80%. Monday morning through the
afternoon, height rises will spread east from the PacNW and produce
a welcome lull in activity through Tuesday.

Pacific Northwest...
Day 3...

Broad zonal flow below a Gulf of Alaska low reaches the PacNW coast
Tuesday night with light to moderate precip with snow levels
generally around 2000ft in WA and 3000ft in OR. Day 3.5 snow probs
(ending 00Z Thur) are generally 40-70% for >6" in the WA/OR Cascades.


...North-Central Plains...
Days 1-2...

Strong trough that pushes over the Pacific Northwest today rides a
150kt zonal jet over the CO Rockies early Monday while high
pressure builds south from the lee of the Canadian Rockies.
Convergence between this high and lee cyclogenesis over KS will
support an expanding area of precipitation over eastern MT and
northeast WY tonight where it is cold enough for snow and then
across western SD through central Neb Monday where temperatures
become increasingly marginal. A general swath of a quarter inch can
be expected through this path, but local enhancements from terrain
(like the Black Hills) and mesoscale banding will result in locally
heavy snow. Day 1.5 snow probs for >4" are splotchy along this path
with generally 20-50% probs from central MT through central SD and
the Pine Ridge of Nebraska (along with 80% probs in the Black
Hills). Those probs continue farther east over SD Day 2, but there
are still latitudinal differences with both the 00Z GFS and ECMWF
still being a bit farther south with accumulating snow over the
Nebraska Sandhills. Furthermore, the developing low over MO Monday
evening may allow some localized banding/dynamic cooling for snow
in southeast Neb/western IA, but it would be hard to overcome the
rather marginal thermals.


...Northeast...
Day 3...

The low tracking over KS/MO on Monday shifts east to the Mid-
Atlantic through Tuesday before shifting up the Northeastern
Seaboard Tuesday night. It's a progressive system, but with the
surface low translating to the coast around Long Island then
strengthening as it tracks northeast, there is a threat for banding
on the back side of the low. Marginal thermals, especially for the
end of the year, are present, but interior elevations in the
banding zone should see some snow accumulation. Please monitor this
storm which will affect New England through Wednesday and may
include some wintry mix in interior valleys.

Jackson
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