FOUS11 KWBC 230757
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Mon Dec 23 2024

Valid 12Z Mon Dec 23 2024 - 12Z Thu Dec 26 2024

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

An active pattern continues this week with mountain snow expected
from the West Coast mountain ranges to both the Northern and
Central Rockies. This stormy pattern in the short range is due to
three atmospheric rivers (ARs) bringing plume after plume of
Pacific moisture into the western U.S.. The first AR will gradually
weaken throughout the remainder of the day, but residual Pacific
moisture and a lack of a meaningfully cold air will keep most
heavy snowfall in the Northern Rockies above 7,000ft. Snow should
taper off over the Northern Rockies by Monday evening.

The second AR arrives Monday evening with IVT values topping 1,000
kg/m/s off the Oregon coast and moisture origins stemming out of
the subtropical Pacific. The initial round of precipitation
arriving Monday afternoon will be primarily rain in northern CA,
western OR, and western WA as snow levels in the OR Cascades are as
high as 8,000ft and even topping 9,000ft in northern CA. The
Cascades and Blue Mountains will have the better odds of staying
mostly snow above 5,000ft. By Tuesday morning, sharper 700-500mb
height falls from the approaching upper trough will force snow
levels to drop as low as 3,000ft at pass level in the WA Cascades
and below 6,000ft in the Sierra Nevada by Tuesday afternoon. The
Sierra Nevada have the best odds for an impactful snowfall event
with the Winter Storm Outlooks now sporting 50% odds of snowfall
exceeding warning criteria for elevations >7,000ft. Periods of
high elevation snow in the Great Basin and Wasatch are likely
Tuesday night but heavier accumulations (>4" will be mostly
confined to elevations at/above 8,000ft. Through 12Z Wednesday, WPC
probabilities show moderate-to-high chances (50-70%) for snowfall
>8" above 7,000ft in the Sierra Nevada, above 5,000ft in the
Cascades/Olympics/Blue Mountains, and above 8,000ft in the Tetons.

The third and more expansive AR arrives Wednesday as a powerful
sub-960mb low tracks towards the British Columbia coast. NAEFS
shows this AR with moisture origins northwest of Hawaii that will
be directed at the Pacific Northwest, including IVT values topping
the 97.5 climatological percentile. Unlike the first pair of ARs,
this one will have a slightly colder air-mass to work with at the
onset while mean 700-300mb winds out of the WSW are better aligned
orthogonally to enhance upslope ascent into the Olympics and
Cascades. This is a recipe for heavy/wet snow in the Olympics and
Cascades Wednesday afternoon and into Wednesday night. Latest WPC
probabilities show high chances (>70%) for >8" of snowfall for
elevations >3,000ft, and similar chances for >12" of snowfall
above 5,000ft. The WSSI-P does show 50-70% odds for Moderate
Impacts in these ranges through Thursday AM, which would include
some of the WA Cascade passes.


...Northern Plains, Great Lakes, to Northeast...
Days 1-2...

A positively tilted 250-500mb trough over the Midwest this morning
is providing sufficient upper-level ascent over the Great Lakes and
supporting a weak area of low pressure tracking towards southern
Wisconsin. Broad 850mb WAA and 290K isentropic glide via SWrly flow
will introduce a slug of Gulf of Mexico moisture (embedded within a
>300 kg/m/s IVT) into the region which maintains sub-freezing
boundary layer temperatures as the precipitation arrives. This
will result in periods of snow on the north side of the low that
stretches from southeast MN and central WI to northern MI today and
into tonight. WPC probabilities show moderate-to-high chances
(50-70%) for snowfall totals >4" across the northern tier of
Michigan's Mitten with low chances (10-30%) or >6". Some light
icing is possible in parts of southeast MN, western WI, southern
MI, and northern IN where there are moderate-to-high chances
(50-70%) for ice accumulations >0.01". With ground temperatures so
cold, even minor amounts <0.1" can cause slick conditions on roadways.

As the storm heads east Monday night, storm system will direct its
anomalous moisture source and modest low-level WAA into the
Northeast. Modest upper-level divergence in the form of a
strengthening 250mb jet streak aloft provides support for the
healthy shield of precipitation throughout the region, while the
air-mass remains quite cold and dry ahead of the storm (temps in
the teens,single-digit dew points). Higher SLRs of 12-16:1 are
anticipated in these areas and soil temperatures are near freezing,
which combined with the event occurring at night, will maximize
snowfall accumulation potential. The storm races off over Nova
Scotia by midday Tuesday and snowfall should taper off by Tuesday
afternoon. WPC probabilities show moderate-chance probabilities
(40-60%) for snowfall >6" in parts of northern NY (including the
Tug Hill) and in the tallest peaks of the Green and White
Mountains. These mountain ranges are favored for the heaviest
snowfall given the added help of upslope flow. Downeast Maine
sports moderate-chance probabilities for >6" of snowfall through
Tuesday morning. For those affected areas listed above, the WSSI-P
has increased its chances for Minor Impacts to moderate-to-high
chances (50-70%) from northern MI on east through northern NY and
into northern New England.

...Mid-Atlantic...
Day 2...

Residents in the Mid-Atlantic will want to monitor the forecast
closely Tuesday morning as the same tongue of moisture bringing
snow to the Northeast could bring a swath of light snow and/or
wintry mix from the central Appalachians to the New Jersey Shore.
Soils temperatures are close to freezing and light ice or snow
accumulations could make for slick travel conditions Christmas Eve
morning. WPC probabilities do show some low chance probabilities
(10-30%) for ice accumulations >0.01" in parts of the DC,
Baltimore, and Philadelphia metropolitan areas Tuesday morning.
Odds of >0.01" ice accumulations (low-to-moderate chances, or
30-50%) are greater to the west of these cities in parts of
northern MD, southeast PA, and the central Appalachians.

Mullinax
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