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

Valid 00Z Tue Dec 24 2024 - 00Z Fri Dec 27 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 next AR will arrive late
this afternoon through this evening with integrated vapor
transport
(IVT) values topping 1,000 kg/m/s off the Oregon coast and
moisture origins stemming out of the subtropical Pacific. This
initial round of precipitation 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 and associated surface cold front will
cause snow levels to crash through out the day. They're expected to
drop as low as 2,500 to 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 50-70% 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 (70-80%) for >8" of snowfall for
elevations >3,000ft, and a 50-70% 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. WPC Winter Storm Outlook values
based on the latest guidance for the Washington Cascades and
Olympics as well as the ranges in eastern Oregon and Idaho have all
increased through Wednesday and Thursday to over 90% of seeing
warning criteria amounts of snow.


...Northern Plains, Great Lakes, to Northeast...
Day 1...

A storm system over the Great Lakes is causing periods of snow from
central Wisconsin through lower Michigan and beginning to spread
into northern New York this afternoon. As the storm heads east
tonight, the storm will direct its anomalous moisture 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 as a secondary coastal low takes over and heads
into the Canadian Maritimes. 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 1...

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.
Soil temperatures are close to freezing and light ice or snow
accumulations could make for slick travel conditions Christmas Eve
morning. WPC probabilities show some low chance probabilities
(10-30%) for ice accumulations >0.01" in parts of the DC,
Baltimore, and Philadelphia metropolitan areas Tuesday morning.
Greater chances are along the Mason Dixon Line between Frederick
and Baltimore, MD through Harrisburg, PA with 20-40% chances of
.01" of freezing rain.


Wegman/Mullinax