Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Tue Dec 24 2024

Valid 12Z Tue Dec 24 2024 - 12Z Fri Dec 27 2024


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

The active weather pattern continues this morning as 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. The ongoing AR will sustain an expansive IVT
above the 90th percentile according to NAEFS this morning that is
supplying ample Pacific moisture over the western third of the
CONUS. As the aforementioned upper trough continues its approach
this morning, snow levels will drop to as low as 2,500-3,000ft 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 in large part due to the anomalous
moisture, the excellent synoptic-scale ascent ahead of the trough,
and SWrly 850-700-500mb winds approaching the 90th climatological
percentile that maximize upslope potential. WPC's Snowband
Probability Tracker snows some impressive hourly rates today that
could eclipse 3"/hr in some cases. WPC probabilities show high
chances (>70%) for snowfall totals >8" above 7,000ft with moderate
chances (40-60%) for >12" snowfall totals at/above 9,000ft in the
central and southern Sierra Nevada. Farther east, 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 5,000ft in the Cascades/Olympics/Blue Mountains.

The next and more expansive AR arrives Wednesday as a powerful
Pacific storm heads for the British Columbia coast. NAEFS shows
this AR, with moisture origins from the subtropical central
Pacific, will be directed at the Pacific Northwest with IVT values
topping the 99th climatological percentile. Unlike the more recent
pair of ARs, this one will have a slightly colder air-mass to work
with at the onset while mean-layer 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 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. The moisture associated with
this AR will spread into the Northern Rockies late Wednesday night
and into Thursday with the heaviest snowfall in the Blue Mountains
>5,000ft and in the Sawtooth/Salmon River Mountains >6,000ft. WPC
probabilities depict high chances (>70%) for snowfall totals >8" at
those elevations through Thursday morning.

This unrelenting jet stream pattern looks to continue across the
Pacific Northwest on Thursday as Pacific moisture continues to be
delivered into the region ahead of the next storm system that
arrives Thursday night. The bulk of the snowfall, just like the
past couple of ARs, will remain mostly at more remote and higher
elevations from the Cascades and Olympics on south to the
Shasta/Trinity Mountains and the Sierra Nevada. This moisture
fetch will advance well inland yet again, reaching the Blue,
Sawtooth, Salmon River, Bitterroots, and Tetons Mountains. In fact,
some locally heavy snowfall may occur in the taller parks of the
Wasatch and northern Great Basin ranges. WPC probabilities show
another high chances event (>70%) for snowfall >8" above 4,000ft
in the Cascades and Olympics, above 5,000ft in the Oregon
Cascades, Trinity/Shasta
in northern California, and Blue Mountains of eastern Oregon, and
above 6,000ft in the Sawtooth and Salmon River Mountains of Idaho.
Throughout the next three days (through 12Z Friday) WPC
probabilities show high chances for snowfall totals >18" in the
taller and more remote reaches of these mountain ranges. Some
impactful snowfall at pass-level and along roadways that wind
around complex terrain may cause difficult to even impossible
travel conditions through the end of the week.


...Northeast & Mid-Atlantic...
Day 1...

The storm system that brought periods of snow to the Great Lakes
continues to track east this morning while directing anomalous
moisture aloft 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 a broad swath 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.

Snow should conclude across northern and western NY by mid-morning,
followed by the rest of northern New England by midday. Downeast
Maine will witness snow the longest and thus sports high-chance
probabilities (>70%) for >6" of snowfall through Tuesday afternoon
as a secondary coastal low takes over and heads into the Canadian
Maritimes. The WSSI depicts mostly Minor impacts from central and
northern New York to eastern Maine with only portions of Downeast
Maine highlighted for seeing Moderate Impacts (hazardous driving
conditions and disruptions to daily life expected).

In the Mid-Atlantic, the same tongue of moisture bringing snow to
the Northeast will generate a swath of light snow and wintry mix
from the central Appalachians to the New Jersey Shore. Soil
temperatures are near freezing and light ice or snow accumulations
may make for slick travel conditions this 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 this morning. Greater chances are 
in portions of the central Appalachians where low-to-moderate
chances (30-50%) are present.


Mullinax