Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Fri Dec 27 2024

Valid 12Z Fri Dec 27 2024 - 12Z Mon Dec 30 2024


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

Prolonged winter weather impacts for terrain in the Pacific and
Interior Northwest persist through the weekend.

Broad troughing centered south of Alaska will continue to shed
shortwave energy through the Northwest through Saturday night
before the trough axis swings inland on Sunday. Convergent flow off
the Pacific is featured ahead of the main trough axis, leading to
rounds of elevated IVT as periodic atmospheric rivers (AR) surge
onshore. This moisture combined with powerful jet streaks aloft
will result in widespread heavy precipitation across much of the
Pacific and Interior Northwest, with the Sunday trough axis
spreading precip down to the central Sierra Nevada.

An ongoing wave will cross the OR coast early this morning with a
notable moisture surge crossing the Great Basin and lifting over
the north-central Rockies into this evening. Snow levels around
6000
ft are expected over NV/UT/CO, around 5000 ft in ID, and remain
around 3000-4000 ft in the Cascades, allowing at least modest
winter impacts at the most passes. WPC probabilities for >6" on Day
1 are high (70-90%) for the higher WA/OR Cascades, Blue Mountains,
Salmon River/Sawtooths, Wasatch, Uintas, Tetons, and into the Park
Range of CO. An additional 18" are likely in the highest terrain.

The next vort lobe arrives into the PacNW coast this evening which
is south of the surface low track into Vancouver Island and under a
150+ kt Wly jet streak. Enhanced ascent from jet streak will help
expand moisture, with upslope flow driving the most intense
precipitation into the Cascades and farther east in the Salmon
River/Sawtooth Ranges. Snow levels rise with this moisture surge
are generally 4000-6000 ft in the Cascades and the north-central
Rockies. Heavy snow is expected in terrain with Day 2 snow
probabilities for >8" 50-90% in the OR/WA Cascades, Salmon
River/Sawtooths and Tetons south through the Wasatch.

Confluent onshore flow with moderate to locally heavy precip
continues to push onshore until the trough passage Sunday
afternoon. Mesoscale forcing for ascent will be more robust Sunday
with the trough axis passage with snow levels generally around
3000 ft in WA with a baroclinic zone across OR where snow levels
should quickly rise to around 6000 ft. Day 3 WPC probabilities
are above 70% for >8" along the spine of the Cascades, yet again
through the Salmon River/Sawtooths, Blue Mountains, expand up
through the Bitterroots and Tetons south through the southern
Absarokas into west-central WY.
Additional 3-day total snowfall of 3-5 feet is likely in the
higher
terrain of the Cascades, Sawtooths, and Tetons.


...Central Appalachians and Northeast...
Days 1-3...

A pair of lows tracking up through the Great Lakes today through
Sunday will bring some freezing rain risk to the Northeast late
tonight and again Sunday night. Surface cold air will be initially
be trapped leading to some light freezing rain accumulations. Day
1.5 WPC ice for >0.1" around around 10% from the Poconos to the
southern Adirondacks with Day 2 up to 10% in south-central NH.

After a brief break in precipitation, a more significant wave of
low pressure will develop across the Ozarks Saturday evening in
response to a potent shortwave trough digging through the primary
trough axis from the Central Plains. A favorably placed jet streak
will leave sufficient diffluence through the right entrance region
to allow deepening of the surface wave as it moves into the
eastern Great Lakes Sunday night. To the east of this low,
precipitation will expand on renewed WAA, while subtle wedging of
the retreating high clings across northern PA, Upstate NY, and
northern New England. Eventually all of the precipitation should
turn to rain as WAA overwhelms the column, but a period of icing is
likely before that time, leading to at least modest accumulations
of ice, particularly over areas with frozen ground. Day 3 ice probs
for >0.1" are only up to 5% in central Maine - will see if these
probabilities continue to trend down in the coming forecast cycles.


Jackson