Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EST Thu Dec 26 2024

Valid 12Z Thu Dec 26 2024 - 12Z Sun Dec 29 2024


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

Active pattern over the Northwest continues into the weekend with
three notable waves through Saturday with a fourth approaching
Sunday. Moisture ahead of these waves generally have ties to the
subtropics north of Hawaii and the surface low tracks are near
50N which will maintain milder than average conditions and snow
levels generally around most pass levels in the Cascades. Each
wave will carry a surge in moisture, peaking around the 95th
percentile (IVT) then waning allowing snow levels fall again. This
will result in impactful snow to the Cascades, and at least into
Friday, the Sierra Nevada and Great Basin, continuing into
Saturday
for the central Rockies.

The heaviest precip of the next three days is going on this
morning with an additional 1 to 2 feet in the Cascades above the
near 4000ft snow level after 12Z. The potent low currently west of
WA quickly diminishes after plowing into Vancouver Island later
this morning and ridging builds this afternoon/evening ahead of the
next wave tonight. Day 1 PWPF for >8" is high, 50-90% over the Blue
Mtns of OR and the Sawtooths of ID up through the Bitterroots.

The low track tonight is a little farther south than the one today
with the not-as-potent low making landfall into the Olympic
Peninsula Friday morning. The moisture focus is over OR and
northern CA, but duration is limited due to the progressive nature
of the shortwave that is able to penetrate inland through the
Rockies. Snow levels over the OR Cascades and Sierra Nevada rise
tonight to 5000-6000ft with heavy snow rates above and Day 1.5
PWPF for >8" is generally 50-90% in the higher terrain. Further
snow over the north-central Rockies continues with 50-80% probs for
>8" over the Wasatch up through the Tetons and again for the
Sawtooth.

After brief ridging late Friday, the next round of precip over the
Northwest is a more prolonged onshore flow centered on WA/OR until
a surface low reaches the OR/CA coastal border late Saturday night.
Snow levels slowly rise in the prolonged moderate precip, to around
4000ft in WA, 5000-6000ft in OR and inland. Day 3 PWPF for >8" is
50-70% in the WA/OR Cascades, Sawtooths yet again, and the Tetons
to the Wind River and ranges along the southern ID/WY border.


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

High pressure shifting off the Eastern Seaboard and a wave coming
up from Texas with origins in the wave reaching the Pacific
Northwest on Friday lifts up the Ohio Valley Saturday night. Warm
air advection moisture and precip shift up the Northeast late that
night. With cold pre-conditions and possibly enough lingering cold
air, freezing rain becomes an increasing threat for the interior
Northeast. Day 3 PWPF for >0.1" ice is up to 20% in southern New
Hampshire.


Jackson