Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
233 PM EST Wed Dec 25 2024

Valid 00Z Thu Dec 26 2024 - 00Z Sun Dec 29 2024


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

Active N Pac will continue for the next few days with waves of
precipitation moving into the West ahead of incoming cold fronts.
These plumes of moisture generally have ties to the subtropics
north of Hawaii, and the storm track of the surface lows will
generally be near/north of 50N. This will maintain milder than
average conditions with snow levels near many passes across the
Cascades as the Atmospheric Rivers (AR) fluctuate in location.
Each wave will carry a surge in moisture with higher snow levels,
peaking around the 95th percentile (IVT) then waning as snow levels
fall again. This will result in impactful snow to the passes at
times (into the northern Rockies as well) and in the mountains all
across the Northwest/Sierra/Great Basin and northern/central
Rockies.

The D1 event will feature the highest QPF and snow, and per the
ECMWF EFI will be the most anomalous along/east of the Cascades
into the Blue Mountains in OR and over eastern WA/northern ID.
Several feet of snow is expected in the higher peaks. WPC
probabilities for at least 12 inches of snow just D1 are high
(>70%) above 4000ft or so in the Cascades and above 5000ft or so
in northeastern OR to central ID.

For D2-3, additional AR events are forecast coincident with a
couple waves in the flow off the N Pacific, aimed farther south
than the event D1 (NorCal/southern OR and points east). Each will
bring another round of snow to the region with little break in
between (6-18 hours at best for areas ~40-45N). D2 event will
focus into NorCal/the Great Basin/Wasatch/central Rockies as the
core of the next AR shifts south and punches well inland (IVT >90th
percentile into western CO late Fri). By D3, the next AR will
shift northward with a focus into central ID/western WY
(Bitterroots to the Tetons) but continuing into the Cascades. WPC
probabilities for at least 8 inches of snow each day are high
(>70%) above about 7000ft in the interior. The active pattern looks
to continue into the medium range.

The probability of significant ice accumulations > 0.25 inches
across the CONUS are less than 10 percent this period.

Fracasso