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

Valid 12Z Wed Dec 25 2024 - 12Z Sat Dec 28 2024


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

This exceptionally busy Pacific wave train will continue to direct
storm system after storm system at the Pacific Northwest and the
Rockies through the end of the week. The next in a series of
Pacific storm systems arrives Wednesday afternoon with the
the divergent left-exit region of a 120kt 200mb jet streak moving
in over the Pacific Northwest. The atmospheric river (AR) peaks
>1,000 kg/m/s off the Northern California coast, directing an
anomalous PW plume (>90th climatological percentile) throughout the
Pacific Northwest Wednesday night and into Thursday. Snow levels
look to start out between 2,500-3,500 ft in the Cascades and
Olympics, then rise to as high as 4,000ft in some areas Thursday
afternoon as snowfall rates diminish. Snow will spread inland
across the Blue, Salmon River, Sawtooth, Tetons, and as far south
as the Wasatch and northern Great Basin. Snow from this system
should taper off in the Tetons and central Rockies by Thursday
night.

The next Pacific storm system arrives Thursday night that
accompanies another IVT topping out around 1,000 kg/m/s that
delivers yet another round of heavy mountain snow from the Sierra
Nevada on north to the Cascades and Olympics through Friday
morning. Snow levels in the Cascades will generally be around the
same levels as the first event while the heavier snowfall totals in
the Sierra Nevada will be above 6,000ft. Similar to the last AR,
this AR will spread anomalous moisture into the Northern and
Central Rockies with most heavier amounts in the higher/more remote
parts of the central and northern Rockies. This will once again be
the case Friday evening when a third AR brings another surge in
Pacific moisture to the Pacific Northwest with more heavy mountain
snow in the Olympics, Cascades, Blue, Sawtooth, and Bitterroot
Mountains.

WPC probabilities for at least 18 inches of snow through the 3-day
period are high (>70%) above about 3000-4000ft from north to
south along the Washington Cascades and in the Olympics. Total
snow accumulations ranging between 4 to 6 feet in the higher
terrain of the Cascades. WPC probabilities for at least 12 inches
through the period are high (>70%) in the higher peaks of the Blue,
Salmon River, Sawtooth, Teton, and Bear River Mountains. The
cumulative 3-day WSSI depicts Major Impacts in the Olympics and
Cascades above 4,000ft, indicating considerable disruptions to
daily life (including dangerous to impossible travel conditions)
are expected through much of the week. Moderate to Major Impacts
are possible as far south as the Trinity/Salmon mountains of
Northern California, the Blue and Sawtooth Mountains, and as far
east as the higher elevations of the Bear River and Wasatch of
northern Utah.


The probabilities for significant ice accumulations across the
CONUS are less than 10 percent.

Mullinax