Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
116 PM EST Fri Nov 10 2023

Valid 00Z Sat Nov 11 2023 - 00Z Tue Nov 14 2023

...Pacific Northwest into the Northern Rockies...
Days 1-3...

A series of back-to-back mid-level waves will rotate towards and
then onshore the Pacific Northwest/Northern Rockies through the
weekend bringing rounds of precipitation and modest snow levels.
The first impulse is progged to surge onshore tonight into early
Saturday coincident with the LFQ of a 130kt Pacific jet streak to
drive ascent from the Pacific Northwest through the Northern
Rockies. The mid-level flow will flatten and pinch behind this
impulse, driving increased moisture onshore noted by probabilities
for IVT exceeding 250 kg/m/s reaching above 90% into WA/British
Columbia according to CW3E. A weakening cold front will also be
pushed onshore and eastward during this time, trailing a leading
warm front, so snow levels will fluctuate significantly during the
precipitation, but may reach as low as 3500 ft by the beginning of
D2 /after 00Z Sunday/. This should keep most of the snowfall
impacts above Stevens and Snoqualmie Passes, but light snow
accumulations are still possible in these areas. Higher
elevations, including Washington Pass, will likely receive more
significant snowfall, and WPC probabilities for more than 12
inches are above 90% in these areas, with locally more than 3 feet
likely in the highest terrain. As this forcing and moisture shifts
eastward late D1 into D2, WPC probabilities for more than 12
inches reach 10-20% near Glacier NP.

After a brief respite in precipitation as the primary forcing
shifts east late Sunday into Monday, a secondary shortwave and
accompanying modest IVT will surge again into the Pacific
Northwest. IVT probabilities from CW3E are only 50-70% for 250
kg/m/s with this second event, and the best ascent looks to lag
offshore until just beyond this forecast period. Additionally, the
more S/SW flow into the coast will support higher snow levels on
the accompanying WAA than with the lead impulse, so snow levels
should remain above 4500 ft on D3. This is likely to limit both
snowfall accumulations and accompanying impacts, which is
additionally reflected by WPC probabilities for 6+ inches of
additional snowfall peaking above 30% in the highest terrain of
the WA Cascades.

Weiss