Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 PM EST Thu Nov 09 2023

Valid 00Z Fri Nov 10 2023 - 00Z Mon Nov 13 2023

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

An active stretch of winter weather is expected for the upcoming
weekend thanks to multiple rounds of atmospheric rivers. This is
due to an exceptionally intense upper low over southern Alaska and
the Gulf of Alaska that is directing a series of upper level
disturbances at the Pacific Northwest. The first atmospheric river
arriving this afternoon will contain an IVT of 250-400 kg/m/s with
vertical ascent supplied by an approaching upper level shortwave
trough and via upslope ascent into the Cascades. However, the mean
flow within the IVT is more out of the SSW, making it not fully
orthogonal to the terrain. WPC probabilities show moderate
probabilities (40-60%) for snowfall totals >6" through Friday
evening at elevations >5,000ft. The second atmospheric river,
which arrives Friday night and persists into Saturday, will be
stronger and its IVT vectors will be more favorably oriented out
of the W-SW. Not only does this support better upslope
enhancement, but the IVT is stronger (peaking around 500-600
kg/m/s early Saturday morning) and the upper level shortwave
trough on approach is both stronger and temperatures are colder in
wake of the cold frontal passage Saturday morning. This will force
snow levels to fall to lower levels (around 4,000ft) and SLRs will
also be higher during the day. WPC probabilities are sporting
low-moderate chances (20-40%) for >8" of snow in the Olympics
while the Cascade have a high chance (70-95%) for snowfall totals
>8". In fact, there are medium-high chances (60-80%) for >12" in
the >6,000ft elevations of the northern Washington Cascades. The
bulk of these heavier snowfall totals are likely to fall north of
I-90 in western Washington. Washington Pass is among the more
notable passes that could pick up over 12" of snowfall through
Saturday evening.

Latest WSSI through Sunday morning depicts Major Impacts for the
higher elevations of the Cascades that is primarily driven by the
Snow Amount algorithm, but there are also Moderate Impacts being
forecast for Blowing Snow. This is due to strong 850-700mb winds
on Saturday that are anywhere from the 90-97.5th climatological
percentile according to NAEFS Saturday morning. The IVT associated
with the second atmospheric river will advance as far inland as
the Northern Rockies midday Saturday and persist into Saturday
evening. A similar tandem of heavy snow and gusty winds are
causing the WSSI to detect Moderate impacts in parts of the
Selkirk and Purcell Mountains, as well as in the Lewis Range
(includes Glacier Nat'l Park). WPC probabilities show low-moderate
chances (30-50%) for snowfall totals >6" in these aforementioned
mountain ranges on Saturday. Expect a brief break in the round of
atmospheric rivers in the Northwest during the day on Sunday.

Mullinax