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

Valid 12Z Fri Nov 10 2023 - 12Z Mon Nov 13 2023

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

Another atmospheric river will take aim on the Pacific Northwest
later tonight into this weekend, bringing unsettled weather and
locally heavy snowfall to parts of the higher elevations of the
Olympics, Washington Cascades, and northern Rockies. In the wake
of the ongoing system, a brief lull in heavier precipitation is
expected for today across the Pacific Northwest, with generally
light amounts expected. After 00Z Saturday, another moderately
strong atmospheric river will impact the region, characterized by
IVT values per CW3E between 500-700 kg/m/s between 00Z-12Z
Saturday. This combined with favorable upper divergence from a
130-150 kt jet streak positioned in the area will help support
locally heavy rainfall mostly across the Olympics but also
extending across the northern WA Cascades. Snow levels between 3-4
kft initially, will rise to around 4500-5500+ ft late tonight as
the core of the atmospheric river arrives. This will bring the
greatest snowfall accumulations mainly to the higher elevations
through 12Z Saturday, with the latest WPC snow probabilities for
at least 4 inches above 50 percent for the highest peaks. After
12Z Saturday, snow levels fall in the wake of the passing front
but with continued moisture transport and lift in the area, this
is expected to bring greater snow accumulations for the WA
Cascades and spreading eastward into portions of the northern
Rockies. WPC snow probabilities for the 24-hr period ending 12Z
Sun are high (>80%) for 12 inches at the highest elevations and
some localized higher totals at the northern Cascade peaks could
top 2 feet before precipitation begins to wind down after 12Z
Sunday. 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 18" of snowfall
through Saturday evening. With this winter storm, the Winter Storm
Severity Index (WSSI) is showing moderate to major impacts
expected for Washington Cascades, especially northern areas. This
is being primarily driven by the Snow Amount/Snow Load but also is
factoring the Blowing Snow due to the very strong winds that are
expected on Saturday. Further east, a similar tandem of heavy snow
and gusty winds is expected late Saturday into Sunday morning and
is causing the WSSI to detect Moderate impacts in parts of the
Selkirk and Purcell Mountains, as well as in the Lewis Range
(includes Glacier National Park). The latest WPC snow
probabilities exceed 60-70% for these mountain ranges with some
localized multi-day totals exceeding a foot likely.

Taylor