Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Fri Oct 22 2021

Valid 12Z Fri Oct 22 2021 - 12Z Mon Oct 25 2021

...Pacific Northwest and Northern California...
Days 1-3

An unsettled and active pattern for the region unfolds through
early next week as a series of shortwave troughs and atmospheric
rivers move onshore. Today, a compact/potent shortwave trough
located just offshore northern California will move onshore with
its associated plume of deeper moisture of near 1.25" PWs pushes
inland. Favorable forcing for ascent driven by upper diffluence,
low level warm air advection, and orographic upslope will produce
widespread precipitation from the Sierra Nevada northward across
Oregon and Washington. Initially higher snow levels early this
morning will crash and fall as the cold front, currently just
onshore, steadily moves east. Snow accumulations today will be
mainly light/minor aside from some slight to moderate
probabilities of exceeding 4 inches at the tops of the Oregon and
Washington Cascades and Mt. Shasta in California.

A brief lull and less active day is in store for Day 2 //ending
12Z Sunday// as a weaker shortwave trough and front pass through
the region. Forcing for ascent driven by the left exit region of a
110 kt jet streak will bring light precipitation to the region
with light/minor accumulations of high elevation snow. WPC
probabilities of 4 inches or more are slight only for the highest
tops of northern CA to OR/WA Cascades.

An extremely strong system is then poised to affect the region
during Day 3 //ending 12Z Monday//. An intense atmospheric river
event that is forecast to exceed the climatological maximum will
surge onshore on the southern edge of a rapidly deepening surface
low and intense upper level divergence as digging trough advances
onshore. This combination of moisture and lift will bring intense
and heavy precipitation to central/northern California where
liquid equivalent amounts for the 24-hour period could total
several inches. Snow levels will be 10-11+ kft initially during
the day but then will fall to around 7-8 kft as the cold front
passes late Sunday into early Monday morning. The overlap with the
deeper moisture/lift and crashing snow levels is expected to
produce significant snow for WA Cascades and the CA Sierra Nevada.
WPC probabilities for 4 inches or more are above 80-90 percent
with above 50 percent probabilities for 18 inches or more for the
Sierra Nevada. Additionally, WPC probabilities for 4 inches or
more are above 50 percent for the central Idaho mountains
including the Sawtooth, Boulders, Pioneers, and the Lost River
Range.

For Days 1-3, the probability of significant icing is less than 10
percent.

Taylor