Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 PM EDT Thu Oct 21 2021

Valid 00Z Fri Oct 22 2021 - 00Z Mon Oct 25 2021

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

A series of moisture plumes will bring widespread precipitation to
the West through the weekend. The first will be associated with a
sharp shortwave moving onshore the CA/OR coast Friday morning
while taking on a subtle negative tilt. This will be accompanied
by a Pacific jet streak, as well as low-level warm/moist advection
ahead of a surface cold front pushing to the east. Deep layer
ascent through height falls, LFQ diffluence, and low-level
convergence will produce robust omega into a region characterized
by PWs of more than 2 standard deviations above the climo mean.
Snow levels during this time will be rather high, above 9000 ft
during the period of greatest forcing, but will crash back to 5000
ft with still lingering snowfall late D1. WPC probabilities for 4
inches are moderate to high in the highest terrain of the WA and
OR Cascades, as well as parts of the Shasta/Trinity/Siskiyous,
with 2-3 ft possibly at the top of Mt. Shasta.

A brief lull in precipitation overnight into Saturday will quickly
be replaced by a second surge of moisture as yet another, weaker,
shortwave rotates within the broad cyclonic trough across the
Pacific Ocean and onshore. This feature will be accompanied by
modest LFQ diffluence and zonal but confluent mid-level flow to
increase column moisture once again. A weakening surface cold
front will also move onshore before dissipating, and another round
of high elevation snow is likely during Saturday. WPC
probabilities for 4 inches are focused in the highest terrain
around Mt Hood, Adams, and Rainier, where they exceed 40%.

Thereafter, an intense atmospheric river is likely to surge
onshore beginning early Sunday with IVT progged to exceeding the
climatological maximum according to NAEFS ensemble tables. This
intense AR will be driven by extremely confluent and moist flow
surging into the West south of a rapidly deepening cyclone near
British Columbia. Intense height falls and jet-level diffluence
will combine with robust WAA to drive strong omega through the
column, while PWs climb to 3.5 standard deviations above the climo
mean. This overlap of intense ascent and anomalous moisture will
produce heavy precipitation from central CA northward into
Washington State, but at the same time snow levels will surge to
above 11,000 ft in CA, and 6000-7000 ft as far north as WA. An
associated strong cold front will race eastward late D3 causing
snow levels to crash quickly, but this will primarily occur after
the D3 period and into Sunday night. WPC probabilities are high
for 4 inches in the WA Cascades, Shasta/Trinity range, and Sierra,
with low probabilities for 8 inches in the Sierra and near Mt.
Shasta. Much more significant snowfall is likely into D4 in these
same ranges.

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

Weiss