Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Wed Oct 20 2021

Valid 00Z Thu Oct 21 2021 - 00Z Sun Oct 24 2021


...Pacific Northwest and Northern California...
Days 2-3...

A sharpening mid-level trough will briefly tilt negatively as an
intense shortwave lifts onshore northern California Friday aftn.
This trough will be accompanied by impressive height falls and
PVA, as well as robust WAA on intensifying southerly flow. In
addition, developing jet streaks both upstream and downstream of
the trough will advect Pacific moisture onshore with lift being
provided through intense upper level divergence maxima. Diffluence
in the RRQ of the upwind jet will drive precipitation and snowfall
into the Cascades of OR and WA and Shastas/Siskiyous of CA D2,
with heavy upslope snow likely. Snow levels ahead of the
associated surface cold front will be 9000-10000 ft, before
falling to around 6000 ft in the post-frontal airmass. However,
most of the snowfall should occur ahead of the front, and WPC
probabilities for 4 inches are above 50%, but confined to the
highest terrain, with Mt. Shasta possibly receiving nearly 2 ft of
snow.

The secondary and downstream jet streak will pulse onshore into
OR/CA D3, bringing a renewed surge of moisture and IVT. This will
produce another round of heavy precipitation within the divergent
LFQ of this jet streak. Snow levels will again climb on WAA, but
remain much lower than with the precipitation on D2. WPC
probabilities for 4 inches are as high as 30-40%, generally above
6000-7000 ft in the terrain from the Olympics southward along the
Cascades and towards Mt. Shasta.  

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

Weiss