Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Sat Oct 23 2021

Valid 12Z Sat Oct 23 2021 - 12Z Tue Oct 26 2021

...Pacific Northwest and Northern California to the Central
Rockies...
Days 1-3

An active pattern continues across the West as waves of heavy
precipitation move onshore from the Pacific Ocean and spread
eastward. Upper level troughing extending from the Pacific
Northwest through the Intermountain West today and an atmospheric
river bringing a steady plume of higher moisture /near 1" PWs
along CA coast/ will provide the necessary moisture and lift in
the region for widespread precipitation with light to moderate
intensity. A relatively progressive flow pattern and snow levels
above 6-7 kft will keep snow totals in check and the latest WPC
probabilities for 4" range from 20 to 40 percent for the highest
peaks of the Cascades.

A significant atmospheric river is then poised to impact the
region late Sunday into Monday //D2 into D3// associated with a
highly anomalous low pressure system and digging mid-level
shortwave trough. Near record atmospheric river moisture will work
with the favorable mid/upper level dynamics to produce widespread
moderate to heavy rainfall. This will lead to a longer duration
event, particularly for the Sierra where perpendicular flow will
drive enhanced orographic ascent. An overlap of crashing snow
levels and remaining moisture will bring heavy snowfall and
intense snow rates late in D2 through D3 for the Sierra as well as
the Sawtooth and Blue Mountains and WA Cascades. At the peak of
the event, the WPC probabilities for 6 inches or more are well
above 90 percent for the Sierra and above 40 percent for the WA
Cascades higher peaks as well as the central Idaho mountain ranges
above 7000 ft. The maximum probabilities for 18 inches or more
reach 60-70 percent for the highest peaks of the Sierra where
storm totals of 3-4 ft are expected.

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

Taylor