Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Fri Oct 08 2021

Valid 12Z Fri Oct 08 2021 - 12Z Mon Oct 11 2021

Day 1...
...Sierra Nevada to the North-Central Rockies...
Mid-level shortwave will push into California this morning with an
impressive moisture feed from the tropics near 130W. Precipitable
water values were near +3 to +4 sigma on the southeast side of a
130kt jet streak from the southwest to the northeast. Snow will
pick up in earnest later this morning through the afternoon in the
higher terrain of the Sierra as the shortwave weakens and moves
northeastward, only to be reinforced from the north by another
vort max. With decreasingly favorable dynamics and transient
height falls, but still locally favorable orographic enhancement,
snow will spread northeastward across the Great Basin (Ruby
mountains in Nevada) to the Rockies (Uintas northward into the
Tetons) with some higher elevations receiving 6-12"+, especially
into the Wind River Range through the day today. WPC probabilities
of greater than 4" are highest there, exceeding 80-90% at and
above 11,000 ft.

Day 2...
...Wyoming...
Small area of 30-50% probabilities of greater than 4 inches of
snow over the Bighorns in Wyoming and an even smaller area (~30%)
over the northern Absaroka Range as the exiting shortwave starts
to deepen on its way into the Dakotas, mostly focused between 12Z
Sat and 00Z Sun.

Day 3...
...Northern Cascades... 
An upper low currently in the eastern Bering Sea will move across
mainland Alaska and weaken as its southern extent digs
southeastward, promoted by a 150-160kt anticyclonically-curved jet
by late Sun into early Mon. This will drive a strong cold front
across the Pacific Northwest with 700mb temperatures quickly
dropping to -10 to -12C over WA/OR. In response, snow-to-liquid
ratios should rise and compensate for decreasing QPF, but still
yielding several inches of snow atop the Cascades. Probabilities
for snow amounts greater than 4 inches are generally 10-50 percent
for the higher peaks of the Washington Cascades as snow levels
drop to around 3000 ft by early Monday.

 
For Days 1-3, the probabilities for significant icing (0.25 inch
or more) are less than 10 percent. 
 
Fracasso