Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 PM EDT Thu Oct 07 2021

Valid 00Z Fri Oct 08 2021 - 00Z Mon Oct 11 2021

...Sierra Nevada to the North-Central Rockies...
Days 1-2...
Upper air analysis shows deep southwesterly flow from the Pacific
into the western U.S. along the leading edge of a
positively-tilted trough centered off of the California coast. 
Farther to the north, an additional shortwave is digging south
along the British Columbia coast toward Washington.  Overall,
guidance has been consistent in showing a mid-level shortwave west
of California, along with a 130+ kt upper jet, translating
northeast across California into Nevada-Utah on Friday. 
Broad-scale ascent along the left exit region of the upper jet,
along with low level frontogenesis, will support a stripe of
organized precipitation from the northern Sierra Nevada to
northern Utah and western Wyoming on Friday.  Snow levels are
expected to be around 8000 ft over the Sierra Nevada Friday
morning, and between 9000-11000 ft for most areas farther east. 
This will initially confine any significant amounts east of the
Sierra Nevada largely to the higher reaches of the Uintas in
northern Utah and the Wind River Range in western Wyoming through
late Friday.  As this leading shortwave lifts northeast across the
north-central Rockies, followed by the more northerly wave digging
south into the Great Basin, snow levels are expected to decrease
Friday evening through the overnight into early Saturday --
supporting the increasing potential for accumulating snows over
the Ruby Mountains in northeastern Nevada, with an expanding
threat across the western Wyoming ranges and northern Utah ranges.
 Day 1 (ending 00Z Saturday) WPC probabilities for snow amounts
greater than 4 inches are 10-40 percent for portions of the
northern Sierra and the Uintas, and 40-70 percent for portions of
the Wind River Range.  For Day 2 (ending 00Z Sunday), WPC
probabilities of 40-70 percent for snow amounts greater than 4
inches are introduced for a portions of the Big Horn Mountains,
while expanding across a broader reach of the Wind River Range,  
Lower probabilities for snow amounts greater than 4 inches are
introduced for portions of the Absaroka and Teton ranges on Day 2
as well.

...Northern Cascades...
Day 3...
An upper low currently north the Aleutians will continue to deepen
as it moves west into Southwest Alaska on Friday before weakening
as it moves farther east on Saturday.  South of the low, an upper
level shortwave is forecast to dig southeast, driving a strong
front across the Pacific Northwest on Sunday.  Day 3 (ending 00Z
Mon) probabilities for snow amounts greater than 4 inches are
generally 10-40 percent for the higher peaks of the Washington
Cascades. 

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

Pereira