Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Tue Oct 26 2021

Valid 12Z Tue Oct 26 2021 - 12Z Fri Oct 29 2021

...Southern California through the North-Central Rockies...
Day 1...

Longwave troughing and embedded shortwave energy digging across
the Four Corners region will bring large scale forcing for ascent
downstream across the central/northern Rockies today and tonight
with the trough axis taking on a negative tilt as it pushes out
into the southern/central Plains by the end of the day 1 period
//12Z Wednesday//. Surface low pressure will advance through the
Great Basin to the northern High Plains with the greatest snow
accumulations confined to elevations above 9000 ft with some
lowering by the time the snow ends late tonight. The greatest
accumulations will be found across the western CO Rockies
including the San Juans. WPC probabilities for 6 inches are
moderate to high across the western CO Rockies.


...Pacific Northwest...
Days 1-3...

The North Pacific Jet will reach the Pacific Northwest later
tonight with shortwave troughing approaching far southern British
Columbia and far northern Washington State. Favorable onshore flow
and a feed of higher moisture combined with the forcing for ascent
will bring widespread precipitation to the region over the next
1-2 days. With snow levels around 5000 ft, there are moderate to
high probabilities for 6 inches for the WA Cascades on Day 1,
slight to moderate probabilities for Day 2 //ending 12Z Thursday//
and then dropping off for Day 3 as ridging aloft asserts itself
ahead of the next system just beyond the current forecast period.
Over the course of the 3 days, the highest peaks of the WA
Cascades could pick up 2-3 ft.


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


Taylor