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

Valid 00Z Sun Oct 24 2021 - 00Z Wed Oct 27 2021

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

An anomalously strong closed 5H low will move eastward towards WA
state Sunday before lifting onshore southern British Columbia
Monday night. As this drifts east, a secondary reinforcing
shortwave will rotate into CA, amplifying into a full latitude
western trough Monday before swinging into the Intermountain West
on Tuesday. This evolution will be accompanied by intensifying
upper jet streaks both upwind and downwind of the main trough
axis, producing large scale vertical motion through robust
diffluence. Additionally, low and mid level flow out of the W/SW
will advect copious moisture onshore as an intense AR, with PWs of
+3 to +4 standard deviations above the climo mean, peaking Sunday
into Monday. The overlap of this impressive moisture plume with
deep layer ascent will lead to widespread heavy precipitation,
which will fall as snow in much of the terrain. The heaviest snow
is likely in the Sierra where orthogonal flow will ideally upslope
into the region, but additional heavy snow maxima are likely in
the Sawtooth and Cascades.

The significant QPF will begin the latter half of D1 /Sunday/ as
IVT shifts eastward to impinge upon the coast. This will be
associated with strong WAA ahead of a cold front which will push
onshore late Sunday. Ahead of this front, the warm air surging
northeastward will drive snow levels to 6000-7000 ft in WA/OR, and
as high as 11,000 ft in CA. This will confine the heaviest snow
above these thresholds on D1, with the most intense snow rates
occurring through upslope enhancement in the Sierra and Sawtooth
Ranges. In these ranges, WPC probabilities for 8 inches are
20-40%, with accumulations elsewhere likely 6 inches or less
except in the high peaks of Mt. Shasta, the Blue Mountains of OR,
and the WA Cascades. Modest accumulations are also likely as far
east as the Wind Rivers and Colorado Rockies on the periphery of
the IVT plume.

D2 /Sunday night and Monday/ is likely to feature the most intense
snowfall with snow rates exceeding 3"/hr, especially in the
Sierra, as the AR shifts directly into the region and snow levels
crash behind the cold front to as low as 6000 ft. WPC
probabilities for 12 inches are 10-30% in the Sawtooth, but more
than 90% in the Sierra. Here, the WSE 10th percentile is over 18"
in the highest peaks, and locally up to 3 feet of snow is
possible. Otherwise, D2 will feature moderate snowfall in the
Cascades and northern CA ranges, as well as the mountains of the
Great Basin where WPC probabilities for 4 inches are as high as
50%.

By D3, the trough axis shifts towards the Four Corners and Central
Rockies, with height falls and PVA acting upon the still anomalous
moisture in place. As this occurs, confluent mid-level flow
beneath a persistent 130kt upper jet streak will continue to drive
impressive PW into the Pacific Northwest. This leaves two areas of
moderate to heavy snowfall Monday. Across the Central Rockies,
including the ranges of NW Wyoming and the Uintas of Utah, WPC
probabilities are moderate for 4 inches beneath the primary trough
axis. In the Pacific Northwest, heavy snow is again likely in the
WA and OR Cascades, with more than 6 inches of snow likely in the
highest terrain.

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

Weiss