Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 PM EST Sun Nov 06 2022

Valid 00Z Mon Nov 07 2022 - 00Z Thu Nov 10 2022

...Western U.S....
Days 1-3...

A multi-day heavy snowfall event is set to unfold in the higher
terrain of the western U.S. as anomalously cold temperatures and
substantial atmospheric moisture work in tandem to produce
widespread wintry precipitation the first half of the week.

Tonight, a powerful upper low off the Pacific Northwest coast will
slowly deepen and push south along the West Coast through Monday
night and into Tuesday. Downstream of the upper trough, large
scale vertical ascent courtesy of supportive jet stream dynamics
and waves of PVA allows for a favorable environment for widespread
precipitation from California to the northern Rockies and northern
High Plains. According to NAEFS, the 500mb heights are approaching
climatological percentiles over western Washington by 12Z Monday,
and coming close to similar values along the western coast of
Oregon by 00Z Tuesday. Heights throughout the column by 18Z
Tuesday (850-700-500-200mb) are all reaching as low as 3-4 sigma.
Meanwhile, a deep surge of 850-700mb moisture flux beneath the
base of the trough will supply the moisture necessary to generate
heavy snow. The latest GFS shows 850-700mb moisture flux anomalies
as high as +3-4 sigma in the central Great Basin and in
south-central California on Day 1, then ballooning to +5 sigma
over the southern Sierra Nevada and lower Great Basin by Day 2.
This is due to a more vigorous 500mb shortwave trough rounding the
base of the longwave trough approaching southern California. 00Z
NAEFS showed 850mb and 700mb winds above the 90th climatological
percentile over southern California by 18Z Tuesday. This will
undoubtedly aid in the intense snowfall rates over the central and
southern Sierra Nevada on Tuesday, as well as some of the most
elevated portions of the Transverse Range. Lastly, NAEFS suggest
temperatures at the 850mb, 700mb, and 500mb levels will range as
cold as 3-4 sigma below normal over the West Coast throughout the
short range. With such anomalous factors at play (heights,
moisture flux, temperatures), the end result is copious amounts of
snowfall for much of the mountainous terrain in the West.

Latest WPC probabilities show 40-60% values for snowfall >12" in
Washington State's Okanogan Highlands, the tallest peaks of the
OR/WA Cascades, and on south into the Shasta, Trinity, and
northern Cascades for Day 1. As the trough deepens further into
Day 2, the best WPC probabilities depict a high risk, or >70%
chance, for snow totals >18" in the central and southern Sierra
Nevada. Farther northeast, portions of northern Nevada and into
the Absaroka, Teton, and Wind River Ranges have up to ~40-50%
probabilities for >8" of snowfall. By Day 3, the upper trough
finally begins to show more eastward movement, resulting in the
axis of heavy snowfall being oriented farther inland towards the
Wasatch, Uintas, and Wind River Range. It is in these ranges where
WPC probabilities for >8" of snowfall are 50-70% on Day 3.

Regarding impacts, the Sierra Nevada stand out the most with WPC's
experimental pWSSI showed 60-70% probabilities for Major impacts.
There's even some 40-50% probabilities for Extreme impacts in the
tallest peaks of the central and southern Sierras for Day 2. These
probabilities are largely driven by the snow amount and snow load
components in the pWSSI. Travel in and around the Sierras should
be avoided if possible with closures and impacts to infrastructure.

In the medium range, in collaboration with WFOs in the Northern
Plains and Upper Midwest, we have initiated Key Messages for the
potential major winter storm closer to mid-week. This is due to
the aforementioned upper trough, which is responsible for the
barrage of heavy snow in the West, ejecting into the Northern
Plains and spawning a winter storm that will generate heavy and
disruptive quantities of wintry precipitation. These Key Messages
are listed below.

Key Messages:

--Confidence is increasing in a swath of heavy snow and a corridor
of wintry mix from an early season winter storm over the northern
Great Plains into northern Minnesota Thursday and Friday.

--Strong winds associated with the system could result in blizzard
conditions.

--The combination of heavy snow and high wind would result in
dangerous travel conditions.

--Uncertainty remains with the timing and location of the storm
track which determines where the axis of heaviest snow and how
much wintry precipitation accumulates.


The probability of significant icing greater than 0.25" is less
than 10 percent.

Mullinax