Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
453 PM EST Mon Nov 07 2022

Valid 00Z Tue Nov 08 2022 - 00Z Fri Nov 11 2022

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

The focus of the synoptic pattern is an impressive upper low
tracking into California on Tuesday. By 12-18Z Tuesday, one could
choose to look at the NAEFS or ECMWF situational awareness tools,
and both will depict height anomalies through the depth of the
troposphere that are 3-4 sigma below normal along the California
coast. Upper level temperatures are also in the same sigma range,
particularly at the 700-500mb levels. This is noteworthy because
as the upper low moves inland on Tuesday, snow levels will drop
from the Shastas and Sierra Nevada on south to the Transverse
Range where snowfall is expected to be measured in feet for Days
1-2. Meanwhile, downstream of the upper trough, robust PVA and
supportive jet streak dynamics from right-entrance region
diffluence over the Great Basin and Intermountain West provides
plenty of vertical ascent to produce heavy snowfall. By Day 2, a
second jet streak rounding the base of the longwave trough on the
West Coast will place its diffluent left-exit region over similar
areas to Day 1. Finally, the trough begins taking on a negative
tilt by Day 3 with residual moisture and upslope flow allowing for
heavy snow in the Wasatch and central Rockies. In short, it is a
multi-day heavy snowfall event from the higher elevations of the
Sierra Nevada and Great Basin to the northern and central Rockies.

Aside from the upper level evolution, there is also an impressive
moisture fetch that originally starts out with a deep Pacific
moisture feed. Both NAEFS and ECMWF PWs are shown to be above the
90th climatological percentiles in southern California. As deep
moisture moves farther inland on Day 2, there is a large footprint
of anomalous 850-700mb moisture flux streaming into the
Intermountain West into Day 2. The 12Z GFS showed a +3-4 sigma
850-700mb moisture flux field over Arizona and aimed at the
central Rockies 12-18Z Wednesday. The tandem of anomalously cold
temperatures and ample atmospheric moisture, copious amounts of
snowfall are anticipated. The Sierra Nevada will see the most
snowfall through Day 1 as WPC probabilities highlight as much as
an 80-90% chance for snowfall totals >18". Heavy snow is on tap as
far south as the Transverse Range where snowfall probabilities for
>8" of snow are as high as 50-60%. By Day 2, it is the higher
terrain of the central Great Basin, the Wasatch, Uintas, and on
north to the Wind River, Absaroka, and Sawtooths where >8"
snowfall probabilities are >60% in some cases. By Day 3, the
Wasatch continue to see as much as 30-40% probabilities for >8" of
snowfall with some of the higher peaks of western Colorado having
as high as 40-50% probabilities for the same totals.

The WPC experimental PWSSI showed >90% probabilities for "Major"
impacts in the central and southern Sierra Nevada on Day 1. In
fact, some probabilities for "Extreme" impacts are as high as
50-60%. Such impacts suggest that travel will be treacherous there
with significant disruptions to daily life and infrastructure.


...Northern High Plains into Upper Mississippi River Valley...
Day 3...

By Day 3, the upper trough tracking across the Rockies begins
taking on a negative tilt at 500mb. Come Thursday morning, the jet
streak rounding the base of the 250mb low will place its diffluent
left-exit region within the divergent right-entrance region from a
jet streak over southern Canada, maximizing upper level divergence
over the northern Plains and Minnesota. The contrasting pressure
gradient from low pressure in the West and a strong dome of high
pressure over the Ohio Valley and Northeast will prompt a strong
LLJ to form over the central Plains, delivering a deep fetch of
850mb moisture flux into the Northern Plains and Upper Midwest. In
terms of snowfall, the heaviest snowfall totals will be positioned
on the storm's backside where a warm conveyor belt intersects the
850-700mb front and orients the heaviest snowfall bands beneath
the TROWAL over the Dakotas. WPC probabilities for >6" of snowfall
are between 50-80% across southeast Montana, much of western South
Dakota, and southwest North Dakota. It is worth noting that there
are some 30-40% probabilities for >12" snowfall amounts in
southwest North Dakota and northwest South Dakota. The latest
PWSSI shows 40-60% probabilities of Moderate impacts in western
South Dakota with the highest probabilities centered in the
northern Black Hills.

There is also an ice component to this impending winter storm. The
dome of Canadian high pressure to the north will supply
sub-freezing surface temperatures across the eastern Dakotas and
northern Minnesota. Meanwhile, the aforementioned strong LLJ
supplies ample moisture aloft, as well as strong WAA. This will
lead to a "nose" of above freezing temperatures above the surface
and cause precipitation to fall as an icy wintry mix. Latest WPC
probabilities for >0.1" of freezing rain are up to 20-30% in
northeastern South Dakota for Day 3. However, deterministic
solutions are painting an even icy scenario as far south and west
as the South Dakota/Nebraska border and as far northeast as the
Minnesota Arrowhead. There remains a good deal of spread in
ensemble model track solutions for where the primary surface low
tracks and the range of thermodynamic profiles in these ensemble
members are leading to an extensive range from little to no ice,
or significant accumulations. As of this forecast cycle, the
highest confidence in icy impacts resides in central and northeast
South Dakota.

Due to the increasing confidence that a major winter storm will
unfold, WPC Key Messages are being produced for the system.

Key Messages:

--Confidence continues to increase that a winter storm will bring
a swath of heavy snow, and a corridor of sleet and freezing rain,
to portions of the northern High Plains, Great Plains, and Upper
Mississippi River Valley later this week. Major impacts may
produce considerable impacts to daily life.

--Intense snow rates will likely produce significant snowfall
accumulations in some areas.

--Strong gusty winds combined with heavy snow could produce
blizzard conditions. Travel may become dangerous with near zero
visibility.

--Freezing rain and sleet are likely to the south of the heaviest
snow. Significant icing is possible which could produce hazardous
travel conditions and at least minor impacts to infrastructure.

--Uncertainty remains with the exact timing, strength, and
location of the storm track which determines where the most
impactful wintry precipitation will occur.

Mullinax