Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
455 AM EST Tue Nov 08 2022

Valid 12Z Tue Nov 08 2022 - 12Z Fri Nov 11 2022

...Western U.S....
Days 1-2...

Anomalously deep longwave trough pinwheeling along the Pacific
Coast will shift onshore as a closed low over Northern California
tonight, and then continue to advect eastward across the Great
Basin Wednesday before negatively tilting into the Great Plains on
Thursday. This robust trough will drive large scale height falls
across the West, with downstream divergence, PVA, and accompanying
upper jet level dynamics supplying deep layer ascent. Within the
downstream confluent mid-level flow overlapping with the Pacific
jet streak, PWs will surge northeastward from the Pacific,
reaching +2 to +3 sigma above the climatological mean from
southern CA through the Central Rockies. This moisture will be
wrung out efficiently as snowfall across much of the terrain of
the west.

For today and tonight, the heaviest snowfall should be focused in
the Sierra where moist flow advects orthogonally into the terrain
to drive tremendous upslope ascent, enhancing the already
impressive lift driven by the upper level dynamic and a surface
wave moving eastward. Snowfall rates according to the WPC snow
band tool could exceed 3"/hr across the Sierra later today, and
WPC probabilities for more than 12 inches are above 95%. In some
of the higher terrain where the most prolonged focus of moisture
occurs, more than 4 feet of snow is expected. Elsewhere on day 1,
heavy snow is likely within the broad but impressive forcing from
the Shasta/Trinity ranges eastward through the Great Basin and
Central Rockies including the Wasatch, Uintas, Wind Rivers,
Absarokas, and even down towards Mt. Charleston and the San
Gabriels/San Bernadinos where WPC probabilities for more than 8
inches are 50-70% or more.

As the trough shifts eastward late D1 into D2, the guidance still
suggests a band of heavier snow will develop arced from the SW to
the NE from the eastern Great Basin through the Central Rockies.
The pattern supports this evolution, and a quasi-stationary band
may drive heavy snowfall rates o 1-2"/hr within the plume of
greatest moisture. This could result in heavy snow even down to
some of the valley floors as dynamic cooling overcomes the modest
snow levels, but the heaviest accumulations should again be
focused in the higher terrain above 4000 ft. WPC probabilities for
more than 8 inches of snow D2 are moderate from the Kaibab Plateau
through the Wasatch and Uintas, with lower chances extending into
the CO Rockies and ranges of NW WY.


...Northern High Plains into Upper Mississippi River Valley...
Days 2-3...

A 500mb shortwave rotating through the base of a longwave trough
across the Intermountain West will pivot northeastward into the
Central and Northern Plains Thursday while developing a negative
tilt. It is likely this feature will then close off near the
Dakotas Thursday evening before slowly lifting northeast towards
Lake Superior by Friday morning. This will produce impressive deep
layer ascent through height falls, downstream divergence, PVA, and
accompanying upper level diffluence within an increasingly coupled
jet structure. The guidance has split its solutions this morning
with how the mid-level wave will evolve. The NCEP models, GFS/NAM,
take the strong shortwave and close it off more rapidly, keeping
it farther south as it lifts across the Plains, while the non-NCEP
suite have become a little faster and less robust with this upper
evolution. It is too early to call this weaker solution a trend,
and the upper synoptic pattern still supports a strong and slower
surface low, but confidence in the exact evolution has decreased a
bit from earlier.

Still, regardless of exactly how strong and fast this system
moves, there is likely to be a widespread swath of heavy snowfall
NW of the low center. This will develop in response to strong WAA
lifting a theta-e ridge into a TROWAL pivoting cyclonically within
the WCB, in conjunction with increasing deformation and strong
fgen driving ascent. What is most noticeable is that as the CAA
commences behind the low, the DGZ becomes exceptionally deep,
300-400mb deep in some guidance, while remaining moist through
290-295K isentropic upglide and supporting strong ascent. For this
reason, SLRs are likely to be above the NBM, and with the
potential for theta-e lapse rates <0C/km, some convective snow
rates in excess of 1"/hr are likely. Where this band sets up and
pivots slowly to the E/NE, an axis of very heavy snowfall is
expected, which right now is progged to be generally from central
SD through eastern ND and into northern MN. This band will feature
intense snow rates and strong winds, with blizzard conditions
likely. While light snow accumulations are likely as the system
begins to strengthen on D2 across eastern MT, the heaviest snow is
likely D3 in the vicinity of this band, where WPC probabilities
for more than 8 inches are above 70%, and locally 18 inches or
more of snowfall is possible. Of course, this could change
depending on how the system evolves, but at least a narrow band of
very heavy snowfall is becoming more likely.

Southeast of this axis of heavy snow, there is likely to be a
stripe of freezing rain and sleet as WAA overruns cold air at the
surface. The setup seems to support more sleet than freezing rain
in most areas, but a narrow corridor from eastern SD through the
Arrowhead of MN may experience at least modest freezing rain
accretions which is reflected by pockets of 20-30% chance for
minor impacts due to ice accumulation in the PWSSI. WPC
probabilities for 0.1" of accretion on D2 are 10-30% near the
James River Valley and the prairies of eastern ND and SD, with D3
probabilities reaching 30-40% across Cook County and the Arrowhead
of MN.

Key Messages for the Northern Plains and Upper Mississippi River
Valley:

--A winter storm will likely 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 late
Wednesday through Friday.

--Bands of heavy snowfall with rates in excess of 1â€/hr will
likely produce significant snowfall accumulations in some areas.

--Strong gusty winds combined with heavy snow could produce
blizzard conditions across the Dakotas, making for dangerous
travel.

--Freezing rain and sleet are likely to the south of the heaviest
snow which could produce hazardous travel 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.


Weiss