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

Valid 00Z Wed Nov 09 2022 - 00Z Sat Nov 12 2022

...Western U.S....
Days 1-2...
An anomalously deep upper trough will continue to impact a large
portion of the western U.S., with heavy mountain snows expected to
continue across the California mountains into the evening hours
while spreading east across the higher elevations of the Great
Basin into the northern and central Rockies overnight.   This
upper trough is currently centered along the West Coast with a
closed center developing along the Pacific Northwest and Northern
California coasts -- supporting large scale ascent downstream. 
This ascent, along with deep onshore flow, and orographic forcing
will continue to support locally heavy snowfall, with rates
topping 3 in/hr across portions of the Sierra into the evening
hours.  As the upper trough moves east, rates are expected to
diminish overnight across the Sierra, with increases farther east
across portions of the Great Basin, and the northern to central
Rockies.  Guidance continues to show heavy snow developing this
evening and overnight across the higher elevations of southern
Nevada and southwestern Utah northward into the southern Idaho,
southwestern Montana, and northwestern Wyoming Rockies.  While the
heaviest amounts are expected to remain over the higher
elevations, favorable upper jet forcing, combined with low-to-mid
level frontogenesis, and dynamic cooling is likely to support some
lower elevation accumulations.  By late tomorrow, guidance
indicates the better forcing is likely to center across the
central Rockies, bringing some heavy mountain snows into western
Colorado that are expected to continue into the overnight.

For the 24 hr period ending 00Z Thu, WPC probabilities indicate
that additional accumulations of a foot or more are likely for
portions of the central to southern Sierra.  WPC probabilities
also show that accumulations of 6 inches or more likely to impact
the San Bernadino Mountains, Mt Charleston, as well as large
portions of the Utah, southern Idaho, northwestern Wyoming, and
parts of the southwestern Montana ranges.  

...Northern High Plains into Upper Mississippi River Valley...
Days 1-3...
By late Wed, models show the longwave trough in the West centered
over the northern Rockies to Four Corners region, with a new
closed center developing over the northern Rockies.  A
well-defined shortwave moving through the base of the larger-scale
trough is expected to begin lifting northeast from the Four
Corners region, assuming a negative tilt as it lifts into the
central Rockies/High Plains.  By late Wed, left-exit region upper
jet forcing translating northeast will help to support a band of
moderate to heavy snow developing east of the northern Rockies
into north-central to northeastern Montana.

The upper low is expected to move east into the Dakotas on Thu,
with a TROWAL, increasing deformation, and low-to-mid level
frontogenesis supporting the development of heavy snow on the
northwest side of the low.  Intense snowfall rates of more than an
inch/hr, along with strong winds, are likely to create blizzard
conditions across portions of the Dakotas.  Guidance continues to
show a deep DGZ, which is expected to help support higher SLRs,
accentuating the potential for heavy snow accumulations.  For the
24 hr period ending 00Z Fri, WPC PWPF indicates accumulations of 6
inches or more are likely across a large portion of north-central
South Dakota and south-central North Dakota eastward into eastern
North Dakota.  Within this area, localized accumulations of a foot
or more are possible, especially across parts of south-central
North Dakota.  These probabilities reflect the latest model trend
toward a slightly faster solution, which places the heavier
amounts farther north and east during this period than the
overnight run.

Southeast of the heavy snow, mixed precipitation, including some
significant sleet and/or freezing rain accumulations, are possible
from south-central South Dakota to northern Minnesota.  For the 24
hr period ending 00 Fri, the WPC PWPF shows some 30 percent or
greater probabilities for ice accumulations of 0.1 inch or more
centered across northeastern South Dakota.

By late Thu and continuing into early Fri, as the low continues to
track east, the larger threat for banded heavy snow is expected to
shift east across northern Minnesota.  Accumulations of 4 inches,
with locally heavier amounts, are likely across northern
Minnesota, before snows diminish late in the day Friday.


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.

-- Moderate to major impacts (top image) may produce considerable
disruptions to daily life. Driving conditions may be hazardous to
dangerous.

-- Bands of heavy snowfall with rates in excess of 1"/hr will
likely produce significant snowfall accumulations in some areas.
Strong gusty winds combined with heavy snow could produce blizzard
conditions across the Dakotas.

-- 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.

-- Strong northeasterly winds over western Lake Superior may lead
to hazardous waves.

Pereira