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

Valid 12Z Mon Nov 07 2022 - 12Z Thu Nov 10 2022

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

The western CONUS will remain extremely active with respect to
wintry precipitation as an anomalously strong trough combines with
a persistent atmospheric river to produce strong ascent, cool
temperatures, and widespread snowfall.

The primary mechanism for this widespread snowfall will be a
closed 500mb low which will drop due south along the Pacific coast
before shifting onshore northern/central California by Wednesday
morning. This low is then progged to open into a full latitude
trough as the core vorticity rotates through the Four Corners by
Thursday morning, resulting in a negatively tilting trough across
the Intermountain West by the end of the forecast period.
Confluent mid-level flow immediately downstream of the primary
trough axis will likely combine with an impressive and
regenerative upper jet streak to drive PWs across the region to as
much as +4 sigma according to the NAEFS ensemble tables. As the
low shifts southward and then opens, drier air will advect in from
NW to SE, and this will result in the longest overlap of forcing
and moisture to occur from the Sierra Nevada northeast through the
Great Basin and towards the Central Rockies.

For the Sierra and other ranges of CA from the Shasta/Trinities
through the San Bernadinos and San Gabriels, heavy snow is likely
D1 and D2 before waning by D3. SLRs initially will likely be low,
even below climo medians due to the Pacific sourced marginal
airmass, but as 700-500mb temps crash within the advecting trough
and omega maximizes due to orthogonal flow upsloping into the
terrain, SLRs will climb to potentially above climo values. This
will result in very heavy snowfall rates which the WPC snow band
tool suggests could reach 3"/hr in the Sierra, 1-2"/hr elsewhere.
The long duration AR resulting in waves of heavy precipitation
will result in exceptional snowfall across the Sierra where 3-day
totals of 3-6 feet are likely in the higher terrain. WPC
probabilities for more than 8 inches of snow are high D1 and D2 in
the Shasta/Trinity ranges, with moderate probabilities dropping
into the Transverse and Peninsular ranges D2, as well as spilling
over as far as Mt Charleston and the eastern NV mountains.

The other impressive area of snowfall will likely be in a SW to NE
oriented band from eastern NV through UT, eastern ID, western WY,
and into the Absarokas of MT. Here, a slow moving fgen band is
likely, which will combine with increasing moist isentropic
upglide and upslope enhanced UVVs to drive a band of heavy
snowfall. The synoptics support a quasi-stationary band with
snowfall rates of 2"/hr possible, with only a slow progression
eastward by D3. There is still quite a bit of uncertainty as to
the timing and placement of this band development, which could
have significant impacts on snowfall amounts, especially in the
lower elevations where the enhanced ascent will likely overcome
marginal low-level thermals and lead to at least moderate
accumulations even in the valleys. However, the greatest
confidence for heavy snow, and the highest snowfall accumulations,
are likely in higher terrain from the Absarokas, Tetons, and Wind
Rivers D1-3, shifting into the Wasatch and Uintas D3. WPC
probabilities are high for 8 inches in these areas, with event
total snowfall likely reaching 2 feet in some areas.


...Northern High Plains into Northern Plains...
Day 3...

While much of the next winter storm will occur after 12Z Thursday
/D4/, there is increasing confidence that a major winter event
will unfold across the Northern High Plains into the Northern
Plains beginning late Wednesday. A shortwave swinging through a
full latitude trough will eject out of the Four Corners late
Wednesday night and then shift towards the Plains. This will
combine with increasingly coupled upper jet streaks to drive
surface cyclogenesis. This surface low will then deepen as it
shifts northeast, generally just beyond this forecast period.
However, increasing downstream WAA driving theta-e ridging
northward will combine with the synoptic ascent to produce an
expanding shield of precipitation, with wintry precip in the form
of snow, sleet, and freezing rain developing from MN westward
through MT. For D3, WPC probabilities for more than 4 inches of
snow are confined to the High Plains of eastern MT, reaching as
high as 20-40%, but will almost certainly expand to cover much of
the area by D4. Additionally, light freezing rain is likely in the
precip transition zone centered near eastern SD, but current WPC
probabilities for 0.1" or more of accretion is less than 5%. There
will likely be adjustments to storm track and intensity during the
next few days which will impact the amounts and placement of
wintry precipitation, but a major winter storm is becoming more
likely for late this week.

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

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.


Weiss