Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
314 PM EST Sun Dec 04 2022

Valid 00Z Mon Dec 05 2022 - 00Z Thu Dec 08 2022

...Western United States...
Days 1-3...

A complex synoptic evolution through the middle of the week will
yield areas of moderate to heavy snow across many regions of the
West. The primary drivers of this evolution will be a slowly
filling 500mb low which will drop along the OR coast tonight
before opening into a positively tilted trough as it shifts into
northern CA Monday, and then continue to slowly advect SE towards
the Four Corners by the end of the forecast period /Wednesday
evening/. The other primary feature will be a larger closed
mid-level gyre centered over northern Manitoba through which
modest shortwaves will rotate cyclonically into the Northern High
Plains and Northern Rockies through mid-week. These features
together will drive periods of height falls for ascent, while jet
streaks downstream of both the northern stream trough and, more
impressively, the southern stream trough, driving pronounced
moisture across the western half of the CONUS. With snow levels
gradually falling due to increasingly cold air filtering down from
Canada behind a front, this will result in widespread, generally
elevation based, snow, with snow levels falling to below 2000 ft
in the Great Basin and points north, and to around 3000-5000 ft
across the Four Corners and CA.

For D1, the heaviest snow is likely to be in two parts of the
region. Across the Sierra and other ranges of northern CA,
orthogonal mid-level flow will also be confluent downstream of the
primary closed low, leading to enhanced moist advection with
upslope flow driving robust ascent. Snow levels will hover between
3000-4000 ft, suggesting significant snow accumulations will
impact many of the area mountain passes as well. WPC probabilities
are high for more than 8 inches from generally Lake Tahoe area
north along the Sierra, and across much of the Shasta/Trinity
region. Locally more than 12 inches is likely in the Sierra. Also
on D1, a lobe of shortwave energy ejecting towards the Northern
Rockies will help spawn a weak surface low within a belt of
enhanced WAA along a northeast advancing warm front. At the same
time, a cold front will approach from CA, and this squeeze will
result in some enhanced frontogenesis, that will become enhanced
into D2. Snow levels northeast of the terrain will be at ground
level, and generally 1000-2000 ft elsewhere. This suggests a
northward advancing band of heavy snow is likely, and with cold
temperatures in place, SLRs will be quite fluffy. WPC
probabilities D1 are high for more than 4 inches of snow
stretching from the Bitterroot Range eastward into the Little
Belts, the Absarokas, and many of the NW WY ranges, where locally
more than 10 inches is likely in the higher terrain. As the
primary upper low off OR shifts eastward into CA and the Four
Corners and weakens, snow will wane on D2 across the Sierra.
However, a stripe of heavy snow is likely to enhance from the
Northern Rockies through the Dakotas as LFQ diffluence, persistent
moisture advection, increasing upslope flow ahead of an advancing
surface high pressure, and a stripe of enhanced and regenerative
fgen develops in response to the upper jet and dual cold fronts
sinking southward. This will result in a prolonged period of
moderate to at times heavy snow, and WPC probabilities are
moderate to high for more than 4 inches from the Northern Rockies
through the High Plains of MT both D2 and D3, with storm total
accumulations approaching 12 inches in central MT, and nearing 2
feet in the Northern Rockies.

D2-D3, there is an even more enhanced signal for heavy snowfall
across the Four Corners, primarily the higher terrain of Utah into
Colorado. The approach of the opening longwave trough will drive
downstream divergence and height falls into UT/CO, with LFQ
diffluence and persistent moist advection additionally driving the
favorable setup for heavy snow. The synoptic features overlapping
should lead to some enhanced mid-level fgen as well as a surface
low blossoming in the lee of the Rockies which will also result in
easterly upslope flow. The setup seems favorable for periods of
heavy snow across much of the CO Rockies D2, sinking southward
into the San Juans, Wasatch, and Great Basin ranges of NV D3. The
heaviest snow is likely in the Park, Gore, and Flat Top ranges on
D2, shifting into the San Juans D3, primarily driven by the fgen
and enhanced upslope. WPC probabilities for more than 8 inches
reach 20-40% across these areas, with more than 4 inches likely in
the surrounding terrain above 4000 ft, with light snow
accumulations likely at Sacramento Pass D3.


For Days 1-3, the probability of significant icing exceeding 0.10
inch is less than 10%.

Weiss