Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 PM EST Fri Dec 30 2022

Valid 00Z Sat Dec 31 2022 - 00Z Tue Jan 03 2023

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

140kt jet out of the eastern North Pacific will carry a frontal
boundary southeastward through CA/NV Day 1 and through the
Intermountain West Day 2. A wave along the front will briefly
pause its movement over NorCal early Saturday as the atmospheric
river makes steady progress. By Saturday afternoon, a well-defined
shortwave will move into central/southern CA and close off into an
upper low over northern AZ late Sunday. This will aid in weak
cyclogenesis through the Great Basin but much more especially as
the upper trough/low reaches the lee of the Rockies Day 3. Also by
late Day 3, yet another system is forecast to reach the West Coast
with another round of snow for the mountains. To the north, weak
mid-level troughing into the PacNW will favor continued light snow
for the Cascades and Idaho ranges Day 1 before diminishing Day 2.

Snowfall on Saturday will be confined to high elevations generally
above 8000ft as the brunt of the precipitation moves through the
Sierra. Snow levels will fall as colder air moves in behind the
front as precipitation wanes. With a strong westerly flow across
the terrain, more spillover than usual will occur into northern
Nevada (PW anomalies +2.5 to +3 sigma) as a jet streak passes atop
the region. Heavy orographically-forced snow will wring out over
the Sierra where more than 12-18 inches of snow is likely. To the
east, locally heavy snow accumulations are likely across the
higher elevations of the northern Nevada mountains, including the
Ruby Mountains, as well as the southern Idaho, northern Utah, and
western Wyoming ranges. Parts of the northern Wasatch, especially,
may be well-positioned to maximize snowfall over the next two days
where multiple feet are quite probable owing to sustained moisture
influx.

As the upper low traverses northern AZ, snow will overspread the
Mogollon Rim into the White Mountains as 850-700mb moisture flux
intersects the terrain. Snow levels will fall below 7000ft with
the most significant snow over 12 inches above 8000ft. To the
north, snow will continue across much of UT into the CO Rockies
but with backing winds as the 700mb low moves through the region,
favoring westerly-facing terrain initially but transitioning to
southerly-facing. Height falls to the north will expand the area
of light snow across WY as WAA drives an increase in snow out of
the southeast. Snow will linger behind the trough axis into Day 3
across the Four Corners region as the system continues to develop
downstream onto the Plains (see below).

At the end of Day 3, another system is forecast to enter the West
Coast, tied to an upper low in the Gulf of Alaska. Generally light
snow is forecast through the end of this period for the southern
OR Cascades into NorCal and the northern Sierra, where more
snowfall is expected into Day 4.

Fracasso

...Central Plains...
Day 3...

Pronounced shortwave ejecting from the Four Corners will take on a
negative tilt and close off as it amplifies into the Central
Plains late Monday. This will be accompanied by a poleward arcing
upper jet streak rounding the base of the longwave trough still
positioned over the Intermountain West, and may at least subtly
couple with a zonal jet streak moving over the Great Lakes. The
overlap of this coupled jet energy and the closing mid-level low
will result in cyclogenesis across eastern Colorado, with this low
then deepening as it advects northeast towards the Upper Midwest
by the end of the forecast period. Guidance still features quite a
bit of spread both in evolution of the mid-level energy and
subsequent response of the track of the surface low, leading to
lowered confidence in the forecast by D3. However, as WAA begins
in earnest downstream of the primary trough it will advect
moisture from the Gulf of Mexico northward, with isentropic ascent
at 290-295K sloping northwestward and lifting into a modest
TROWAL. Where this moisture can rotate into the strengthening
deformation axis NW of the surface low, a swath of heavy snow is
likely within a pivoting  band, but placement of this axis is
still highly variable among the models. The guidance has trended a
bit SE since yesterday which seems reasonable based on the
intensity of the synoptic forcing, and an ensemble mean approach
seems most reasonable at this time for D3, and into D4, beyond
this forecast period.

Where the heaviest snow develops within the deformation NW of the
low and beneath the intensifying TROWAL, snowfall will likely be
heavy at times driven by mid-level fgen and an isothermal layer
noted in regional forecast soundings beneath the DGZ to support
aggregate maintenance and higher SLRs. Despite the spread, WPC
probabilities for more than 6 inches of snow are above 40% for the
southeast High Plains of WY east of the Laramie Range, into the
Nebraska Panhandle and far SW South Dakota. Additionally, east of
the heavy snow, mixed precipitation is likely both due to warm air
rotating in atop colder surface temperatures, but also within an
impressive dry slot progged to surge northeast, drying the DGZ and
resulting in freezing rain/drizzle. Again, confidence in the
placement of this mixed p-type and freezing rain zone is lower
than normal, but WPC probabilities for 0.1" are 5-10% from central
NE through the Buffalo Ridge of MN. Additional heavy snow and
freezing rain is likely just beyond the current WWD forecast
period.

Weiss