Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
442 AM EST Fri Dec 09 2022

Valid 12Z Fri Dec 09 2022 - 12Z Mon Dec 12 2022

...Upper Midwest..
Day 1...

An impressive closed low will move quickly eastward across the
Upper Midwest today, eventually shearing out to the east over the
eastern Great Lakes tonight. Accompanying this low, a strung out
subtropical jet will buckle briefly over the Midwest to produce
enhanced LFQ diffluence, which will combine with height falls to
produce enhanced vertical ascent through tonight. Downstream of
this wave, transient but impressive WAA will drive moisture
advection and further enhance lift, with the end result being a
band of impressive snowfall rates of 1-1.5"/hr according to the
WPC prototype snow band tool. Although this will advect quickly to
the east, this intense snowfall rates should quickly accumulate,
with the greatest snowfall likely centered across southern WI
where both the NBM and WSE feature a trend upward with the
overnight guidance. WPC probabilities for 4+ inches are 20-40%,
but isolated amounts above 4 inches are possible. As the system
continues to shift eastward, it will encounter much drier air
leading to just light snowfall accumulations pivoting into lower
Michigan.


...Northeast...
Day 3...

A fast moving northern stream mid-level shortwave amplifying
across the Great Lakes on Sunday will dig southeast across New
England and the Mid-Atlantic while opening Sunday night and paired
with enhanced upper diffluence as a fast moving jet streak
reaching 130kts arcs zonally from the OH VLY into southern New
England. Pronounced synoptic ascent within these features will
combine with increasing but transient 285-290K isentropic upglide
atop a surface wedge to spread expanding precipitation across the
region. The available moisture will be modest noted by mixing
ratios within the 285K layer of just 2g/kg and PW anomalies that
are just normal for the date. However, intensifying forcing both
through the isentropic ascent and along an inverted trough that
may develop as a wave of low pressure blossoms offshore, will
wring out efficiently available moisture, likely resulting in
light to moderate snowfall from the eastern Great Lakes through
Southern New England. While there is still considerable
uncertainty into exactly how all these features will evolve and
work together, there has been an increase in snowfall among the
guidance tonight, and WPC probabilities reflect that increase.
Although most snowfall should generally be light, WPC
probabilities for more than 4 inches reach above 40% in Upstate
NY, highest in the Catskills. Lighter snowfall may spread as far
southeast the I-95 corridor, but accumulations are expected to be
minimal.


...Pacific Northwest and California through the Great Basin
Rockies...
Days 1-3...

...Widespread heavy snow likely across the West through early next
week...

A leading shortwave moving onshore the Pacific Northwest this
morning will be replaced quickly by shortwave ridging tonight.
This will be followed on Saturday but an anomalously strong upper
low shifting into CA, and then expanding across much of the
western third of the CONUS on Sunday. The result of this evolution
will be significant height falls to lower snow levels, impressive
synoptic ascent through upper diffluence, mid-level divergence,
and spokes of PVA, with a strong atmospheric river (AR) for which
both the ECENS and GEFS ensembles have probabilities for IVT above
500 kg/m/s. While heavy snow is likely from the Olympics southward
along the length of the Cascades and into the northern CA ranges,
with WPC probabilities high for more than 6 inches, the most
significant snowfall is likely D2-3 across the area.

As the mid-level low drops along the Pacific coast and amplifies
into a large gyre shifting onshore the west coast, downstream WAA
and associated moisture flux through a Pacific jet streak and
confluence in the mid-levels will spread heavy snowfall across
much of the west, with extreme snowfall likely in the CA ranges.
Across the Sierra, but also into the Shasta/Trinity/Siskiyou
region, snowfall rates will likely reach 3"/hr at times as show by
high probabilities in the WPC prototype snow band tool, with many
hours of these rates likely. This will result in exceptional
snowfall D2 as WPC probabilities for more than 18 inches eclipse
95% near Mt Shasta and along the length of the Sierra, with
locally more than 5 feet likely in the higher terrain. Snow levels
will climb to 5000-6000 ft within the most intense WAA ahead of
the warm front, but then will crash back to just 2000-3000 ft,
even to just 1000 ft or less in some areas, behind the subsequent
cold front. Although the heaviest snow is likely in CA D2, WPC
probabilities for more than 8 inches are also high farther north
into the Sawtooth range of ID, the Blue Mountains of OR, and along
the international border near the Okanogan Highlands of WA into
the Northern Rockies.

During D3 as the upper low moves more onshore and encompasses much
of the western CONUS, periods of heavy snow will continue into the
CA ranges, but also spread as far east as the Great Basin and
Wasatch Range, south into the Transverse Ranges of CA and the
northern Mogollon Rim, with snow also spreading northward on
spokes of PVA into the eastern Snake River Valley and much of
eastern Oregon. WPC probabilities for more than 8 inches are above
80% on D3 in the Sierra as well as parts of the Great Basin and
southern Wasatch of UT, with moderate probabilities extending into
the Uintas and San Gabriels. Light snow exceeding 2 inches will
likely cover nearly the entire Great Basin and the Intermountain
West. With snow levels falling D3, light accumulations are even
possible at some of the southern CA passes including Tehachapi,
Tejon, and Cajon passes.


Key Messages for December 9th-15th winter storm from the Pacific
Coast through the Upper Midwest:
--A significant storm with high winds and heavy precipitation,
including mountain snow, will begin this evening and impact
California and the Great Basin through the weekend.

--Heavy snowfall rates of 1-3"/hr are likely at times, primarily
on Saturday. This will result in extremely dangerous travel,
especially across mountain passes. If you must travel, prepare for
rapidly changing conditions, and carry winter driving supplies.

--Snowfall totals of 1-3 feet are possible across much of the
higher terrain, with locally more than 5 feet expected in the
Sierra.

--As the system moves east, a strong surface low is expected to
develop across Colorado and then track from the Central High
Plains to the Upper Midwest through the middle of next week.

--While some uncertainty persists, confidence is increasing that
strong winds and significant snows will produce hazardous impacts
across much of the Central/Northern Plains and into the Upper
Midwest.


Weiss