Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Sat Jan 07 2023

Valid 12Z Sat Jan 07 2023 - 12Z Tue Jan 10 2023

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

Multiple shortwaves and associated atmospheric rivers (AR) will
plague the western CONUS into next week as an active Pacific jet
continues. A persistent and re-loading closed low centered south
of the Gulf of Alaska will be responsible for the shedding of
these shortwaves, with each subsequent impulse tracking SW to NE
and weakening as they approach the coast. While Days 1 and 2 will
likely feature modest energy approaching the area, the feature on
D3 is progged to be more substantial and amplified, with the
associated trough axis and subsequent height falls likely reaching
as far inland as the Great Basin by the end of the forecast
period. The AR ensemble forecasts suggest modest probabilities
from both the ECENS and GEFS for IVT exceeding 500 kg/m/s D1 and
D2, but increase to support IVT above 750 kg/m/s moving onshore CA
D3. This is also when the NAEFS ensemble tables predict IVT above
+4 standard deviations from the climo mean, supporting snowfall
that has a ECMWF EFI of 0.7-0.8 and shift of tails of 1 across the
Sierra, suggesting renewed exceptional snowfall especially D3
despite snow levels rising above 7000-8000 ft in CA.

While rounds of heavy snowfall are likely each day through the
period, the heaviest snow and most significant impacts
today/tonight and Sunday will focus across the northern/central
Sierra, and more impressive into the Shasta/Trinity/Siskiyou
region of northern CA. Snow levels are progged to be around 4000
ft, and WPC probabilities are above 80% for 12+ inches both days,
and in the higher terrain, especially near Mt. Shasta, 2-day
snowfall will likely exceed 5 feet, with more than 3 feet in the
northern Sierra. Snow levels being modestly high will spare some
of the lower passes, but impactful snow exceeding 6 inches is
likely at both Black Butte Pass and Donner Pass this period.
Additional heavy snow will spread across the OR and WA Cascades,
Olympics, Blue Mountains, and into the Sawtooth/Salmon River where
WPC probabilities for more than 6 inches of snowfall are moderate.

By D3 /Monday and Monday night/ the core of the more intense AR
shifts into CA, accompanied by WAA driving snow levels up above
7000 ft in CA, and 4000-5000 ft elsewhere. The impressive moisture
will spill well north and east of CA through much of the West, but
the heaviest precip is likely to occur in the Sierra. With snow
levels so high, SLRs are progged to be near the 25th percentile
from the Baxter climatology, suggesting a very heavy and wet snow,
which is also supported by high probabilities for extreme impacts
in the pWSSI due to snow load. This could become an extremely
hazardous situation across the Sierra as despite the low SLR, WPC
probabilities are above 90% for 12 inches, and some areas may
receive 3-4 feet of this heavy and wet snow just on D3, especially
across the central and southern Sierra south of Lake Tahoe.
Farther to the north, spillover moisture and increasing ascent
through downstream divergence and height falls ahead of the
approaching trough axis will spread heavy snow across the
Cascades, Blue Mountains, Sawtooth/Salmon River ranges, and into
NW WY where WPC probabilities for more than 6 inches are moderate
to high, and locally more than 12 inches is likely in the higher
terrain.

Weiss


...Key Messages for Early January Atmospheric Rivers...

--Next Atmospheric River to Arrive Tonight: Heavy rain and
mountain snow returns late tonight in northern California,
spreading to central California Saturday.

--Threat of Heavy Rain, Flooding Returns This Weekend: The
cumulative effect of successive heavy rain events will lead to
additional considerable flood impacts this weekend, including
rapid water rises and mudslides, over northern and central
California, and portions of far western Nevada. Minor to isolated
major river flooding is expected through the weekend into next
week, with record river levels possible.

--Dangerous Mountain Travel Resumes Saturday: 1-3 feet of
additional snowfall this weekend, with blowing snow at times, will
lead to dangerous travel above 4000 feet in the mountains of
northern and central California.

--Increasing Risk of Infrastructure Impacts and Avalanches:
Mountain communities will be at increasing risk of infrastructure
impacts and avalanches as snow load continues to increase. By
Sunday, two week snow totals should approach or exceed 100 inches
in many of the higher elevation areas of the Sierra Nevada.

--Stronger Atmospheric River to Arrive Monday: A more intense
atmospheric river will arrive over California and western Nevada
Monday, with additional heavy precipitation and damaging winds
expected to persist through Tuesday. Flash flooding and debris
flows are possible over burn scar areas. Another round of heavy
precipitation is likely later next week.