Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Sun Jan 08 2023

Valid 00Z Mon Jan 09 2023 - 00Z Thu Jan 12 2023

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

Continued troughing out of the eastern North Pacific into the
western CONUS will maintain a very active pattern with an intense
atmospheric river (AR) event into California on Monday.
Combination of incoming 120kt jet with 130+ kt jet streak,
embedded mid-level shortwaves lowering heights through the D1
period, and a strong influx of moisture off the Pacific (PW values
nearing an inch or +3 sigma toward the snow level and IVT values
over 500 kg/m/s) will yield a significant amount of snow for the
Sierra with appreciable snow over the crest. As the mean trough
axis moves inland, moisture anomalies will still remain well above
normal through the Great Basin to the central Rockies (+2 sigma)
despite a waning connection to the Pacific. By D3, the system will
push onto the Plains in a weakened state as another Pacific system
spreads more warm advection light snow into the NorCal terrain.

Impressive D1-2 AR event will see a surge in snow levels as the
core of the moisture plume sinks from northwest to southeast,
driving levels to around 8000ft. However, snow levels will start
and end in the 3000-5000ft range before the strong WAA and during
the post-secondary FROPA CAA. Snowfall of several feet is likely
for the higher terrain of the Shasta/Trinity/Siskiyou region and
especially into the Sierra with high snowfall rates 3-5"/hr
possible per the WPC Snowband Probability Tracker above 7000ft.
With such a strong AR and accompanying warm surge, driving 700mb
temps to -2C, SLRs will likely be well below climo for January
yielding a very dense, heavy, and compact snow. This is shown
explicitly through the probabilistic WSSI for snow load which
shows high probabilities of extreme impacts mostly due to snow
load. To the north, lighter snow is likely via broad southwest
flow and and embedded shortwave lifting through the PacNW/northern
Great Basin. This will yield several inches of snow across the
Cascades, Blue Mountains, and into the Sawtooth Range/Salmon River
Mountains in Idaho, as well as the western WY (especially by D2).
WPC probabilities for at least 6 inches of snow are more than 50%
for these areas.

By later D2 into D3, snow will expand through the Great Basin into
Utah and eventually the CO Rockies as height falls move eastward.
Southwest flow and incoming height falls with the upper trough
will promote a broad area of light to moderate snow over higher
elevations (snow levels 5000-7000ft from north to south),
including the Uintas/Wasatch where over a foot is likely. In
addition, the high peaks in southern AZ/CA (Spring Mountains, San
Bernadinos, and San Gabriels) will likely see more than a foot of
snow as well. WPC probabilities for at least 6 inches of snow east
of the Sierra are high (>70%) above 7000-8000ft, and are moderate
(>40%) for at least 4 inches of snow above about 6000ft. This
includes the San Juans into southwestern CO as well by D3 as the
mid-level center moves across the Four Corners region.

The probability of significant icing greater than 0.25 inches is
less than 10 percent.


Fracasso


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

--Strong Atmospheric River Arrives Sunday Night: A powerful
atmospheric river arrives Sunday Night, peaking in intensity
Monday into Tuesday over California and western Nevada with
additional heavy rain, heavy mountain snow, and damaging winds up
to 60 MPH expected. Flash flooding and debris flows are possible
over burn scar areas. Scattered power outages are also likely.

--Threat of Heavy Rain, Flooding Continues: The cumulative effect
of successive heavy rain events will lead to additional
considerable flood impacts, including rapid water rises and
mudslides, over northern and central California, and portions of
far western Nevada.  Isolated major river flooding is expected
through much of this week, with record high river levels possible.

--Dangerous Mountain Travel To Continue Through Tuesday: 1-4 feet
(localized amounts greater than 6 feet possible above 7000 feet)
of additional snowfall through Tuesday with blowing snow at times,
will lead to dangerous travel above 4000 feet in the mountains and
passes 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. Two
week snow totals have reached or exceeded 100 inches in many of
the higher elevation areas of the Sierra Nevada.

--Emergency Preparedness: Never drive across flooded roads â€
turn around, donâ€t drown †and be sure to have both an
emergency preparedness kit and evacuation plan in place. More
heavy precipitation returns late week.