Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Mon Dec 13 2021

Valid 12Z Mon Dec 13 2021 - 12Z Thu Dec 16 2021

...Northern Plains...
Day 3...

A closed mid-level low lifting rapidly northeast out of CO will
work in tandem with intense upper diffluence within the LFQ of an
amplifying jet streak to drive surface cyclogenesis with rapid
deepening on Wednesday. This low will race northeast from the lee
of the Rockies early Wednesday to Ontario, Canada by Thursday
morning. Guidance still feature above normal spread in the spatial
and temporal track of this low, leading to below normal confidence
in both the placement and amount of heavy snow. However, it is
likely that synoptic ascent will become robust, with enhancement
likely through a pivoting deformation axis collocated with a
pronounced TROWAL wrapping cyclonically to the west of the surface
low. Where the most intense ascent occurs, a band of moderate to
heavy snow is is likely. Due to the current model spread, WPC
probabilities for more than 4 inches are just 10-30%, and range
from the High Plains of WY northeast towards the Arrowhead of MN,
but it is possible snowfall rates could become heavy within the
deformation band leading to greater accumulations. Future model
runs will better handle this evolution.


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

A mid-level trough will sharpen off the Pacific coast today and
Tuesday, finally ejecting eastward to surge onshore Tuesday
evening. This trough will then amplify into a negative tilt by
Wednesday across the Great Basin before closing off and lifting
into the Plains D3. Increasingly confluent mid-level flow and WAA
ahead of surface fronts moving eastward within the flow will
combine with a robust Pacific jet streak to drive ample moisture
onshore noted by PWs reaching +1 to +2 standard deviations above
the climo mean. This setup will support a prolonged AR lifting
into the West, and IVT is progged to exceed the 99th
climatological percentile on Tuesday. Snow levels D1 will range
from 1500 ft in the Pacific Northwest, to 5000-6000 ft within the
AR and spreading into the Central Rockies. However, the approach
of the trough and the eastward progression of the associated cold
front will drop snow levels down to near sea level in the Pacific
Northwest, and to just 1000-2000 ft elsewhere across most of the
West by Wednesday.

For D1 /Monday and Monday night/ the heaviest snow is likely from
the OR Cascades southward through the Shasta/Siskiyou/Trinity area
and into the Sierra Nevada where the most intense ascent will
produce snowfall rates that may reach 3"/hr, especially in the
Sierra due to the nearly orthogonal flow within the AR driving
intense upslope into that range. WPC probabilities are high for 18
inches across most of the Sierra, with locally in excess of 5 ft
likely above 6000 ft. Heavy snow exceeding 18 inches is also
moderate to high in the northern CA ranges, and also into the
Sawtooth of ID where upslope flow out of the Snake River Valley
will enhance ascent.

By D2, the heaviest precipitation is likely to shift east into the
Great Basin and Central Rockies, south into the Peninsular and
Transverse ranges of Southern CA, while continuing in the Sierra.
WPC probabilities for at least 18 inches is again high in the
Sierra D2, and an additional 1-3 ft is possible from near Tioga
Pass southward. WPC probabilities for 12+ inches are moderate in
the higher terrain of the San Bernardino Mountains, many of the
ranges across NV including Mt. Charleston, and into the Wasatch,
San Juans, Uintas, and Absarokas.

By D3, as the primary trough shifts into the Plains, an impressive
closed low will dive along the British Columbia coast towards WA
before opening as it shifts eastward. This will be accompanied by
another surge of moisture into the Pacific Northwest, with
precipitation diving back into the Sierra and northern CA ranges
late D3 after only a brief respite. With snow levels expected to
be quite low Wednesday, even some lowland accumulations are
possible across WA and OR, and WPC probabilities for 0.1" are
10-30% for both Seattle, WA and Portland, OR. Elsewhere, WPC
probabilities are high for 8 inches across the Olympics, OR
Cascades, northern Sierra, and other ranges of northern CA on D3.
3-day total snowfall across the Sierra may reach 8 feet in the
highest terrain.

For Days 1-3, significant icing is not expected.


~~~Key Messages for Dec 11-14 Winter Storm across the Western
U.S.~~~

...Ongoing Atmospheric River will bring heavy precipitation to
much of the Western U.S....
Heavy precipitation will spread across OR and much of CA today,
before expanding into the Great Basin on Tuesday.

...Heavy mountain snow likely...
Significant snow accumulating to 1-3 feet is likely in the Oregon
Cascades, northern California ranges, and parts of the Northern
Rockies and Great Basin mountains. Heavy snow of 3-6 feet, with
locally higher amounts, is likely in the Sierra Nevada where
snowfall rates may reach 3"/hr

...Widespread impacts to travel and infrastructure...
Travel may become impossible, especially across the mountain
passes, and is discouraged. Some road closures are expected.
Whiteout conditions, downed trees, and power outages are all
likely. Avalanches will be possible across the Sierra.

...Heavy rain expected for portions of California...
Several inches of rainfall will create the potential for flash
flooding and debris flows, particularly near recent burn scar
areas and along the central California coast.


Weiss