Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Tue Dec 14 2021

Valid 12Z Tue Dec 14 2021 - 12Z Fri Dec 17 2021

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

Amplifying mid-level trough off the West Coast will sharpen this
aftn as a potent vort lobe swings through the base, driving the
entire trough onshore this evening. This trough is progged to
close off Wednesday near the Four Corners before ejecting into the
Plains, with brief mid-level ridging filling in its wake.
Thereafter, yet another potent trough, this one likely manifesting
as a closed low dropping along the British Columbia Coast, will
surge southward into Oregon Thursday morning bringing yet another
round of heavy precipitation to the West. As this mid-level
pattern evolves, moisture will be plentiful as WAA ahead of a cold
front combines with Pacific jet energy to drive PWs to +2 standard
deviations over CA and the Great Basin/Four Corners on D1, with a
second PW plume advecting onshore D3 with the next system. Snow
levels within, and in the vicinity, of the most robust IVT will
climb to around 6000 ft. However, elsewhere across much of the
West, snow levels will be quite low, less than 2000 ft, and at
times reaching sea level.

For D1 /Tuesday and Tuesday night/ the heaviest snow is likely
again in the central and southern Sierra where robust and moist
700mb will angle orthogonally into the ranges to upslope
efficiently and produce snowfall rates of 2-3"/hr. This will
create heavy additional accumulations, and WPC probabilities for
18 inches or more are high in the Sierra where an additional 1-3
feet is likely. The impressive overlap of moisture and forcing
will spread heavy snow as far south as the terrain of southern CA
and into much of the Great Basin and Central Rockies, with WPC
probabilities greater than 70% for 12+ inches in the Wasatch,
Uintas, Wind Rivers, Absarokas, San Bernardinos, and San Juans. In
the lower elevations of the Great Basin and points east, lighter
snow accumulations are also possible as intense forcing within the
DGZ could produce some intense snow rates to dynamically cool the
column and allow accumulating snow into the valleys. WPC
probabilities are modest for 2" across much of the Intermountain
West, with shadowed valleys east of the terrain least likely.

During D2 the primary trough shifts east with some shortwave
ridging cause precipitation to wane, and WPC probabilities for 6
inches are confined to the higher peaks of the San Juans and CO
Rockies. However, the next system and associated AR returns to WA,
OR and CA early D2 spreading heavy snowfall once again into the
region. With snow levels very low, much of the precipitation away
from the coast and outside of the lowest valleys will be snow, but
even Portland, OR and Seattle, WA have a 10-30% chance for greater
than 0.1" of snow D2. However, the heaviest accumulations are once
again expected in the Sierra and northern CA ranges where WPC
probabilities for 12 or more inches are high, and locally 2 feet
is possible. In the WA and OR Cascades, and Olympics, WPC
probabilities are moderate for 12 inches on D2. Lingering snow
into D3 will accumulate heavily, at least early in the period,
across the Sierra and OR Cascades, shifting into the Sawtooth and
Tetons, where WPC probabilities for 8 inches are moderate.
However, ridging across the Pacific is likely to expand east
during D3, bringing and end to the precipitation by Friday morning.


...Northern Plains and Upper Midwest...
Days 2-3...

A closed mid-level low racing out of the Four Corners will
interact with a second shortwave dropping from Alberta over the
Northern Plains/Upper Midwest on D2 /Wednesday and Wednesday
night/ before lifting into Canada on Thursday. These features will
be accompanied by impressive upper diffluence within the LFQ of a
poleward arcing jet streak of Pacific origin, which will also
spread increased PW noted by anomalies of +5 standard deviations
lifting into WI, but still +1 to +3 in the cold sector over ND/MN.
These together will drive surface cyclogenesis in the lee of the
Rockies, and this low will deepen rapidly as it races northeast to
be in Ontario by Thursday morning. The guidance continues to
suffer from much greater than usual spread both in time and space,
leading to lowered confidence in the forecast for snowfall.

However, there are likely to be two areas of snow. The first is
associated with the trailing shortwave which should bring some
light to moderate snow Wednesday across northern SD an much of ND,
where WPC probabilities are 10-30% for more than 4 inches of snow.
The second region of heavy snow will be just W/NW of the surface
low where an intense deformation axis pivots along the western
edge of a pronounced TROWAL wrapping cyclonically around the low.
This should produce a band of precipitation changing from rain to
heavy snow, or all heavy snow as strong CAA commences. Again, the
guidance is spread as to the timing and east-west location of this
feature, but a brief 6-8 hour period exists where intense snowfall
rates are possible, and WPC probabilities for more than 4 inches
are as high as 40% D2 and D2.5 across W/NW Minnesota. Note the
90th percentile from both the NBM and WSE are near 8", so locally
much higher maxima are possible which needs to be monitored with
future updates.


...New England...
Day 2...

WAA ahead of the potent low pressure in the Midwest will spread
precipitation northeast across New England Wednesday and Wednesday
night. As this occurs, cold Canadian high pressure will slowly
retreat in response to mid-level divergence ahead of the
approaching trough. Despite the cold high pressure lifting off to
the east, cold air will likely remain trapped in some of the
elevated valleys across NH and ME, and a period of freezing rain
is likely. WPC probabilities are 10-20% for 0.1" of accretion in
northern NH and much of interior ME.


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

...Heavy mountain snow...
Additional heavy snowfall of 1-3 feet is likely in the Sierra
today before snowfall shifts east into the Great Basin and Four
Corners region tonight. 1-2 feet of snow is possible in the higher
terrain as far east as the San Juan Mountains.

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

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


Weiss