Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 PM EST Tue Dec 08 2020

Valid 00Z Wed Dec 09 2020 - 00Z Sat Dec 12 2020

...Pacific Northwest to the Northern Rockies...

Days 1-3...

An upper level shortwave crosses from the eastern Pacific onshore
and inland into the northern Rockies tonight into early Wed.,
continuing on to the northern Plains Wed night-Thu. A jet max
cross the WA Cascades tonight and then inland through the northern
Rockies Wed, with divergence in the favorable right rear quadrant
for ascent as the jet crosses the terrain.
The shortwave will advect enhanced moisture across the northwest
and northern Rockies, which supports snow at higher elevations in
upslope areas. Snow levels will initially be high but gradually
lower with the height falls and cold front. The system is forecast
to be progressive so snowfall will be modest, with several inches
in portions of the northern WA Cascades and western MT Rockies.  

As the moisture from the above forcing spreads eastward, it will
encounter a cold surface high stretched east of the WA Cascades
and into the Columbia Basin. This cold air will be slow to scour,
so an area of light freezing rain is likely across parts of
central WA on Tuesday.

The next shortwave approaches the WA/OR Coast Thursday night
bringing an enhanced moisture plume, but modest ascent. A weak mid
level wave progresses across the region, with modest lift
resulting in renewed chances for light snowfall to the Cascades of
WA and OR. WPC probabilities for Thu night-early Fri are low for 4
inches of new snowfall.


...Western to Northern New York...
Day 1...

A fast moving shortwave diving out of Ontario into Quebec will
drive a secondary front across Lake Ontario and western to
northern new York. Boundary layer flow will become westerly Wed
with the westerly flow turning upslope in the Tug Hill and western
Adirondacks, resulting in a swath of moderate snowfall.  Several
inches of snow are expected, with a complimentary burst of
boundary layer vertical velocities forecast by the GFS centered on
18z Wed.
However, the flow starts to veer, weaken, and result in less lee
shore convergence Wed evening, and then drier air advection aloft
cuts down on snow showers Wed night, bringing the event to a
close.

...Southwest/Central High Plains...
Day 3...

A cutoff low will gradually cross the southwest Wed night, the
southern Rockies Thu, and the high Plains Thu night and central
Plains Friday. Low-mid level moisture advection and ascent ahead
of the approaching wave, supported by 300 mb divergence maxima,
lead to mountains snows starting in the higher elevations of AZ
and then increase in coverage/intensity crossing the ranges of
NM/southern CO.  The highest amounts, locally around a foot of
snow, are expected  in the highest terrain of the Sangre De
Cristos of northern NM/southern CO. Several inches are possible in
the White Mountains, and Mogollon Rim. On Fri, light snows extend
onto the central Plains as low pressure beings to develop,
resulting in a front increasing in strength.  Amounts and
probabilities are low through Friday for the Plains. 

Petersen