Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 PM EST Sat Jan 02 2021

Valid 00Z Sun Jan 03 2021 - 00Z Wed Jan 06 2021

...Northeast...
Days 1-3...

The approach of a mid/upper level shortwave currently analyzed
over the mid-MS River Valley and low pressure forecast to develop
over eastern NC due to a residual baroclinic zone will bring
another round of wintry precipitation to portions of the Northeast
and New England late on day 1 (Sunday) through day 3 (Tuesday). A
lack of deep phasing and separation between the mid-level wave and
low will keep precipitation relatively light over portions of
PA/interior NY where a few inches of snow are forecast. But as the
low approaches the Gulf of Maine, it will slow or even retrograde
back to the west/southwest as its caught up in the
deepening/closing off mid-level energy. This could support a
prolonged period of snow across coastal NH to much of Maine in th
day 2/3 time-frame, though there remains considerable uncertainty
in the evolution and QPF. Deterministic runs of the latest
guidance did trend upward, particularly for Maine where there is a
signal for winter storm criteria to be met or exceeded. However,
given the uncertainty and model spread still, the blend favored
the ECENS/ECMWF runs. This system bears watching.


...Pacific Northwest/California/Northern Rockies...
Days 1-3...

A couple distinct shortwaves tracking through California and the
Pacific Northwest over the next few days will bring impressive
mountain snowfall to portions of the WA Cascades and other favored
ranges. The combination of deep/long moisture axis and large scale
forcing for ascent. Snow levels will vary through the weekend,
rising as high as 5000 ft ahead of each cold front on the WAA, and
crashing back to 2000-3000 ft in the wake of the frontal passages.
WPC probabilities are high for exceeding 12 inches across the WA
Cascades in the day 1 period. For day 2, as the moisture plume
continues across WA and also spreads inland and south, WPC
probabilities for exceeding 12 inches reach the Sierra Nevada
range while for day 3, the central to northern Rockies will see
the highest probabilities. All told, for the 3 days, some very
impressive totals are forecast with snowfall of more than 5 ft is
possible in the WA Cascades, with 2-3 ft likely in the other
ranges.


The probability of significant icing (0.25" or more) is less than
10 percent all 3 days. However, some light icing is expected in
the Laurel Highlands of PA and mountains of western MD on Sunday
morning.

Taylor