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

Valid 12Z Sat Jan 02 2021 - 12Z Tue Jan 05 2021

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

Two distinct storm systems will bring impactful winter weather to
the Northeast through the weekend and into early next week.

The first system is a low pressure which will race quickly
eastward beneath a deamplifying shortwave. The exact track of this
low will determine the rain/snow line, but there is good model
agreement that much of Northern New England will remain all snow,
with a snow to mix to rain scenario likely in central and southern
parts of New England. The initial precipitation could be heavy at
times as WAA and modest isentropic ascent spread northward, but
this should be a snow to mix to rain scenario for much of the
region outside of the northern tier. However as the low moves
eastward it will shut off the WAA and colder/drier air working in
from the NW behind it will combine with modest jet streak coupling
to drive the potential for heavier banded snow across interior
Maine, possibly dropping closer to the coast towards the end of
the event. HREF probabilities indicate an increasing chance for
1"/hr rates this aftn, and the heaviest snow accumulations will
occur where this banding sets up. WPC probabilities indicate a
high chance for 4 inches across most of north and central Maine
and Coos County, NH, with isolated accumulations of 8" possible.

After brief shortwave ridging and a dry Saturday night/Sunday, the
second storm system will approach from the southwest. This system
will develop along the residual baroclinic gradient left in the
wake of the first low along the trailing cold front, with
cyclogenesis developing in response to renewed height
falls/mid-level divergence and the LFQ of a subsequent jet streak
arcing poleward from the Deep South. While there is uncertainty
into the exact track of this second low as it will depend somewhat
on the evolution of the first system, it is quite likely that a
low will move offshore the Mid-Atlantic and then pass near the
Benchmark before lifting into the Gulf of Maine Monday night. As
this occurs, the mid-level shortwave will deepen and close off,
potentially capturing the surface low which will be slowing due to
downstream ridging. The ridging downstream could be significant
enough to block eastward progress of this low, and while there is
quite a bit of spread in the ensembles by D3, a track more NW of
the operational runs seems likely as the downstream ridge gets
reinforced and the upper trough interacts with this low. Current
WPC probabilities are modest for 4 inches both D2 (Catskills and
terrain of SNE) and D3 (Maine and NH), but there is potential for
quite a bit more, especially in Maine, where a stalling or
retrograding low with persistent theta-e advection may occur and
CIPS analogs suggest a greater than 60% chance for 6" of snow.
After coordination with the offices believe the ensemble approach
showing a subtle upward trend in probabilities is best at this
time.


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

A persistent fetch of moisture /AR/ characterized by IVT reaching
750 kg/m/s will impact the west coast through early next week.
While the max progged IVT onshore is around 500 kg/m/s, several
embedded shortwaves within the flow combined with the persistent
Pacific jet energy and associated surface cold fronts will spread
periods of heavy precipitation across the West.

Three distinct shortwaves are likely to move onshore this period
to aid in deep layer ascent. These are progged to move onshore
this morning, again Sunday morning, and a third more robust
shortwave with slightly more amplitude is progged for Monday
night. Each of these will be accompanied by a cold front, and the
combination of low-level convergence, height falls, periods of WAA
as mid-level flow backs ahead of each shortwave, and periodic
upper divergence maxima will drive ascent across much of the West,
weakening as impulses move eastward with time. 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 these fronts. This suggests the heaviest snow will be
confined above 5000 ft, and focused in the terrain which will be
more orthogonal to the 700mb flow leading to stronger orographic
ascent. WPC probabilities on day 1 are high for 8 inches in the WA
and OR Cascades, as well as the Sawtooth, Bitterroots, and
Northern Rockies. Local maxima will likely exceed 2 ft in the
Cascades. On day 2, probabilities wane a bit due to weaker
forcing, but still suggest a moderate potential for 8 inches in
the Cascades of WA. By day 3 the stronger shortwave lifts onshore
spreading heavy snow once again into the Cascades and as far east
as the Northern Rockies, but with this last feature having greater
amplitude, heavy snow is also likely as far south as the Sierra.
WPC probabilities are high for 8 inches in these ranges, with
again local maxima exceeding 2 ft likely. Impressive 3-day
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.

Weiss