Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Sun Jan 03 2021

Valid 12Z Sun Jan 03 2021 - 12Z Wed Jan 06 2021

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

A low pressure system moving up the coast will spread
precipitation across the Northeast beginning this morning and
persisting through Tuesday across Maine. This low will lift along
the coast and into the Gulf of Maine by Tuesday, driven by the LFQ
of a modest jet streak south of New England, and a
deepening/closed mid-level low pivoting into New England from the
west. WAA/isentropic ascent will drive a rain/snow/sleet/freezing
rain mixture across parts of PA and NY, with snow gradually
developing across SNE and the Poconos/Catskills this aftn. The
overall forcing is modest and of short duration, and the guidance
has shown a downward trend in QPF. This has led to a decrease in
WPC probabilities for 4 inches on D1, which are now highest over
the Catskills but below 30%. By D2, this low will skirt southeast
of the Benchmark and into the Gulf of Maine, while the upper low
pivots over NY and New England. Some light snow is likely beneath
the upper low, but additional accumulations are expected to be
minimal. The most challenging aspect of the forecast beings late
D2 into D3 as the upper low continues to shift eastward, and the
surface low gets blocked by ridging to its east. This suggests
that the upper low may capture the sfc feature and cause it to
retrograde, with theta-e advection pushing precipitation into
Maine through D3. Guidance has again backed off on QPF as the
surface low tracks further east than previous runs, but there is
still some potential for heavier snowfall to pivot westward into
Maine on D3. WPC probabilities for 4 inches of snow are generally
10-20% on D2 for far eastern MA into southern ME, and then pick up
to 30-40% on D3 across Downeast Maine which has the best chance
for snowfall pivoting back to the west late in the forecast period.

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

Persistent Pacific jet energy will transport moisture as an AR
into the West Coast through Tuesday, with a series of embedded
mid-level shortwaves and associated cold fronts driving ascent for
precipitation. One cold front moving onshore this morning will be
followed quickly by a second Monday morning, and yet a third late
on D3. While the mid-level flow will be generally zonal within the
AR to transport moisture eastward, backed flow will precede each
impulse/front to drive WAA and mid-level divergence to aid in
ascent. This suggests that while periods of snow showers are
possible at anytime in the terrain through all 3 days, the
heaviest snowfall is likely to occur in waves associated with each
system. WPC probabilities on D1 are high for 8 inches in the
Tetons associated with the first impulse, and moderate to high in
the WA Cascades and Olympics ahead of the second front. More
significant moisture spreads onshore D2 with the second wave which
is more amplified. This will drive heavy snow above generally
5000ft across the Sierra, northern CA ranges, Cascades of WA and
OR, and eastward into the Blue Mountains, Sawtooth, Bitterroots,
and Northern Rockies. WPC probabilities for 8 inches are high
across all these ranges, and may exceed 2 ft in the higher peaks.
A brief lull will occur D3 as the second impulse sheds east and
the next in this train approaches late. WPC probabilities on D3
are high for 8 inches only in the Olympic Range.

With snow levels falling to as low as 2000 ft behind each cold
front, even some of the passes such as Stampede, Snoqualmie, and
Stevens, could receive 3-day snowfall of 6-12" by the end of the
forecast period, but the lowlands will remain just rain with this
event.


The probability of significant icing (0.25" or more) is less than
10 percent all 3 days.
Weiss