Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Wed Jan 11 2023

Valid 12Z Wed Jan 11 2023 - 12Z Sat Jan 14 2023


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

The active period across the West continues into the weekend as
the powerful Pacific jet persists west to east, with multiple
mid-level impulses and associated atmospheric rivers (AR) shifting
into the West.

The first of these will be amplifying across the Central Rockies
today, closing off across the Central Plains tonight and working
together with the LFQ of a modest jet streak to help deepen a lee
cyclone moving to the east. PW anomalies of +1 standard deviation
above the climo mean will gradually move east, providing
sufficient moisture to be wrung out by the synoptic lift and
upslope flow into the CO Rockies. Additionally, as the low
develops, a strong deformation axis may pivot across the High
Plains to enhance ascent locally in eastern WY and eastern CO. The
most likely area for more than 4 inches of snow D1 will be in the
higher terrain of CO, including the Park Range and Flat Tops,
where locally more then 1 foot is likely. Into the High Plains,
banded snowfall could vary considerably, but locally more than 4
inches is possible, especially across eastern WY into far NE CO.

After a brief period of shortwave ridging envelops the West behind
this lead shortwave, another amplifying longwave trough digs
towards the coast shedding shortwave energy onshore Thursday into
Friday. While heavy snow will gradually wind down across the
Sierra D1, this will result in additional heavy snow and an
expansion of precipitation coverage from the Sierra northward
through the Shasta/Trinity/Siskiyou region D2-3. Snow levels will
climb on the pronounced WAA within this AR due to the more
northward trajectory of the associated shortwave, reaching
7000-8000 ft across most of the area west of the Great Basin.
Heavy snow will fall across much of the terrain above these levels
though, with WPC probabilities for more than 6 inches reaching
50-70% across the WA Cascades D2, and the Sierra/northern CA
ranges D3. Most of this snow will occur above pass level, but
could impact the Sierra passes, yet again, on D3.


...Appalachians and Great Lakes through the Northeast...
Days 2-3...

A low pressure emerging from the lee of the Rockies in CO will
slowly deepen as it shifts northeast out of the Southern Plains
Wednesday and into New England on Thursday. As this low moves
northeast, especially on Thursday, it will begin to deepen more
rapidly in response to phasing northern and southern stream energy
driving an amplifying longwave trough shifting to the Atlantic
seaboard Friday night. As the low deepens and shifts into
Ontario/Quebec, moisture will advect northward on WAA from the
Gulf of Mexico/Atlantic and spread a shield of precipitation
northward. At the same time, forcing for ascent will intensify
through height falls and favorable upper diffluence within the LFQ
of a poleward arcing jet streak. Together, this will result in two
areas of heavy snow.

The most significant snowfall is likely across northern New
England, primarily the terrain of NH and ME, where strong WAA will
lead to at least a short duration of heavy snowfall late Thursday
into Friday, primarily in the higher terrain. This will be before
the WAA surges a warm nose northward to cause a changeover to
mixed precip and eventually rain, before the dry slot causes
precip to wind down on Friday. Snowfall rates will likely be
impressive, exceeding 1"/hr, within the most pronounced WAA thanks
to enhanced ascent into the DGZ with an increasingly isothermal
layer beneath it to support large aggregates. This will result in
snowfall accumulations which the WPC probabilities indicate have a
moderate risk for exceeding 6" in the White Mountains, and could
approach 10" in Aroostook County, ME.

Farther to the south and west, CAA behind the departing low will
produce an environment that will become increasingly favorable for
lake effect snow (LES) southeast of the Great Lakes, with moderate
to heavy upslope snow likely along the Central/Southern
Appalachians. Despite modest 850mb-sfcT differentials and shallow
inversion depths, favorable low-level lapse rates with enhanced
ascent and moisture across the Lakes should result in periods of
heavy snow rates within the most impressive LES bands.
Additionally, a band of increased deformation W/NW of the surface
low as the trough amplifies in its wake will result in some
enhanced snowfall from the L.P. of MI northeast, potentially into
the St. Lawrence Valley. Lastly, despite some model differences in
how the trough will evolve leading to variations in the low/mid
level flow behind the system, a significant upslope snow event
appears likely from the Smokies through the Central Appalachians.
WPC probabilities for more than 4 inches across these areas reach
as high as 50%, highest in the upslope flow regime across
NC/TN/WV, with a secondary max possible in the deformation band in
lower Michigan.


The probability of significant icing greater than 0.25 inches is
less than 10 percent.

Weiss