Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EST Thu Jan 27 2022

Valid 00Z Fri Jan 28 2022 - 00Z Mon Jan 31 2022


...Southern Appalachians through the Mid-Atlantic...
Days 2-3...

...Significant Nor'easter likely for the Mid-Atlantic and
Northeast...

Forecast confidence continues to increase in a significant snow
event, but the details of placement and amounts remains uncertain.

Dual shortwaves moving across the central part of the country are
progged to phase near the TN VLY and absorb into an amplifying
longwave trough across the East Coast. This trough is then likely
to tilt negatively and then close off as it approaches New
England. At the same time, a pair of jet streaks downstream of the
primary trough axis will intensify and become favorably coupled
(overlap of LFQ and RRQ of the subsequent streaks), in tandem with
the closing of the mid-level trough. All this is likely to occur
atop the natural baroclinicity offshore the East Coast, leading to
surface cyclogenesis off the NC/SC coast, which is then likely to
lift northeast while explosively deepening, potentially below
970mb as it moves somewhere near the Benchmark (40N/70W) Saturday
and then into the Canadian Maritimes Sunday. NW of the
surface-850mb low track, heavy snow is likely with deformation and
frontogenetical banding likely to enhance snowfall rates and
accumulations.

While there is good agreement in a strong low pressure moving up
the coast, there remain two distinct camps in the placement which
lead to significant differences in the winter impacts. The
EC/12kmNAM/CMC suggest a track further west with a "capture" type
scenario as the mid-level low closes off. This spreads significant
snowfall further west, and leads to a more intense deformation
axis down the coast as far as northeast NC. The GFS/3kmNAM/GEFS
are much further east and progressive, with less snow across the
Eastern Seaboard. While the trend even in these further east
models has been subtly NW today, they still lie well east of the
consensus of the other guidance, and a blended approach is still
preferred. This leads to a big "boom or bust" potential for
snowfall, which is reflected by very large spreads in the
10th-90th percentiles in both the WSE and NBM. Despite that, there
is high confidence in at least moderate to heavy snow from the
DelMarVa peninsula through southern NJ, across eastern Long Island
and into much of eastern New England where WPC probabilities for
more than 6 inches are generally above 80%. There may be a sharp
cutoff to the west, but the I-95 corridor from Washington, D.C. to
New York City could also receive several inches of snowfall, with
light snow possible as far south as central NC. Where heavy snow
is most likely, from eastern Long Island through southeast New
England including Boston, MA and into eastern ME, locally 2 feet
of snow is possible where any banding sets up.

Additionally, there remains the potential for locally heavy
accumulations farther west across the central and southern
Appalachians.  As a cold front associated with the northern stream
trough drops into the region, WPC probabilities show the potential
for some localized amounts of 4 inches or more across portions of
the West Virginia, southwestern Virginia, and along the
Tennessee-North Carolina border.


...Pacific Northwest...
Day 3...

A strung out shortwave will advect southeast into Washington State
Sunday night embedded within moist confluent mid-level flow. This
feature will dig quickly into the region accompanied by a modest
Pacific jet streak to surge moisture into the WA Cascades.
Moisture ahead of this feature will increase as noted by PWs of +1
standard deviation above the climo mean, upon which this forcing
will drive heavy snowfall, primarily across the terrain of WA
state above 4000 ft. WPC probabilities for more than 6 inches are
above 50% in the WA Cascades, which could bring some accumulation
to Washington and Stevens Passes.

For days 1-3, the probability of significant icing is less than 10
percent.

Weiss


~~~ Key Messages for Jan 28-30 East Coast Winter Storm ~~

-- A winter storm is likely to produce significant snow
accumulating to more than 12 inches across eastern Long Island and
eastern New England Friday night through Sunday. Local maxima may
exceed 20 inches.

-- Heavy snow rates combined with strong winds could produce
blizzard conditions across eastern Long Island and along much of
the New England coast. Whiteout conditions could create nearly
impossible travel, while strong winds will likely lead to
scattered power outages and some damage.

-- Significant coastal impacts are possible, including coastal
flooding and beach erosion.

-- Significant impacts will also likely extend south along the
Mid-Atlantic coast where moderate to heavy snow and gusty winds
are possible. Difficult travel is likely.

-- Very cold temperatures will likely follow the storm with
dangerous wind chills possible Saturday night.