Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
240 AM EST Fri Feb 17 2023

Valid 12Z Fri Feb 17 2023 - 12Z Mon Feb 20 2023

...Upstate New York and New England...
Day 1...

Positively tilted shortwave with associated strung out vorticity
lobe will continue to advect northeast through New England today,
exiting offshore into the Atlantic by Saturday morning. This
feature will be accompanied by coupled upper jet streaks and a
surface wave moving progressively to the east. Together, this will
provide ample ascent into an airmass with well above climo PWs
thanks to downstream moisture advection, resulting in heavy
precipitation across Upstate NY and central/northern New England.
The low-level thermals will be modest for frozen precipitation due
to a warm nose aloft on strong WAA, and this will yield a mix of
snow and sleet/freezing rain from the Adirondacks eastward across
the northern half of New England, with heavy snow likely to be the
predominant p-type in northern ME. As the wave pulls away, CAA in
its wake will rapidly drag cooler air southeastward such that a
changeover to snow will occur across the area, but with limited
additional snow accumulation. The guidance continues to struggle
with the exact strength and depth of this warm nose, but forecast
soundings suggest the potential for significant freezing rain in
the Adirondacks, with heavy sleet accumulations likely across
central ME thanks to a cold depth as high as 850mb. Heavy snow
rates of more than 1"/hr are likely in northern ME where WPC
probabilities are greater than 80% for 6 inches, with sleet
accumulations up to 1 inch possible across portions of central ME.
Freezing rain resulting in ice accretions above 0.1" is also
likely in some areas of northern New England and south-central ME.

Behind the system, strong N/NW flow will produce an environment
favorable for at least modest LES south of Lakes Michigan and
Erie, with additional upslope snow across the central
Appalachians. WPC probabilities for more than 2 inches of snow are
highest across the higher terrain of WV.



...Pacific Northwest, Northern Rockies through Central Rockies...
Days 1-3...

An active period of winter weather begins today as a shortwave
dives out of British Columbia into the Pacific Northwest and then
into the Northern Rockies on Saturday. This initial shortwave is
progged to have limited amplitude and remain progressive,
combining with subtle upper diffluence into a region of normal to
slightly below normal PWs to produce light to moderate snowfall on
D1 from the WA Cascades through the Northern Rockies where WPC
probabilities for more than 4 inches are as high as 50%.

Behind this lead shortwave, a second more intense trough and
associated vorticity maxima rotates southeastward out of Alberta
with greater height falls and more impressive deep layer ascent,
aided by much more intense jet level diffluence along the LFQ of
an approaching Pacific jet streak digging from the Gulf of Alaska.
This increase in moisture will be even further enhanced as
700-500mb flow becomes impressively confluent with persistent NW
flow from the northern Pacific. The overlap of this mid-level
confluence with the aforementioned jet streak will drive PW
anomalies to as high as +2 sigma by D3, which in a region of
continued synoptic level ascent will result in increasing coverage
and intensity of snowfall from the Cascades through the Northern
Rockies and then diving towards the Central Rockies on D3. While
the most intense and impressive snow of this period looks to hold
off until D4 and D5 (beyond the current WWD forecast), D2 and D3
will be a precursor to an extended period of heavy wintry
precipitation falling as snow above generally 1500-2000 ft. WPC
probabilities D2 and D3 for more than 6 inches reach above 40% for
the WA Cascades and points southeast through the Northern Rockies,
the Absarokas, the ranges of NW WY, and as far south as the Park
Range of CO.


Weiss