Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 PM EDT Tue Apr 26 2022

Valid 00Z Wed Apr 27 2022 - 00Z Sat Apr 30 2022


...Pacific Northwest through the Northern Rockies ...
Days 1-3...
A pair of low amplitude shortwaves are expected to produce mainly
light, high elevation snows as they move across the northern
Cascades and northern Rockies tonight and Wednesday.  A more
amplified shortwave is then expected to dig into the Northwest on
Thursday.  This will initially bring some generally light
accumulations to the Oregon Cascades and Blue Mountains as it
moves across the region on Thursday.  As the system continues to
amplify and track east, heavier snows are forecast to develop
across portions of the northern Rockies on Friday.  Guidance shows
broad ascent afforded by favorable upper jet forcing, combining
with upslope on the north side of a developing 700mb low, to
produce heavy snows across parts of western to central Montana and
northern Wyoming on Friday.  WPC probabilities indicate snow
accumulations of 8 inches or more are likely for portions of the
western to central Montana ranges by late Friday.


...Northeast...
Days 1-3...
A compact upper low will move across the Great Lakes overnight. 
This will produce some synoptically driven light amounts as it
moves across the region, followed by lake effect snows as it moves
east of the lakes.  While heavy accumulations are not expected,
local amounts of an inch or two are possible, especially along the
higher terrain in western New York in the lee of Lake Erie.

This system is expected to assume as negative-tilt as it continues
east across the Northeast Wednesday night.  As a blocking pattern
develops downstream over the northern Atlantic, this system is
expected to linger near the Gulf of Maine/Bay of Fundy/Nova Scotia
through Friday.  Low level frontogenesis overlapped by favorable
upper dynamics will support precipitation developing on the
northwest side of the low across northern New England on
Wednesday.  As the air column cools, rain changing to snow is
expected Wednesday afternoon, with snow continuing across portions
of Maine into Friday.  WPC probabilities indicate that by late
Thursday snow accumulations of 4 inches or more are likely, with 8
inches or more possible, from far northern New Hampshire across
much of northern Maine.   

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

Pereira