Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 PM EDT Fri Apr 24 2020

Valid 00Z Sat Apr 25 2020 - 00Z Tue Apr 28 2020

...Northeast...
Days 2-3...

The southern stream component of a broad trough swinging across
the southern Plains this afternoon will lift northeast across the
Ohio Valley Saturday night before closing off over PA/MD Sunday.
As the system translates energy to a coastal low off the northern
Mid-Atlantic Sunday, a reinforcing northern stream shortwave will
merge into this system, slowing the northeast progression Sunday
night into Monday before the system shifts east of Maine Monday
night. The threat for snow begins in the deformation zone dynamics
near the PA/NY border as the low takes on a double-barreled
structure as energy translates to the Mid-Atlantic coast Sunday
and continues into Monday along this particular structure (and at
high NY and New England elevations). Given good clustering of the
low evolution and track, a preference is given to the 12Z ECMWF
for the position of the deformation axis. Given this is a Day 2/3
forecast, there is certainly an expectation for the exact axis/low
track to shift somewhat from the current progs. 

As some modest coupled jetting develops, the primary surface low
should intensify, which when combined with Canadian High pressure
over Ontario will allow cold air to funnel southward into New
England. While the column may initially be marginal for anything
but rain, as the deformation band intensifies Sunday afternoon,
increased forcing and column cooling is likely to cause a p-type
changeover from rain to snow. First this will occur in the terrain
and Sunday night snow is possible even down into the valleys where
forcing is strong and cause enough dynamic cooling of the lowest
levels. The highest snow totals are likely in the higher
elevations, generally above 2000 ft with Day 2.5 snow
probabilities moderate for 4 or more inches in the  Adirondacks,
Greens, and low probabilities in the Catskills. Moderate Day 3 WPC
probabilities for 8 or more inches are across the Presidential
Range portion of the White Mtns in NH. Low to moderate Day 2.5
probabilities for 2 or more inches are across the Alleghenies of
western NY and lower portions of the Adirondacks, Catskills,
Greens and Whites. However, where this band is most intense,
particularly Sunday night into Monday morning before the strong
sun is present could see inches of accumulating snow no matter the
elevation.


...Pacific Northwest...
Days 1-3...

A series of troughs will swing south of a gyre over the Gulf of
Alaska starting tonight with mid-level trough axis that reaches
Vancouver Island Saturday morning and the next reaching Vancouver
Island Sunday night. Both troughs will be preceded by 1" or higher
PW which will keep snow levels elevated in the pre-trough heavier
precip over western WA, around 6000ft tonight into Saturday, and
around 5000ft Sunday night. Therefore Day 1 snow probabilities for
6 or more inches are limited to the highest WA Cascades and Day 3
are limited to mainly the northern WA Cascades and Mts. Rainier
and Olympus.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson