Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 PM EDT Tue Apr 21 2020

Valid 00Z Wed Apr 22 2020 - 00Z Sat Apr 25 2020


...Northeast...

Day 1...
A negatively tilted trough extending from a low centered near the
Ontario/Quebec border will continue to swing east across Quebec
tonight. A secondary low will develop over coastal Maine this
evening along the occlusion from the parent low and lift north
into New Brunswick tonight. Any heavy snow will be limited to
Maine this evening ahead of the secondary low where the Day 1 snow
probabilities are moderate for 4 or more inches over far
northeastern Maine. Particularly cold air (for late April) is
spilling in behind the system and lake effect snow from Lake
Ontario could be around 2 inches southeast of Syracuse.


...WA Cascades/Northern and Central Rockies...

Days 1-3...
A pair of shortwave troughs riding a 130kt westerly jet arrive
from the Pacific to the WA coast Wednesday and Wednesday night.
The second one amplifies a bit by the time it crosses the northern
Rockies Thursday night. Pacific moisture and ascent from these
troughs and snow levels around 5000ft brings locally heavy snow in
windward terrain of the highest WA Cascades later Wednesday, and
the northern Rockies of ID/MT/and northwest WY Wednesday night
into Thursday where Day 2 snow probabilities for 6 or more inches
are high in the Mission/Swan Ranges of Northwest MT, the
Bitterroots along the ID/MT border, and the Tetons in WY. A
northwesterly turn in the jet Thursday shifts the precip focus to
the rest of the WY and northern CO Rockies through Thursday night
where snow levels will be 6000 to 7000ft with Day 3 probabilities
generally moderate for 6 or more inches, highest in the Medicine
Bow Mtns in southeast WY.

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

Jackson