Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 PM EDT Fri Apr 17 2020

Valid 00Z Sat Apr 18 2020 - 00Z Tue Apr 21 2020

...Upstate New York and New England...

Day 1...

A positively tilted mid-level trough will race eastward from the
Great Lakes tonight into Saturday, driving a low amplitude surface
wave across PA and then south of Long Island Saturday morning.
Warm advection ahead of this feature will become robust as
southerly flow intensifies, spreading moisture SW to NE across New
York and into Southern/Central New England. The atmospheric column
will be marginal for frozen precipitation outside of the higher
terrain, but as frontogenesis maximizes and becomes intense
overnight, dynamic cooling will likely overcome the marginal
thermal structure to change precipitation over to snow across most
if not all of Southern New England. During this time, snowfall is
likely to become heavy from the Catskills and points east, with
snowfall rates likely exceeding 1"/hr and may reach 2"/hr
according to HREF probabilities. Despite the rapid motion of the
wave, heavy banded snow will likely produce widespread 2" or more
of snow even in the lower elevations from Buffalo, NY all the way
to Cape Cod, MA, and this is represented by moderate to high WPC
probabilities. The heaviest snow, which may exceed 6 inches,
should be confined to the higher terrain of the Catskills,
Berkshires, and Worcester Hills, where forcing is stronger and
colder temperatures will exist.


...Northern Rockies...
Days 1-2...

Mid-level trough will rotate through the broad cyclonic flow
tonight through Saturday before shifting towards the Great Lakes
late Saturday. Anticyclonic post-cold frontal flow will promote
upslope flow for eastern slopes of the MT Rockies including
Glacier NP Saturday, and then northern WY Saturday night into
Sunday. Ascent will also be provided by modest 700-500mb height
falls through the first 24-36 hours. Day 1 WPC probabilities are
high for 4 inches in the vicinity of Glacier NP, shifting into the
higher terrain of the Absarokas and Big Horns on day 2.
 

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

Weiss