Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EDT Mon Mar 29 2021

Valid 12Z Mon Mar 29 2021 - 12Z Thu Apr 01 2021

...Northern to Central Rockies...
Days 1-2...

An anomalously deep closed 500mb low moving across southern Canada
will amplify a longwave trough across the West, with a surface
cold front dropping southeast beneath it. Height falls, modest
upper diffluence within the RRQ of the accompanying upper jet
streak, and 700-500mb warm and moist advection will spread snow
showers from the Northern Rockies this morning to the Central
Rockies tonight, with enhancement likely in the CO Rockies late D1
into D2. This enhancement is likely due to secondary shortwave
energy rotating atop the region combined with upslope flow behind
the cold front. WPC probabilities for 6 inches are high in the
Northern Rockies near Glacier NP D1, and low in the CO Rockies.
While probabilities on D2 remain low in the CO Rockies, the 2-day
totals may eclipse 8 inches in isolated locations along the Front
Range.


...Northeast...
Day 3...

An area of low pressure developing across the Mid-Atlantic will
lift northeast Thursday along a slowing cold front. This low will
strengthen in response to rapid height falls as an anomalous
mid-level trough advects east from the Great Lakes, and an
intensifying divergence maxima within the RRQ of an upper jet
streak. Large scale ascent will spread precipitation across the
Mid-Atlantic and Northeast, but WAA ahead of the front suggests
all precip will be rain initially. However, as the low deepens and
the cold front drops slowly southeast, the column will cool
rapidly to allow a p-type transition from rain to snow. This is
likely to be additionally enhanced by robust fgen/deformation NW
of the low. While there remains uncertainty into how much precip
will occur as the column cools enough for snow, this enhanced
ascent through the fgen should provide at least a burst of
snowfall, enough to overcome the antecedent rain and warm
conditions, especially in the terrain. WPC probabilities for 4
inches are as high as 30-40% in the Adirondacks and Tug Hill where
post frontal upslope flow will enhance snowfall, and 10-20% across
much of Upstate New York and into Vermont.


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

Weiss