Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sun Apr 19 2020

Valid 00Z Mon Apr 20 2020 - 00Z Thu Apr 23 2020



...Sierra Nevada through the Four Corners...
Days 2-3...

Multiple mid-level shortwaves will rotate southeast through the
broad cyclonic flow providing ascent from the Sierra through the
Four Corners. A weak impulse will rotate across the Great Basin
tonight before dissipating by the end of D1, with a much stronger
impulse lifting onshore CA early Tuesday, and closing off across
the Four Corners into Wednesday. While the leading shortwave will
bring some snow to the Wasatch and San Juans, accumulations should
be light. The secondary impulse is accompanied by much better
moist advection as 700mb flow backs to the SW, providing ascent as
well to the region, aided by modest jet level diffluence moving to
the east. WPC probabilities for 4 inches are low in the Sierra D2,
but high in the Wasatch, San Juans, and parts of the CO Rockies,
generally above 8000 ft. Isolated amounts to 8 inches are
possible. By D3 the strongest forcing moves to the east, with
lingering light snows continuing in the mountains of CO.


...Northern New York and New England...
Day 3...

An area of low pressure moving across eastern Canada will drag a
cold front from the Great Lakes and then across New York and New
England into Wednesday. This low is accompanied by a sharpening
mid-level trough, which has gotten stronger in recent model
guidance, and is now progged to tilt negatively and close off over
Ontario Province. As this closes off, a secondary wave of low
pressure may develop as the boundary occludes, aiding in ascent
across New England. Additionally, strong WAA ahead of this feature
will spread moisture northward, but the period of intense forcing
has limited duration. Precipitation may be primarily rain
initially due to marginal thermal structure, but cold advection
should occur in time to change over the precipitation to snow,
with further snowfall likely in the favored upslope regions. WPC
probabilities for 4 inches are moderate in the terrain of far
northern VT eastward into much of central and northern ME.


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

Weiss