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

Valid 12Z Tue Apr 21 2020 - 12Z Fri Apr 24 2020


...Northeast...
Day 1...

An upper low will continue to move east across southeast Canada
today.  Models show the system beginning to assume a negative tilt
with a strong surface cold front sweeping across much of the
Northeast on Tuesday.  By late this afternoon and evening, a
secondary low developing near coastal Maine is forecast to lift
north into Atlantic Canada overnight.  Overall, expect generally
light snow accumulations across Upstate New York and central to
northern New England, with locally heavier totals possible across
portions of the higher terrain.  WPC probabilities of 50 percent
or greater for snow accumulations of 4 inches or more across
portions of the White Mountains of NH and across much of interior
northeastern Maine.

...Central and Southern Rockies...
Day 1 ...

A progressive southern stream shortwave is advecting moisture out
of northern Arizona and New Mexico up into southern Utah and
Colorado.  Difluent flow aloft with embedded upper divergence
maxima combines with upslope flow and moisture advection today to
produce snow showers across the mountains of southern Utah,
Colorado and far northern New Mexico.  WPC PWPF does show some low
probabilities for accumulations of 8 inches or more across
portions of the San Juan Mountains in CO, due to longer duration
of the snow and better defined 300 mb divergence this afternoon.
The event winds down as the upper trough moves east onto the
Plains Wed.

...WA Cascades/Northern and Central Rockies...
Days 2/3...

An arriving pacific jet maxima the 00z ECMWF shows moving onshore
during the day Wed into WA spreads both moisture and ascent across
WA and then into ID and western MT to adjacent northwest WY. 
Locally several inches of snow are possible in windward terrain.
The flow gradually turns northwest Thu with the 300 mb jet maximum
stretching from ID to WY and western CO.  While widespread heavy
amounts are not expected, the persistence of moisture across the
ranges of WY does allow longer duration snows. WPC PWPF does show
the potential for localized accumulations of 6 inches or more on
Thu in the ranges of western WY, the Bighorns, and Medicine Bow
Peak in southeast WY.  

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

Petersen