Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Mon Apr 20 2020

Valid 12Z Mon Apr 20 2020 - 12Z Thu Apr 23 2020


...Sierra Nevada through the Four Corners...
Days 1-2...

The models indicate an upper trough moves slowly east across the
Sierra through the Four Corners, continuing across the Southern
Rockies Tue and high Plains Wed. A period of pooled moisture and
ascent crosses
the CA Sierra Nevada late today and this evening, moving east into
southern Nevada and Utah by Tue morning, then across southern CO
Tue. WPC probabilities for 4 inches are low in the Sierra D1, but
moderate to high in the CO San Juans, and parts of the CO Rockies,
generally above 8000 ft. Isolated amounts to 8 inches are
possible. On Wed the mid level trough moves east across the Plains
where it is too warm for snow. Snow ends in CO as ascent fades and
drier air aloft moves into the region.


...Northern New York and New England...
Day 2...

An area of low pressure moving across eastern Canada will drive 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. A secondary wave of low pressure may develop
along the triple point, aiding ascent in NH and ME.  Low level
southerly flow will spread warm air advection and moisture
northward across NH and ME.  Wet bulb cooling and a period of
moderate ascent should allow snow to occur in northern NH and
interior ME.  Most guidance has a quarter to half inch liquid
equivalent in the form of snow.  As a result, WPC probabilities
for 4 inches are moderate in the northeast kingdom of VT to the NH
White Mountains and eastward into much of central and northern ME.
The event winds down as the low departs Wed.

Northern to Central Rockies...
Day 3...

The models indicate a zone of confluent flow develops over the
northeast Pacific with a flat wave projected to bring moisture
onshore and inland Wed.  As the wave progresses inland, it crosses
higher elevations of ID and southwest/south central MT and then
into northwest WY Wed night/Thu morning. Moisture and lift coming
into the terrain should result in a period of snow, with the
higher probability of four inches shown in the mountains of
southwest MT to northwest WY.
The steady progression of the wave should minimize chances of a
heavy snow event, as no one area gets prolonged moisture and lift.

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

Petersen