Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
517 AM EST Wed Feb 24 2021

Valid 12Z Wed Feb 24 2021 - 12Z Sat Feb 27 2021



...Pacific Northwest to the Northern Rockies...
Days 2-3...

A brief break in between systems is expected today, with little
potential for heavy snow.  Heavy snows return during the day on
Thursday as another shortwave trough and associated frontal band
approach the region, with low-mid level moisture convergence near
the frontal boundary
supporting long duration snows, resulting in heavy snow
accumulations likely across the Olympics and northern Cascades,
expanding east into the Blue Mountains and portions of the
northern Rockies late Thursday through Friday. 

The upper trough passage and lowering heights/snow level in the OR
Cascades, Boise Mountains in ID sets the stage for heavier snows
on Day 3/Fri. The confluent flow produces a 300 mb jet max
crossing OR and southern ID to northern NV and UT, with upper
divergence maxima near the jet and in the left exit region
producing prolonged ascent in windward terrain of the OR Cascades,
Blue Mountains, Boise Mountains, and ten the Tetons of WY.

Two day snow totals covering Thu through Fri night/early Sat
morning are likely to be two to four feet across a large portions
of the WA Cascades, with 1-2 feet common in the Blue Mountains,
Bitterroot Range, and mountains of northwest MT.

...Central Rockies/High Plains...
Days 1/2...
A positively titled longwave upper level trough is forecast to
move southeast from Wyoming and Utah across CO Thursday.
The forward flank jet maxima Wed afternoon/overnight contains
upper divergence maxima, providing lift starting in northern CO
and working south to the Sangre De Cristo and San Juan mountains
by Thu morning. As the system approaches, low level easterly flow
will support developing snows across the High Plains and along the
foothills and Front Range. Several inches of snow area expected in
the foothills and Front Range, focused by well defined boundary
layer moisture convergence maxima and associated ascent.  Lighter
snows are expected on the high Plains, with the GFS under weighted
as it shows more snow spreading east on to the Plains. The event
winds down on Thu as the upper trough moves south across NM,
followed by drying/sinking air aloft. A few inches of snow are
expected in the mountains of NM before the upper trough departs.

...The Northeast...
Day 1...
A progressive, but dynamic shortwave will move out of the Great
Lakes today, crossing southern Quebec and Maine tonight before
exiting New Brunswick Thu morning. Several inches of snow are
expected across the Adirondacks of northern New York, northern
Green Mountains of VT,and Maine as a swath of 850-700 mb
warm/moisture advection moves across these areas in advance of the
wave.
Once the wave passes, the mountains of northern NY/VT have
post-frontal west to northwest flow that produces snow showers
that add a few more inches of snow.

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

Petersen