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

Valid 00Z Thu Feb 25 2021 - 00Z Sun Feb 28 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...
A passing shortwave ridge will continue to promote dry weather
across much of the region through the early part of the period. 
However, a shortwave trough and associated frontal boundary that
is forecast to move into the region on Thursday is expected to
mark the onset of a period of unsettled weather, resulting in very
heavy snow accumulations across portions of the high terrain.  A
well-defined shortwave and associated frontal band dropping
southeast into the region on Thursday is expected to produce heavy
snow accumulations across the Olympics and northern Cascades by
the late afternoon hours.  As heavy snows continue across these
areas, heavy accumulations are also forecast to develop farther to
the east late Thursday into the early Friday, with impacts across
the Blue Mountains and along the northern and central Idaho
ranges.  As this leading shortwave continues to dig to the
southeast, upstream energy embedded within strong northwest flow
will drop across the Northwest, supporting additional periods of
heavy snow through Friday into Saturday before an upper ridge
begins to build east into the region.  Storm accumulations of
several feet are likely across the higher elevations of the
northern Cascades.  Accumulations of 2 feet or more are likely for
portions of the Blue Mountains, as well as the the Bitterroot
Range.

...Central Rockies and High Plains...
Day 1...
As an upper level shortwave approaches from the west, snows are
expected to develop across the Colorado Rockies Wednesday night. 
Low level easterly flow developing east of the mountains is
expected to support developing snows over the High Plains into the
foothills as well.  Guidance has shown a slight northerly shift
with the wave, edging the potential for heavier snows farther to
the north along the I-25 corridor and Front Range.  Models have
also trended more amplified with the wave over the past day as
well, signaling greater potential for significant snowfall
accumulations across a broader area.  Latest WPC PWPF shows snow
accumulations of 4-inches or along much of the northern Front
Range and across the Denver Metro onto the Palmer Divide.

...Maine...
Day 1...
A progressive, but dynamic shortwave moving east of the northern
Great Lakes this afternoon is expected to generate a period of
heavy snow as it moves across northern Maine Wednesday evening and
overnight.  Latest guidance has trended upward with amounts, with
strong low-to-mid level frontogenesis expected to support a period
of moderate to heavy snows moving east across the region.  Several
HREF members are showing the potential for snowfall rates of 0.5-1
inch/hr within developing bands moving across the region beginning
late this evening.

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

Pereira