Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 AM EST Thu Feb 25 2021

Valid 12Z Thu Feb 25 2021 - 12Z Sun Feb 28 2021


...Pacific Northwest through the Central Rockies...
Days 1-3...
A shortwave dropping into WA this morning will drive a surface
frontal wave onshore, and initiate an extended period of unsettled
weather across the Pacific Northwest. This first shortwave will
traverse the Pacific Northwest and move into the Central Rockies
by Friday as broad troughing envelops the Intermountain West. As
this first trough digs southeastward, a second shortwave with
associated elongated vorticity will stream down from British
Columbia immediately on the heels of the first system. This second
wave will amplify more significantly than the first as it expands
towards the Four Corners by the end of the period.

These shortwaves will be embedded within confluent mid-level flow
and periods of upper diffluence as Pacific jet energy sheds down
from the Gulf of Alaska. This persistent flow will provide
sufficient moisture for heavy snowfall, and the trajectories will
be nearly ideal for upslope enhancement into the Cascades,
Olympics, Blue Mountains, ans Bitterroots. This is evidenced by
prolonged W/NW 700mb flow, and nearly continuous and robust omega.
While the heaviest snow is likely across WA/OR/ID, lighter snows
are expected to spread into the ranges of the northern Great
Basin, Utah, and CO D2 into D3. WPC probabilities for 12 inches
are high in the Olympics, Cascades, Blues, and Bitterroots D1 and
D3, and the higher terrain may exceed 5 feet of snow during the
period. With snow levels falling to around 2000 ft, even the
Cascade Passes including Stevens and Snoqualmie may receive 3 ft
of snow. Further south and east where moisture and ascent weaken,
WPC probabilities are moderate for 6 inches D2 into the Tetons and
northern Wasatch, extending into the CO Rockies on D3 while waning
quickly across the Cascades D3 when forcing shifts away from the
area.

...Central Appalachians through the Northeast...
Days 2-3...

A wave of low pressure beneath a weakening shortwave impulse will
lift out of the Deep South and into the Mid-Atlantic Friday with
weak secondary low development likely occurring off the NJ coast
Saturday. This secondary low will then lift northeast off the
coast through the weekend. Initially, WAA ahead of the first wave
and along a warm front will spread precipitation northward, with
cold high pressure briefly wedging down into the Southeast.
However, this high is likely to retreat quickly in response to
mid-level divergence and warm advection, and precipitation is
likely to start as snow/ice before changing to all rain as far
north as central New England by Saturday.

Some light snow is likely at precip onset across much of the area,
but a change to freezing rain with light accretions is likely from
far NW North Carolina along the Appalachians and into the Laurel
Highlands D3 where WPC probabilities for 0.1" are as high as 20%.
This should eventually change to rain however. Further north, and
primarily on D3, moderate snow may overspread New England, and
linger long enough across the terrain of Northern New England
above 2000 ft that WPC probabilities are low for 4 inches in the
higher terrain of the White Mountains.

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

Weiss