Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Thu Mar 25 2021

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

...Northern, Central, and Southern Rockies...
Days 1-2...

A broad mid-level trough will sharpen and drift slowly southeast
from the Pacific Northwest, through the Great Basin, into the Four
Corners, and eventually towards the Southern Plains by Sunday.
Above this trough, Pacific jet energy will push onshore leaving
periodic divergence maxima across the Western CONUS, shifting
southeast with time as the trough digs eastward. The combination
of height falls and divergence will spread precipitation in the
form of rain and snow showers across much of the inter-mountain
west during the forecast period. The heaviest snow will be
associated with the strongest height falls, but also coincident
with the most robust 700-500mb WAA on SW flow ahead of the trough
axis. Additionally, wave of surface low pressure are likely to
develop and move across the West beneath the trough, enhancing
lift at times.

WPC probabilities for snowfall greater than 6" are high on D1
across many of the ranges in MT/ID southward through the Great
Basin and Four Corners, with the heaviest snow likely in the San
Juans where locally more than 12" is possible. By D2 the total
forcing and moisture begins to wane, but residual moderate
probabilities for 6" of snowfall continues across the ranges of WY
and CO Rockies/San Juans. By late Saturday, the forcing has
shunted off to the east and a respite to the moderate snow should
occur across the region.


...Great Lakes to Northern New England...
Days 1-3...

A negatively tilted shortwave trough ejecting northeast through
the Ohio Valley Friday morning is progged to close off and lift
quickly through Northern New England by Saturday morning. This
feature will be accompanied by a coupled jet structure and strong
upper divergence maxima to drive surface cyclogenesis near the
Missouri Valley, with rapid strengthening likely as the low shifts
towards New England and then into Atlantic Canada this weekend.
The environment is generally too warm for snowfall across much of
this area, however, a cold front sinking southward from Canada
combined with some dynamic cooling of the column should allow for
rain to change to freezing rain briefly, and then snow, across
parts of WI and MI, and then Northern New England. Freezing rain
accretions could reach up to 0.1" in far northern MI and eastern
WI, and WPC probabilities for 0.1" are as high as 20%. The
heaviest snow from this low is expected across northern Maine
which will have the longest overlap of precipitation with
sufficiently cold thermals for snowfall. WPC probabilities are as
high as 40% for 4 inches on D2 across far northern ME.

A secondary shortwave will follow quickly behind this first
impulse, moving across the Northern Plains and towards the Great
Lakes Saturday night /D3/. This shortwave is also likely to spawn
weak cyclogenesis, but the antecedent column is expected to be
sufficiently cold enough for an area of light to moderate snow
from Minnesota into the U.P. of Michigan. WPC probabilities on D3
are less than 20% for 4 inches of accumulation.

Weiss