Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Fri Mar 26 2021

Valid 12Z Fri Mar 26 2021 - 12Z Mon Mar 29 2021


...Western U.S....
Days 1 & 3...

Through tonight, areas of high elevation snow will persist across
the terrain of the Four Corners as modest height falls continue in
response to a longwave trough axis shifting southeast across the
region. Weak to moderate ascent through these height falls and WAA
on SW flow ahead of the trough axis will promise periods of heavy
snow, generally above 5000-6000 ft. The weak surface low
associated with this ascent will gradually fill tonight, which
when combined with the eastward progression of the deeper ascent
will bring an end to the snowfall. WPC probabilities for 6 inches
are low to moderate and confined to the terrain of the CO Rockies.

On Day 3, a potent shortwave embedded within moist confluent flow
will race eastward accompanied by intensifying upper divergence
ahead of a SE diving jet streak. Pronounced moisture advection
ahead of the jet streak and within the confluent mid-level pattern
will spread precipitation across the Pacific Northwest Sunday,
while height falls, divergence, and low-level convergence along a
frontal band will produce ascent for snowfall across the region.
Snow levels ahead of this front will be quite high as tropically
sourced moisture spreads inland, but should fall dramatically as
the front races SE with time late on D3. WPC probabilities for 6
inches are quite high in the Olympics and Cascades, spreading into
the northern Rockies as well. Locally more than 12" is likely in
the Cascades.


...Great Lakes to the Northeast...
Days 2-3...

A shortwave dropping out of Alberta, Canada Saturday will move
eastward towards the Great Lakes while strengthening. As this
feature shifts across the Upper Midwest Saturday evening, a weak
wave of low pressure may develop beneath it with a cold front
dragging in its wake. Warm advection ahead of the trough axis will
spread precipitation northward, and as the column cools behind the
front rain changing to snow is likely, with some modest
accumulations possible across MN and into the Western Great Lakes.
WPC probabilities for 4 inches are generally less than 20%,
highest in the Minnesota Arrowhead on D2.

As this shortwave continues to progress eastward, it will interact
and potentially phase with southern stream energy coming out of
the Plains on Sunday. This will drive a more intense surface low
through the Ohio Valley and into southeast Canada late Sunday into
Monday. Impressive moisture transport from the Gulf of Mexico will
spread northward ahead of this system spreading precipitation
northward into New England. Much of this precipitation is likely
to be rain as WAA overwhelms any modest cold air in place.
However, for parts of far Northern New England moderate to heavy
snow is possible, and WPC probabilities are high for 4 inches,
primarily north of I-95 and U.S. 201 in Maine.


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

Weiss