Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
435 AM EST Sun Feb 28 2021

Valid 12Z Sun Feb 28 2021 - 12Z Wed Mar 03 2021

...Upper Midwest into the U.P. of Michigan...
Day 1...

A fast moving but rapidly deepening surface low pressure will lift
northeast from MN this morning into Ontario by Monday morning.
This low will be driven by a potent shortwave and the LFQ of a
strengthening upper jet streak progged to reach 170kts. This
robust deep layer ascent allows for rapid surface pressure falls,
and this ascent will be aided by intense mesoscale forcing as
well. As the low moves northeast, an intense band of frontogenesis
is likely to stream northeast, coincident with the saturating DGZ,
and beneath a developing TROWAL, albeit with modest intensity.
This suggests that within a broader footprint of light to moderate
snow, a heavy but narrow snow band will develop and lift
northeast. Within this band, snowfall rates of 1-2"/hr are
possible as shown by recent HREF guidance and suggested by CSI
potential shown in cross sections. These rates should overcome the
progressive nature of the surface low, so despite a short
duration, WPC probabilities are high for 4 inches across northern
WI and into the U.P. of MI, where local amounts near 8" are
possible.


...Northeast...
Days 1-2...

Low pressure and its associated mid-level trough will move
northeast through Ontario, leaving SW flow and WAA into the
Northeast. Embedded within this flow, a weak shortwave is progged
to lift across PA and into New England Sunday night, which will
enhance precipitation across the region. The column is marginally
cold enough for wintry precipitation, and WAA is expected to
overwhelm most of the area leading to primarily rainfall. However,
far northern ME is likely to remain cold enough for some modest
snow accumulations Sunday night and WPC probabilities are as high
as 30% near the Canadian border. Additionally, some lingering cold
air in the terrain of the Catskills, Adirondacks, and Whites could
lead to some modest accretions of freezing rain with WPC
probabilities indicating less than 20% chance for 0.1".

After this system moves away, a more potent shortwave/closed low
will race across Northern New England Monday night driving an
arctic front southward across New England. An area of snow will
likely accompany this feature as ascent is provided through rapid
height falls and PVA. However, somewhat limited moisture and fast
progression should limit snowfall accumulations, with WPC
probabilities for 4 inches less than 20%, and confined to the
terrain of the Adirondacks and Greens which should benefit from
some upslope enhancement. Additionally, some lake effect snow is
likely E/SE of Erie and Ontario, but forecast soundings indicate
the DGZ struggles to saturate so LES accumulations are forecast to
be modest. Behind the arctic front, the environment looks primed
for snow squalls across Upstate New York, parts of New England,
and potentially into northern PA. While accumulations from these
squalls should be limited, briefly intense snowfall rates and
gusty winds are likely Monday night.


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

Weiss