Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Mon Mar 21 2022

Valid 00Z Tue Mar 22 2022 - 00Z Fri Mar 25 2022

...Southern Rockies into the Central Plains...
Days 1-2...

A surface low pressure emerging from the lee of the Southern
Rockies tonight will lift northeast into the MS VLY by Tuesday
evening, and then slowly drift further northeast Wednesday as the
upper trough evolution becomes complex. While this low is not
expected to become exceptionally strong, forcing will be
impressive through robust WAA out of the Gulf of Mexico and strong
jet-level diffluence. The atmospheric column is likely to be too
warm for much snow across most of this area, at least initially,
so the pronounced WAA on 50kts of southerly 850mb wind will lead
to an expansion of rainfall. However, a trailing cold front will
drop southeast immediately behind the wave of low pressure, and
the associated strong CAA will aid in dynamically cooling the
column to cause a changeover from rain to snow. During this
transition, fgen will sharpen in response to the pinched
baroclinic gradient beneath the LFQ of the jet streak, and this
may align with a stretched deformation axis NW of the surface low.
Where these features overlap, a period of heavy snow is likely,
with snowfall rates possibly reaching 2"/hr thanks to CSI and as
forecast by the WPC prototype snowband tool. The guidance has
become more aggressive with snowfall accumulations due to the
rapid changeover, and this is despite low-level temps that will
crash to just around 0C. However, the more rapid changeover
combined with intense ascent should allow snowfall to accumulate
rapidly, and WPC probabilities for 4+ inches have increased on D1
to more than 50%, highest in the TX/OK Panhandle and into western
KS. Locally, more than 6 inches is possible in any of these bands.

As the low pulls further E/NE on D2, forcing is expected to weaken
so only light accumulations reaching around 2 inches is forecast
from eastern NE into the Buffalo Ridge of SD/MN, and residual
light snow is also likely into D2 across the higher terrain of NM
within the broad upper trough axis, where storm-total snowfall
will likely exceed 6 inches in some places.


...Northern Plains through the Great Lakes into the Northeast...
Days 1-3...

A northern stream shortwave trough will cross ND tonight and then
shifts towards the Great Lakes into D2, while eventually merging
with southern stream energy coming out of the Plains to drive a
complex but large scale closed low across the Plains. The
amplified mid-level pattern will allow deep southerly flow to
advect anomalous moisture northward ahead of the northern stream
low, and then more impressively in advance of the phased upper
gyre and associated surface wave ejecting out of the MS VLY. While
moisture will be plentiful, the column will be marginally cold
enough for snow, and a period of freezing rain is likely from
eastern MN through the western Great Lakes in response to the slow
retreat of Canadian high pressure.

On D1, WPC probabilities for 4 inches of snowfall are 10-30%
across northern MN, with a local maxima above 50% in the Arrowhead
where moist advection off Lake Superior will efficiently upslope
into the Iron Ranges to increase snowfall. While some freezing
rain is also likely in the Arrowhead D1, this is likely to
transition to snow the latter part of D1, and more impressively on
D2 in response to deeper moisture out of the Gulf lifting
northward. WPC probabilities on D2 for 4+ inches are above 80% in
the Arrowhead, and storm total snowfall could exceed 12 inches in
some places. Heavy snow will also extend eastward into the Great
Lakes on D2, with WPC probabilities for more than 4 inches
reaching as high as 50% in the eastern U.P. of MI.

As the warm nose overruns the cold surface air, a period of
freezing rain is likely in eastern MN and the U.P. of MI D1 where
WPC probabilities for 0.1" of accretion reach above 50%, highest
in the arrowhead, and then shift eastward with decreasing
probabilities in the northern L.P. of MI on D2. Finally, as the
best WAA shifts into the Northeast on D3, WPC probabilities
indicate a better than 40% chance for 0.1" of freezing rain in the
Adirondacks, Catskills, Berkshires, and Green Mountains.


Weiss