Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Wed Mar 30 2022

Valid 12Z Wed Mar 30 2022 - 12Z Sat Apr 02 2022

...Central Rockies...
Day 2...

A weakening shortwave moving onshore British Columbia on Wednesday
morning will drop southeast into the Central Rockies by Friday.
Modest height falls and PVA will accompany this feature as a
vorticity impulse swings through the base of the trough on D2. A
modest upper jet streak of 90-110 kts will accompany this feature
to enhance deep layer ascent through LFQ diffluence, but total
deep layer omega will remain transient. Moisture into the region
will increase Friday as noted by an increase in mid-level RH and
PWs rising to +0.5 standard deviations above the climo mean. The
modest ascent and rapid progression of the system keeps snowfall
accumulations relatively light, but WPC probabilities for more
than 6 inches are 20-40% in the Absarokas and Big Horns on D2.


...Upper Midwest, Great Lakes, Northeast...
Days 1-3...

Two low pressure waves will spread periods of wintry precipitation
during the next few days. The first is ongoing this morning as a
low pressure moves northeast out of Iowa and weakens. Pronounced
warm advection will continue out of the south spreading a moisture
plume into the Great Lakes and Upper Midwest. This WAA will
overrun cold surface air leftover from a retreating high,
producing an extended period of light to moderate freezing rain
from northern WI through the northern L.P. of MI and into the U.P.
The HREF suggests some areas may exceed 0.25" of freezing rain,
but this may be overdone due to runoff from periodic heavy rates,
and periods of drying which may limit total QPF in some areas. The
WSE and NBM are in pretty good agreement, resulting in a forecast
and PWPF that features probabilities for 0.1" above 30%, but less
than 5% chance for 0.25". Further to the NW where the column will
be colder, the guidance has become a bit more robust with an axis
of deformation snow pivoting from eastern SD and the Buffalo Ridge
northeast into the Arrowhead of MN and the western U.P. WPC
probabilities for more than 4 inches of snow have increased
slightly and are 20-30% on D1.

As this first low weakens, a secondary low emerging out of the
Southern Plains will deepen more rapidly in response to better
height falls within a large mid-level gyre, and accompanying upper
diffluence where the downstream jet streaks interact in a coupled
fashion. This should allow this secondary low to intensify much
more significantly as it lifts through the Great Lakes on D2.
Impressive WAA from the Gulf of Mexico will surge PWs to as high
as +3 standard deviations above the climo mean as 850mb flow
reaches above 60 kts. This will manifest as an impressive theta-e
ridging lifting into a TROWAL around this secondary low, and this
TROWAL may pivot along the NW flank in tandem with an impressive
axis of deformation. The guidance has come into much better
agreement that this will lead to a band of precipitation which
will change rapidly from rain to heavy snow Wednesday night into
Thursday. Although the antecedent conditions are less than
favorable for accumulation, snowfall rates will likely reach more
than 1"/hr as shown by the HREF probabilities and the WPC
prototype snowband tool. Dynamic cooling of the column during this
transition may allow changeover to occur faster than guidance, and
the WPC probabilities for more than 4 inches D1 and D2 above 20%
stretch from southeast MN through much of WI and into the
western/central U.P. Locally, probabilities are above 70%, and
storm total snowfall may reach 10 inches in the U.P. where
additional enhancement from Lake Superior is likely.

As the parent shortwave trough lifts into the Canadian Maritimes
on D3, CAA on impressive NW flow will likely produce moderate to
heavy upslope snow in the far northern reaches of New England and
Upstate New York. WPC probabilities for 4+ inches of snow are
5-10% in the Adirondacks, and 20-30% in the mountains of NH and ME.


...Great Lakes, Ohio Valley, Northeast...
Day 3...

A potent cold front racing across the area Thursday and Friday
will leave pronounced CAA in its wake, while an associated
mid-level closed low moving out of the Great Lakes and into New
England spins a vorticity lobe and height falls southward across
the region behind this front on Friday. The combination of
increasing instability due to steepening lapse rates and
impressive low-level fgen behind the front could produce scattered
to widespread heavy snow showers. Analysis also suggests an
increasing potential for at least isolated squalls embedded within
the more widespread snow showers. This is noted by an overlap in
guidance of high 0-2km fgen, 100-200 J/kg of SBCape, theta-e lapse
rates <0C/km, and a region of snow squall parameter values as high
as +3, focused across the Ohio Valley into the interior Northeast
on Friday. While this is outside the current high-res windows
which will likely pick up on the squall potential better, this
threat should continue to be monitored as the event evolves during
the next few days.

Weiss