Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
213 PM EST Mon Feb 28 2022

Valid 00Z Tue Mar 01 2022 - 00Z Fri Mar 04 2022

...Pacific Northwest...
Days 1-3...

A potent atmospheric river (AR) will wane tonight as the best IVT
shifts eastward in response to the Pacific jet streak shifting
into western Canada, but moderate to heavy precipitation will
persist much of D1 as WAA ahead of a surface cold front keeps PWs
of at least +2 standard deviations above the climo mean. The
impressive WAA will keep snow levels quite high, above 7000 ft in
most places, so snowfall across the Pacific Northwest D1 will be
well above pass level, and WPC probabilities for more than 6
inches are above 40% only in the northern WA Cascades and far
Northern Rockies near Glacier NP.

A brief lull in precipitation is expected the first half of D2,
but renewed moderate to heavy precipitation is likely D3 as the
aforementioned jet streak pivots to place the RRQ overhead, in
conjunction with height falls indicating the approach of a closed
mid-level low moving overhead the Strait of Juan de Fuca and Puget
Sound Thursday. WAA ahead of this feature will increase moisture,
and as snow levels fall beneath the upper trough to around
3000-4000 ft, heavy snow is likely to come down the terrain moreso
than on D1. WPC probabilities for more than 6 inches are highest
in the northern CA ranges near Mt. Shasta and the Blue Mountains
of OR where they are above 40%. These falling snow levels may also
allow for several inches of snowfall at some of the higher passes
including Marias, Siskiyou, and Willamette.


...Northeast...
Days 1-3...

Broad cyclonic flow across the east will surround a closed
mid-level low centered near Hudson Bay through Thursday. Weak
shortwaves will rotate around this trough through the period,
driving two clipper-type lows from the Great Lakes through New
England. The first of these is expected to move across late
Tuesday into Wednesday, with the latter low moving overhead on
Thursday.

With the first wave, a brief period of WAA snow will occur ahead
of the wave, with cold advection behind the associated front
leading to some upslope snow and LES. Overall the snow is expected
to be light, but WPC probabilities for 4+ inches are as high as
20% in the Tug Hill due to LES off Lake Ontario, and along the
spine of the Greens and into the Whites where upslope enhancement
is likely D1. On D2 some light to moderate snow may expand across
eastern Maine as a modest surge of Atlantic moisture lifts onshore
with PWs rising to +0.5 standard deviations above the climo mean.
WPC probabilities on D2 are generally just 5-10% for more than 4
inches across eastern Maine.

The second clipper which will drop across the Great Lakes and move
through New England early Thursday is expected to be more robust
as it is driven by stronger height falls and modest diffluence
within the LFQ of an approaching upper jet streak. WAA ahead of
the wave will spread some snowfall northeastward, but again the
heaviest snow may be due to LES and upslope ascent within the
trailing CAA. The guidance still features some considerable
differences in the latitudinal placement of the wave, but an
increase in moisture, robust LES, and Froude numbers that fall to
well below 1 implying blocked flow suggest an increase in QPF and
snowfall D3 across the region. WPC probabilities are 20-40% for 4+
inches downwind of both Lake Erie and Ontario, in the upwind side
of the Adirondacks and Greens, and into the Northeast Kingdom of
VT. Later on D3, the clipper may try to deepen just east of Maine
in response to more robust synoptic ascent, and a secondary
maximum in snowfall is possible across Downeast Maine through
Thursday.


For Days 1-3, the probability of significant icing of at least a
quarter of an inch is less than 10 percent.


Weiss