Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Fri Apr 15 2022

Valid 00Z Sat Apr 16 2022 - 00Z Tue Apr 19 2022

...Central West Coast region to the Northern Rockies...
Days 1-2...

Dual potent mid-level shortwaves will spin onshore the West Coast
Saturday morning, one into Washington State and the other into
central CA. These will at least weakly interact into one trough,
and this feature is likely to lift quickly eastward through the
Great Basin and into the Northern/Central Rockies by Sunday.
Aloft, a Pacific jet streak will shift eastward behind these
features, enhancing column moisture, and fueling an expansion of
precipitation as an associated wave of low pressure moves from the
west coast into the Northern Plains. This overlap of moisture and
ascent will manifest as snow, generally above 2000-3000 ft in
OR/WA/ID, and 4000-5000 ft in CA. Above these levels, heavy snow
is likely, especially where the mid-level flow is orthogonal to
the ranges to drive increased upslope ascent. WPC probabilities on
D1 are above 80% for more than 6 inches in the Sierra, the
Shasta/Trinity ranges, the OR Cascades, and the Sawtooth of ID.
Heavy snowfall is likely on D1 at many of the Passes within these
ranges. Locally more than 12 inches is possible in the higher
terrain.

For D2, the best ascent and moisture overlap shifts northeast into
the Northern Rockies, and WPC probabilities for more than 6 inches
are high for the ID Panhandle and western MT terrain, including
near Glacier N.P. Additionally, the guidance is aggressive in
driving a band of 700-600mb fgen northward beneath the potent
shortwave. This occurs with a fully saturated column noted on
regional soundings, and could support snowfall rates in excess of
1"/hr with some convective potential early D2 (00Z-12Z Sunday).
This snowfall could impact Lookout Pass Saturday evening producing
hazardous travel conditions. By D3 this larger system shifts
eastward into the Plains bringing an end to the heavy snow across
most of the West, but another shortwave approaching late D3 could
bring heavy snow again into the Olympic Range where WPC
probabilities for more than 6 inches are above 50%.


...Northern Plains and Upper Midwest...
Days 2-3...

A sharpening shortwave will race quickly along the Canadian/U.S.
border Saturday night and become subtly negatively tilted as it
advects towards the Great Lakes Monday morning. This feature will
be accompanied by modest upper diffluence to help drive a surface
low from Wyoming into Wisconsin late in the weekend. A warm front
extending from this surface low will remain south of the area, but
elevated WAA will surge moisture northward and lift isentropically
along the 290K-295K surfaces to produce an expanding area of
precipitation from eastern MT through the Dakotas and into the
Upper Midwest. This system is fast moving and SLRs are likely to
be modest due to April sun, warm advection, and some subtle drying
noted in soundings just above the DGZ which will limit total
snowfall. However, the pronounced isentropic ascent should produce
at least moderate accumulations, and WPC probabilities are high
for more than 4 inches across ND, with locally up to 8 inches
possible on D2. As the system progresses eastward towards the
Great Lakes D3, the overlap of moisture and forcing broaden and
weaken, such that snowfall is expected to be more light to
moderate, and WPC probabilities for more than 4 inches fall to
20-40%, highest along the Arrowhead of MN Iron Ranges where moist
upslope flow from Lake Superior will likely enhance totals.


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

A cold front moving across the Northeast on Saturday will be
accompanied by modest moisture and a rapidly cooling column. This
front will be driven eastward by a lobe of vorticity swinging
eastward from the Great Lakes to deepen a shortwave trough over
the region. The combination of height falls and PVA interacting
with the low-level baroclinic gradient may lead to a wave of low
pressure development, which is progged to enhance moisture along
and just behind the front. While much of the region will be too
warm for snowfall, on D1 WPC probabilities for more than 4 inches
are as high 30% in the higher terrain of the Tug Hill Plateau and
Adirondacks. On D2, the wave of low pressure ejects to the
northeast but a trailing shortwave will pivot southward from
Canada into the trough and provide secondary ascent Saturday night
and Sunday. This will likely lead to some enhanced snowfall due to
upslope flow on NW winds in the higher terrain of VT/NH and ME,
with some additional enhancement possible in far northern Maine as
a broad deformation axis tries to pivot southward behind the
departing low pressure. Guidance is very split on the intensity of
this deformation axis, with the NAM being much more aggressive
than other models. While this could lead to higher snowfall in
Aroostook County, ME, in general WPC probabilities on D2 are
10-30% for more than 4 inches of snow in the upwind terrain.

Additionally late Saturday night into Sunday, some enhanced
instability beneath the upper trough may help produce scattered
convective snow showers across PA/NY and into New England. There
is some support of embedded isolated snow squalls as well, but
most of the activity should be snow showery with briefly heavy
snow rates.

For Days 1-3, the probability of 0.10 inch or more of ice is less
than 10 percent.


Weiss