Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Sat Apr 16 2022

Valid 12Z Sat Apr 16 2022 - 12Z Tue Apr 19 2022

...West Coast to the Northern Rockies...
Days 1 and 3...

Water vapor imagery early this morning shows two shortwaves of
interest, a closed low off the Pacific Northwest while a sharper
piece of energy racing underneath it toward northern California.
Both of these features move inland through the day 1 period and
should reach the northern Rockies by 12Z Sunday. Additional
forcing for ascent is brought by a Pacific jet streak shifting
eastward behind these features while at the surface low pressure
organizes and moves from the west coast through the Northern
Plains. The combination of moisture and lift, enhanced where the
flow is orthogonal to the terrain, will bring widespread heavy
snow to portions of west coast through the Northern Rockies. Snow
levels, generally 2-3 kft in OR/WA/ID and 4-5 kft in CA are
expected. WPC probabilities for 6 inches during day 1 are high for
the Sierra, the higher peaks of the OR/WA Cascades, and the
Sawtooth of ID and further north across the ID Panhandle into
northwest MT including near Glacier N.P.. Locally, amounts greater
than 12 inches are possible, particularly for the CA Sierra and
Sawtooth of ID.

Another closed upper low approaches the Pacific Northwest during
the day 3 period with the associated surface low swinging toward
and up the coastline before approaching British Columbia.
Widespread forcing for ascent and plentiful amounts of moisture
will yield precipitation including heavy mountain/terrain snow
confined mostly to the WA Olympics, WA/OR Cascades where the
latest WPC probabilities are generally in the moderate range
(40-60 percent) but for the highest peaks of the Olympics and
Cascades in WA, localized totals in excess of a foot will be
possible.


...Northern Plains and Upper Midwest...
Days 1-2...

Shortwave trough racing along the Canadian/U.S. border becomes
neutrally to negatively tilted as it approaches the Northern
Plains and Upper Midwest Sunday into Monday. Low pressure is
expected to track from Wyoming through the Dakotas into the Upper
Midwest. Strong isentropic lift ahead of the low will bring a
large area of precipitation Sunday while a narrow focused area of
prolonged deformation band precipitation just north/northwest of
the low track is expected over northern ND. A quick thump of snow
could total few inches on the leading edge while the
deformation/frontogenetical band area could pick up close to
warning level snows. The latest WPC PWPF shows a solid swath of
30-40 percent probabilities for 6 inches or more with a localized
signal for 8 inches (5-10 percent along the U.S./Canadian border).
As the system progresses eastward towards the Great Lakes, 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 20-30 percent, except
for localized higher probabilities along the Arrowhead of MN Iron
Ranges where moist upslope flow from Lake Superior will likely
enhance totals.


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

Precipitation developing along/behind a cold front that will
slowly move through the region today/tonight could mix with or
changeover to wet snow, particularly for the higher elevations
areas of upstate NY into New England. Shortwave energy is expected
to close off over the Great Lakes with an embedded lobe of
vorticity swinging through. This enhanced lift combined with the
lift associated with the front and modest moisture in place is
likely to bring a band of precipitation. With dynamic cooling
taking effect, temperatures marginally supportive of wet snow
across the higher terrain areas could accumulate a few inches
before ending late Saturday into early Sunday. The greatest WPC
probabilities for 4 inches are across the high peaks of the
Adirondacks and the highest peaks of NH and ME.

On Day 3, shortwave energies traversing the CONUS during D1-2
begins to phase and take on a negative tilt as it approaches the
Great Lakes and Northeast Monday night into Tuesday. As the trough
axis sharpens, low pressure originating out of the Southeast and
Mid-Atlantic is expected to deepen rapidly and track just offshore
the Northeast. The favorable forcing for ascent aided by a coupled
jet streak over the region will lead to a widespread area of
precipitation northwest of the low track. Temperatures are
expected to be marginally supportive of wet snow, generally in the
lower 30s, and an elevation driven snow event is increasingly
likely. The system will be moisture rich though should move
through fairly quickly Monday night into Tuesday. Higher
elevations of the central Appalachians, NY Catskills, Adirondacks,
Green, and White Mountains stand the best chance for several
inches of wet snow and the latest WPC probabilities show slight to
moderate probabilities of 4 inches or more. The potential
lift/forcing could overcome the marginal temperatures, especially
given the timing during the overnight into early morning hours and
there remains quite a bit of spread in the model guidance both in
QPF and thermals. At least some minor impacts due to snow are
possible.

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


Taylor