Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 PM EST Thu Dec 24 2020

Valid 00Z Fri Dec 25 2020 - 00Z Mon Dec 28 2020



...Great Lakes/Ohio Valley/Central Appalachians..
Days 1-3...

An anomalously deep upper trough will rotate across the Ohio
Valley and through the Eastern Seaboard through Saturday. Beneath
this feature, height falls, PVA due to elongated vorticity lobes,
and the upper diffluence of a cyclonically rotating jet streak
will drive a wave of low pressure to develop along a strong arctic
cold front. As this low lifts northeast into Ontario, and the
arctic front advects eastward, precipitation will change from rain
to snow, with heavy accumulations possible in some areas. Behind
the front, strong CAA across the Great Lakes will produce LES.

On Day 1, through Friday night, the heaviest snow is likely just
NW of the cold front across the OH Valley and towards Lake Erie.
It is in this region where strong fgen will drive ascent into the
deepening DGZ to produce intense snow rates of more than 1"/hr as
progged by the HREF snowband probability tool. While cold air
chasing moisture is rarely a setup for heavy snow, the signal has
now been present for many runs that anafrontal snowfall will
develop both due to mid-level divergence leading to ascent NW of
the front, but also through mid-level theta-e advection driving
the ridge into the eastern OH VLY. These features together will
overcome the antecedent hostile environment due to very warm temps
and heavy rainfall, so snowfall should rapidly accumulate across
this region. Upslope into the terrain of WV/PA will enhance
snowfall, as will developing LES SE of Lake Erie as the flow
becomes northerly behind the system. WPC probabilities for 4
inches are greater than 50% from far SW VA northward through WV,
eastern OH, and western PA. The heaviest snow on D1 is likely SE
of Lake Erie due to lake enhancement, and isolated amounts to 12
inches are possible here. Elsewhere, local max above 6" are
possible in the terrain of WV/PA or where any banding can persist
otherwise.

As the low pulls away, strong CAA across the Great Lakes will
drive LES Friday into the U.P. and L.P. of MI, expanding to Lake
Erie and Ontario D2, and persisting across Erie and Ontario D3.
Although the setup favors multibands off Michigan D1, pronounced
instability and rising ELs support heavy snow rates within any
bands, and WPC probabilities are high for 4 inches in SW and NW
Michigan, with local amounts over 8" possible where repeated
banding occurs. By D2/3, single and intense bands may develop
along the lengths of Erie and Ontario. Snowfall rates well in
excess of 1"/hr are likely, and intense forcing combined with
significant instability and even some theta-e lapse rates less
than 0 suggest the potential for thunder-snow as well. While the
bands may fluctuate slightly as the winds gradually back from NW
to W, there is good agreement in single band heavy snowfall
exceeding 12" downwind of Lake Erie and Ontario. WPC probabilities
for D2 and D3 are moderate for 12 inches downwind of these Lakes,
aided by topography in the Tug Hill. 3-day totals may reach 2' in
some locations.


...Pacific Northwest/Northern CA/Northern Rockies...
Days 1-3...

A deep trough will approach from the Pacific, accompanied by a
robust 130+kt Pacific jet streak which will transport moisture
onshore CA and Pacific NW Friday, continuing eastward into the
Northern Rockies by Saturday. Height falls accompanying a
shortwave embedded within this trough, upper diffluence, and
low-level convergence as a front shifts eastward draped from a NE
moving wave of low pressure will combine to produce ascent, aided
by the approach of the 700mb wave. This will produce periods of
heavy snow in the terrain from the Sierra northward through the
Cascades and including the Shastas/Siskiyous. WPC probabilities
for 6 inches peak on D2 across these ranges, and 3-day snowfall
may reach 2 feet in the higher peaks.

As forcing and moisture continue to the east, snow is also
expected in the Boise Mountains and Sawtooth and Clearwater ranges
of ID into northwest MT, where moisture and warm advection are
accompanied by ascent centered in the dendritic growth zone,
favoring an extended period of snow, which may be heavy at times.
Probabilities for 6 inches shift into ID D2, and then further east
into MT/WY D3. Local maxima approaching 2 feet are possible,
highest in the Tetons of WY.  

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss