Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 PM EST Fri Dec 25 2020

Valid 00Z Sat Dec 26 2020 - 00Z Tue Dec 29 2020


...Great Lakes..
Days 1-2...
CAA embedded within intensifying cyclonic flow behind a departing
arctic front and wave of low pressure will drive increasing
coverage and intensity of Lake Effect Snow through Saturday. The
heaviest snow is likely D1 into D2 before subtle shortwave ridging
begins to bulge up from the SW. This will veer the winds to a less
favorable fetch direction while also shutting off CAA, both
requirements for significant LES.

While modest accumulations of LES are expected in the NW L.P. and
eastern U.P. of MI on D1, the heaviest snow is almost certainly
going to occur downwind of Erie and Ontario. On D1, flow beneath
the EL will be generally W/SW with weak directional shear,
favoring multibands east of Lake Erie. However, as D1 progresses
and advances into D2, the flow across Erie is likely to back more
to the SW becoming more supportive of a single band along the
entire fetch of Erie, with nearly unidirectional westerly flow
across Ontario. Both of these fetches are supportive of intense
snowfall rates as the band wavers slowly. The exact placement of
this band can be problematic to forecast even on D1, but the high
res guidance suggests the maximum snowfall will be just south of
Buffalo off Erie, and into the Tug Hill off Ontario. These high
res models can have a little southward bias sometimes, and the
RGEM is a bit further north and used to potentially offset the
high res just a bit. In the most intense bands, snowfall rates
will approach 2"/hr or more with high SLRs, and the presence of
theta-e lapse rates less than 0 suggests the potential for
thundersnow as well. WPC probabilities on D1 are moderate to high
for 12 inches SE of Erie and E of Ontario, with additional heavy
snowfall likely the first half of D2. Event total snowfall could
reach 2 feet in the Tug Hill Plateau, and while Buffalo itself may
be just north of the heaviest snow, amounts near 18" are likely
east of Erie as well.

Day 3...
A wave of low pressure merging with a mid-level impulse ejecting
out of southern Canada will drive a clipper type low across the
Midwest and Great Lakes Sunday night into Monday. Forcing with
this feature looks transient and is weak, so coverage and
intensity of snowfall is likely to be modest. However, a very cold
airmass will support high SLRs, and flow behind the system as it
departs Monday /D3/ will create some enhanced LES across the U.P.
and northwest L.P. of MI. Some modest synoptic snows through a
weak deformation axis combined with post-low LES could produce
accumulations in excess of 4" in a few locations. WPC
probabilities are as high as 40% for 4" in the eastern U.P.,
slightly lower in the NW L.P. near Traverse City.
   

...Pacific Northwest/CA/Northern Rockies...
Days 1-3...
A potent 130kt+ Pacific jet streak will approach the California
coast tonight providing robust moisture transport into the West. A
weakly coupled jet structure around this primary jet streak, and a
modest shortwave embedded within confluent flow will drive ascent
into the Pacific NW, and produce a weak wave of low pressure and
associated cold front moving into the Intermountain West. Deep
layer ascent from low-level convergence and upper diffluence will
spread snowfall across much of the terrain from the Northern
Sierra, through the Cascades, and eventually into the Northern
Rockies and surrounding terrain by D2. The heaviest snowfall is
likely in the ranges which are orthogonal to the mid-level flow,
with WPC probabilities indicating high probabilities for 8 inches
in the Sierra, Cascades, Olympics, and Tetons through D2. Isolated
amounts over 18" are likely.

Additionally, some light freezing rain is expected within the
Columbia Gorge and into the Columbia Basin tonight, but ZR
accretion should be only a few hundredths of an inch.  

Sunday into Monday, a new and more robust upper low off the CA
moves slowly east southeast towards the coast and then towards the
Great Basin by the end of the period.  As the upper trough
approaches, the upper jet maxima develops and moves onshore,
continuing inland into the central to southern Sierra.  The jet
will drive both moisture advection and ascent as embedded upper
divergence maxima drive ascent Sunday night into the central to
southern Sierra Nevada mountains. Additionally, the upper low
moving into CA will drive enhanced ascent through mid-level
divergence and height falls, along with periods of intense PVA
ahead of any embedded vort lobes. This should produce heavy snow
on D3, focused in the Sierra but also dropping into the ranges of
Southern CA as mid-level temps cool. WPC probabilities are high
for 4 inches across these ranges and into the Great Basin. Amounts
in excess of 12" are likely in the Sierra.

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

Weiss