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

Valid 12Z Fri Dec 25 2020 - 12Z Mon Dec 28 2020


...Great Lakes/Northeast..
Days 1-3...

Through Friday night, heavy snow is expected as cyclonic flow
crosses Lake Erie, with lee shore convergence and upslope
conditions creating bands of locally heavy snow.
An area of a foot of snow is expected close to lake Erie in
northwest PA and southwest NY. The uncertainty is residence time
over any one area, as changing wind speeds and directions alter
the axis/orientation. A few solutions even keep the heaviest bands
over Lake Erie by Sat morning.
Snowfall rates of 1-2"/hr are likely, and steep sfc-850 mb lapse
rates suggest the potential for thunder snow as well. Snow amounts
of 12-18" are anticipated downwind of Lake Erie and Ontario. 

The low-mid level flow off Ontario is southwest today, favoring
the bands of snow shifting towards the northeast corner of lake
near Watertown.  The models differ on the axis/orientation of the
bands and residence time.  Better agreement develops on Sat., as
the flow backs towards the west.  This increases both the cross
lake trajectories and residence time to gather heat and moisture
fluxes from Lake Ontario, plus increases the upslope flow coming
into the Tug Hill.  Snow winds down Sun as dry air aloft and
sinking motion reduce snow shower coverage and intensity.

Strong cold advection across the Great Lakes will drive lake
effect snow showers today in the eastern upper peninsula and
western lower peninsula of MI today.  There is a single band
coming onshore in southwest lower MI that is forecast to continue
into Christmas Day with a south drift continuing.  Drying aloft
Sat and weakening low level convergence brings the snow coverage
and intensity lower.

Light snow develop in MN and extends to the UP of Michigan late
Sun as a 700 mb wave crosses the region and produces an area of
light snow in northern MN across northern WI to the UP of MI.  Low
level northwest flow crossing Lake Superior late Sun produces a
lake enhancement in the UP of MI, with near shore convergence
producing added forcing for ascent.    

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

A deep trough will approach from the Pacific, accompanied by a
robust 110-130 kt Pacific jet streak which will transport moisture
onshore CA and Pacific NW today.  A coupled jet region supports
upper divergence maxima across western OR and WA, allowing heavy
snow to also occur in the WA Olympics and the southern OR
Cascades. 
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. This will produce periods of heavy snow in the
terrain from the Sierra northward through the Cascades and
including the Shastas/Siskiyous.

Light freezing rain is possible in portions of the Columbia River
basin east of Portland to Hood River and Arlington OR to adjacent
WA State as a mid level inversion just above freezing allows
falling precip to melt, leading to freezing rain where temps are
below freezing. Cooling aloft eliminates the inversion tonight, so
the precip type transition zone should then become rain vs. snow.

As the upper jet spreads east, upper divergence maxima spread
inland across the Blue Mountains of OR and into 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. During Saturday, the forward
flank of the jet crosses southern ID into northwest WY. 
Accompanying moisture advection leads to snow developing, and
becoming heavy at times. Amounts are forecast to be highest in the
Tetons of WY.  

On Sunday through Monday morning, a new upper low off the CA moves
slowly east southeast towards the coast.  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.  Several inches of snow are
expected.

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

Petersen