Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 PM EST Sat Dec 19 2020

Valid 00Z Sun Dec 20 2020 - 00Z Wed Dec 23 2020


Days 1-3...
...Pacific Northwest to the Northern Rockies...
Strong deep layer westerly flow is expected to advect deep
moisture towards the Pacific Northwest coast Saturday night,
supporting the development of heavy precipitation across portions
of western Washington and Oregon.  This precipitation will include
snow for the higher elevations of the Cascades, and high snow
levels should keep significant accumulations above pass levels. 
The moisture surge is expected to be transported well inland and
result in enhanced snowfall for the higher terrain of the Blue
Mountains in eastern Oregon, as well as the northern Rockies. 
Rising snow levels in this region are expected to limit heavy
accumulations mostly to the higher peaks.

There will be some decrease in precipitation rates by late Sunday
owing to an abatement of onshore flow and less synoptic scale
forcing.  However, a lingering frontal boundary and coinciding
axis of deep moisture will continue to support additional locally
heavy amounts, including mountain snows for the Cascades and
northern Rockies.  This front is expected to lift north ahead of
an amplifying wave approaching the Washington coast Monday
morning.  This organized storm system will support an additional
round of enhanced precipitation across the northern Cascades and
northern Rockies as the front lifts back across the region on
Monday.  Snow levels are expected to once again increase ahead of
the low, before dropping rapidly as the low and a strong trailing
cold front move across the Northwest and then the northern Rockies
by late Monday into early Tuesday.  As a result, additional heavy
snow accumulations are likely across the northern Cascades, and
then northern Idaho/western Montana by this time. 

...Upper Midwest...
An Alberta Clipper type surface low is expected to track from
North Dakota to Michigan from Sunday night and into Monday.  A
swath of mainly light snow, owing to warm air advection and
amplifying 500mb shortwave trough, can be expected from northern
Minnesota to northern Wisconsin, including portions of the Upper
Peninsula of Michigan.  Amounts on the order of 1 to 3 inches,
with perhaps some locally higher amounts, are likely across this
region.

...Central Appalachians...
Cold air advection on the backside of the departing low from the
Great Lakes region is expected to produce some lake effect snow
showers downwind of Lake Erie and Lake Ontario, mainly during the
00Z-12Z time period on Tuesday.  In addition, upslope flow across
the spine of the central Appalachians in eastern West Virginia
will likely lead to light to moderate snow showers generally
during that same time period, with accumulations generally under 4
inches expected. 


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

Hamrick/Pereira