Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EST Wed Dec 09 2020

Valid 00Z Thu Dec 10 2020 - 00Z Sun Dec 13 2020


...Southwest to the Southern and Central Rockies...
A closed low currently centered west of Southern California and
Northern Baja is expected to transition to an open wave and move
east across the region on Thursday into early Friday.  This will
bring high elevation snow, with several inches of accumulation
possible to the Rim Country and White Mountains region of Arizona,
while spreading north and east into the Rockies on Thursday.  Snow
is likely to continue across the northern New Mexico and southern
Colorado ranges through the overnight.  Precipitation is expected
to wane, as the remnant shortwave associated with the upper low
moves east of the Rockies Friday morning.  However, a northern
stream trough amplifying through the Great Basin into the Rockies
may bring some additional light accumulations to the region during
the day on Friday.  Mostly dry conditions are expected late Friday
into early Saturday, before another system drops southeast into
the central Rockies late Saturday.  Three day totals of 6-12
inches are expected across a large portion of the southern
Colorado and northern New Mexico ranges, including the Jemez, San
Juan, Sangre de Cristo and Elk mountains.  Similar totals are
expected across the White Mountains in eastern Arizona.

...Central Plains to the Great Lakes...
The remnant shortwave associated with the weakening upper low
moving across the Southwest on Day 1, along with an amplifying
trough moving east of the Rockies, will support precipitation
extending northeast from Kansas and Nebraska to Lower Michigan on
Friday, with mainly light snow accumulations expected along the
northern edge of the developing precipitation shield.  Then by
Friday night and continuing into Saturday, a coupled upper jet
along with low-to-mid level frontogenesis is expected to support
heavier accumulations on the northwest side of a deepening low as
it moves from the mid Mississippi valley toward the Great Lakes. 
While confidence in the details is limited, the heaviest totals
through late Saturday are most likely to occur across portions of
far northern Illinois into southern Wisconsin and central Lower
Michigan.  The latest WPC PWPF suggests significant potential for
snow accumulations of 4-inches or more across the region.  Given
the strong forcing and potential for mesoscale banding, locally
heavier totals can be expected.

...Pacific Northwest/Great Basin/Northern Rockies...
Prior to impacting the central Rockies on Friday, a well-defined
shortwave is expected to slide southeast from the Pacific
Northwest into the Great Basin late Thursday into early Friday. 
This will be followed by a second shortwave dropping into the
Northwest by late Friday, that is then forecast to move east
across the northern Rockies on Saturday.  While the second system
is expected to generate more widespread coverage -- with
accumulating high elevation snow moving east from the Cascades
through the much of the Great Basin into the Rockies late Friday
and Saturday -- neither system is expected to produce widespread
heavy amounts.  Meanwhile, warm advection precipitation ahead of a
deepening low moving toward the Gulf of Alaska, will likely bring
additional high elevation snow into the Cascades and the Sierra,
with some signal for mixed precipitation and accumulating ice
across the interior valleys of far northern California, western
Oregon and Washington.

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

Pereira