Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EST Thu Dec 10 2020

Valid 12Z Thu Dec 10 2020 - 12Z Sun Dec 13 2020


...Arizona to the Southern Rockies...
Days 1/2...

A closed low currently centered over the northern Baja of
California will open into a wave as it shifts east-northeast
across AZ/NM today and southern Rockies tonight across the region
on Thursday into early Friday. This will bring high elevation snow
(generally above 7500ft), with several inches of accumulation
possible to the Rim Country and White Mountains of AZ, while
spreading to the CO/NM border tonight. 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 will bring
some additional locally heavy accumulations to the region with
lower snow levels during the day on Friday. Two day totals of 6-18
inches are expected for 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 on Day 1.


...Central Plains to the Great Lakes...
Days 2/3...

The remnant shortwave associated with the weakening upper low
moving across the Southwest through tonight, along with an
amplifying trough moving east of the Rockies Friday, will support
development of a low pressure system over the central Plains
Friday night before shifting northeast across the southern Great
Lakes Saturday/Saturday night. Increasing baroclinicity along the
leading frontal boundary will promote a widening snow swath over
KS/NE which will morph into comma head precip Friday night as the
reinforcing northern stream wave deepens/becomes negatively tilted
which will prolong the residence of snow over southern NE before
shifting along the frontal boundary to lower MI through Saturday.
Low probabilities for 6 or more inches are present in Day 2 over
southeast NE and western IA then on Day 2.5 and Day 3 across
southwestern to east-central WI and moderate over the northern LP
of MI on Day 3. A tight gradient in rain/snow is expected to
develop ahead of the surface low with wrap around snow behind
which should provide Chicagoland some accumulating snow late
Saturday.


...Northeast...
Day 3...

As the low shifts northeast across the eastern Great Lakes
Saturday night, increasing baroclinicity/frontogenesis is expected
in the leading swath of precip from the Adirondacks across
northern Maine. Surface cold air drainage from the 1036mb high
shifting east across northern Quebec with low level warm air
advection with a warm nose well above 0C at 850mb leads to mixed
precip/icing potential, particularly over northern New England.
There is a 30 percent chance or so for a tenth inch of ice in
northern NH and far western Maine in the Day 3 ice probabilities.
Snow probabilities for four or more inches are limited to far
northern Maine on Day 3.


...Pacific Northwest/Great Basin/Rockies...
Days 1-3...

A trough extending from a deep-yet-compact low over the Gulf of
Alaska shifts onto the WA/OR coast this afternoon before digging
southeast across the northern Great Basin to the southern Rockies
by late Friday that is then forecast to move east across the
northern Rockies on Saturday. Little Pacific moisture accompanies
this wave and snow levels are fairly high (3000 to 5000ft) with
only moderate probabilities for 4 or more inches in the OR
Cascades Day 1 and western/southern CO Rockies on Day 2.
The remnants of the Gulf of Alaska low itself cross southeast over
Vancouver Island late Friday then the system also digs southeast
across the northern Great Basin before reaching the southern
Rockies by late Saturday night. More moisture and lower snow
levels (generally 2000 to 3000ft) bring much more widespread and
heavier snow along the path with moderate probabilities for 6 or
more inches to the southern WA/OR Cascades on Day 2 and southeast
OR/northern NV ranges on Day 2.5, and the Wasatch and western CO
Rockies on Day 3.
Finally, warm advection precipitation ahead of a deepening low
moving toward the Gulf of Alaska, will bring widespread precip
from western WA to far northern CA Saturday night. Snow levels
will drastically rise to over 7000ft in the subtropically sourced
moisture plume with Day 3 snow probabilities only for -the highest
Cascades.

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

Jackson