Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sun Dec 26 2021

Valid 12Z Sun Dec 26 2021 - 12Z Wed Dec 29 2021

...Northern Plains and Great Lakes...
Days 1-3...
A pair of shortwaves lifting out of the base of a broad trough
centered over the western U.S. are forecast move across the region
Sunday night and Monday.  The leading wave, along with sufficient
low level warm advection, will support snows developing and
spreading northeast across the Dakotas and Minnesota Sunday night.
 This will be quickly followed by the second and more amplified of
the pair lifting across the region on Monday.  Low pressure
deepening and tracking northeast from the northern High Plains
will support windy conditions as well.  As the low tracks through
the Dakotas into Minnesota, an inverted surface trough developing
on the northwest side of the low is expected to focus heavy snows.
 For the two period ending 12Z Tuesday, the latest WPC
probabilities indicate amounts of 6 inches or more are likely
across a large portion of central to eastern North Dakota and the
adjacent Minnesota counties.  There also remains a strong signal
for a secondary max along the lake shores in the Minnesota
Arrowhead, where a period of easterly flow beginning Sunday night
and continuing into Monday is expected to produce some lake
enhanced totals.  WPC probabilities indicate that totals of 6
inches or more are likely there as well.  South of the heavier
snows, a period of mixed precipitation, including freezing rain
producing light ice accumulations, is expected across portions of
South Dakota through southern Minnesota and Wisconsin into Lower
Michigan and northern Indiana and Ohio. 

Shortwave ridging behind the departing system will provide a brief
interlude before Monday night into Tuesday before snow returns to
the Upper Midwest by late Tuesday.  Models show the western trough
axis shifting east into the Plains, with a well-defined shortwave
moving across the northern Plains on Tuesday.  At this point,
models are not suggesting widespread heavy totals, but instead a
broad area of generally light accumulations across the Upper
Midwest.

...Northern Mid Atlantic to the Northeast...
Day 2...
Mixed precipitation in the warm advection pattern ahead of the
northern Plains/Great Lakes system is expected to spread from the
eastern Great Lakes into western New York, Pennsylvania, and
northern Maryland Monday morning.  While mostly light ice
accumulations are expected, WPC probabilities indicate some
heavier totals of 0.10 inch or more are possible, especially along
the Allegheny Mountains in central Pennsylvania.

...Western U.S...
Days 1-3...
A broad upper trough with several embedded shortwaves will
continue to support a very unsettled weather across much of the
western U.S., over the next couple of days.  Also, an arctic
airmass spreading south will support low elevation snows from the
Pacific Northwest into California. 

On Sunday into early Monday, additional accumulations of 1-2 feet
are likely along the Oregon Cascades and the Klamath Mountains. 
While lighter accumulations are expected farther to the north, a
very cold airmass in place will support accumulating snows down to
sea level across western Washington. 

Heavier totals are once again likely for the Sierra.  Although
ongoing intense rates are expected to diminish later this morning,
rates are expected to build once again as westerly flow and
forcing increases ahead of an approaching amplified shortwave. 
Heavy snows are expected to develop once again and shift south
from the northern into the southern Sierra on Monday.  Several
more feet of snow are possible before snows begin to diminish
Tuesday night. 

In addition to the heavy snow across the Sierra, some locally
heavy accumulations are expected to return to Southern
California's Transverse and northern Peninsular ranges Monday into
early Tuesday.  Locally heavy totals are also possible farther
east across portions of the Great Basin and northern Arizona into
the central Rockies.

On Tuesday and continuing into early Wednesday, as the upper
trough shifts east and onshore flow diminishes, the threat for
heavy precipitation is expected to wane along the West Coast. 
However, models show a fairly well-defined shortwave moving
through the base of the eastward moving trough, supporting some
locally heavy totals along the Mogollon Rim and White Mountains in
Arizona to the southwestern Colorado and north-central New Mexico
ranges.


~~~Key Messages for Western U.S. Heavy Snow Through Tuesday and
Cold Through Midweek~~~
Periods of very heavy snow will continue through Monday for the
Sierra Nevada with heavy snow north through the Oregon Cascades
and Coastal Ranges. Heavy snow continues across the Intermountain
West through the central/southern Rockies through Tuesday.

Increasingly cold air will allow for locally moderate snow
accumulations to sea level across the Northwest through Sunday
night. The foothills and elevated valleys of California and the
Great Basin will also see light to moderate snowfall through
Monday.

Travel will remain dangerous and is discouraged, especially across
the Sierra Nevada and along other mountain passes where long
duration closures remain likely. Whiteout conditions, downed
trees, and power outages are all likely. Dangerous avalanches are
likely in the Sierra Nevada, Washington Cascades, Northern through
Central Rockies, and Wasatch.

Extremely cold air will overspread the Northwest through midweek,
with daily record cold temperatures likely for many coastal
locations. Low temperatures below 0F will be common east from the
Cascades through the Northern Plains through Wednesday.


Pereira