Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Mon Dec 27 2021

Valid 12Z Mon Dec 27 2021 - 12Z Thu Dec 30 2021

...Upper Midwest/Great Lakes...
Days 1-2...
System currently impacting the northern Plains and western Great
Lakes is forecast to lift across northern Minnesota and the
northern Great Lakes later today, with additional heavy snows
expected to remain confined mostly to northern Minnesota and the
U.P. of Michigan.  WPC probabilities indicate that additional
accumulations of 6 inches or more are likely along Minnesota's
North Shore of Lake Superior, where easterly flow ahead of the
advancing system is expected to produce some lake-enhanced totals.
 Some of the HREF guidance continues to show snowfall rates of
over an inch/hr continuing into the late morning.   This easterly
flow will also favor Michigan's Keweenaw Peninsula, where some
lake-enhanced totals are likely as well.

A brief period of dry weather early Tuesday will be followed by
the return of winter weather as another shortwave moves across the
region Tuesday night.  Models continue to signal mostly
light-to-moderate snows with this system.  WPC probabilities for
accumulations of 4 inches or more are in the 40-70 percent range
at best, with these higher probabilities confined to central
Wisconsin and along Minnesota's North Shore.

...Northern Mid-Atlantic to the Northeast...
Day 1...
Warm advection ahead of the current Great Lakes system will
encroach upon a cold air wedge extending from the Northeast into
the Mid Atlantic, setting the stage for mixed precipitation across
the region this morning.  Still anticipating mainly light snow and
ice accumulations from western and north-central Maryland through
Pennsylvania into the Northeast.  WPC probabilities continue to
show some higher probabilities for ice accumulations of 0.10 inch
or more centered along Pennsylvania's Allegheny Mountains.

...Western U.S...
Days 1-3...
As an amplified positively-tilted shortwave pivots across the
region, more heavy snows are expected for the Sierra Nevada today.
 HREF guidance continues to suggest impressive snowfall rates
shifting south during the day, with WPC probabilities indicating
widespread additional accumulations of 1-2 feet, with locally
heavier amounts likely.  Locally heavy snows are also expected to
develop in the Transverse and and northern Peninsular ranges later
today.  Snows are expected to diminish across California, with
locally heavy amounts developing across the Great Basin, northern
and central Arizona and the central to southern Rockies as the
upper trough moves east later today into Tuesday.

Meanwhile, a series of shortwaves diving south into the base of
the broader scale trough is expected to continue.  Models show one
shortwave beginning to amplify as it moves south from the Pacific
Northwest to the Northern California coasts on Wednesday, with a
closed low forming west of Central California by early Thursday. 
This will support yet another round of heavy snows for the central
to southern Sierra into the Transverse and northern Peninsular
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 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 along
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