Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sat Dec 25 2021

Valid 12Z Sat Dec 25 2021 - 12Z Tue Dec 28 2021

...Northern Plains and Upper Midwest...
Days 1-3...
Models continue to show two rounds of winter weather impacting the
region.  The initial round is expected to develop later today as
an upper shortwave ejects east from the western U.S. trough --
moving across the northern Plains this afternoon.  This upper
forcing along with low-to-mid level frontogenesis to the north of
a weak surface wave is expected to produce a stripe of light to
moderate snows centered across southeastern North Dakota and
far-northeastern South Dakota into northwestern Minnesota.  This
will be a progressive system, limiting the potential for
widespread heavy amounts.  However, the latest HREF guidance does
indicate the potential for developing snow bands, producing
accumulation rates of 0.5 in/hr.  WPC probabilities also indicate
a notable threat for accumulations of 4 inches or more especially
over southeastern North Dakota.

The second system is expected to develop across the region on
Sunday as a more amplified shortwave lifts along the leading the
edge of the western U.S. trough, assuming a negative tilt across
the central to northern Plains.  This system is expected to
generate more a widespread threat for heavy snowfall accumulations
and windy conditions across the region beginning Sunday and
continuing into Monday.  WPC probabilities indicate widespread
potential for snowfall accumulations of 4 inches or more across
much of central and eastern North Dakota and northern Minnesota. 
WPC probabilities also show the heaviest accumulations most likely
to center near the North Dakota-Minnesota border.  Models show
broad warm advection/isentropic ascent bringing snows into the
region on Sunday.  Strong low-to-mid level convergence northwest
of the surface low, overlapped by strong upper forcing, is
expected to support snowfall continuing into Monday before the
system lifts out north of the upper Great Lakes.  A period of
easterly flow across Lake Superior beginning Sunday night and
continuing into Monday will likely support a secondary heavy
snowfall max across the Minnesota Arrowhead.  For the both the
Minnesota Arrowhead lake shores and areas along the North
Dakota-Minnesota border, WPC probabilities indicate that
accumulations of 6 inches or more are likely. 

...Northeast...
Day 1...
A wintry mix, with accumulating ice and light snow will continue
to develop through the morning and continue through much of the
day across portions of the region.  Scattered light precipitation
at the onset will followed by steadier precipitation as a surface
wave, accompanied by deeper moisture moves across the Northeast
later today.  While cold air aloft will be quickly swept away,
temperatures at the surface are expected to remain below freezing
-- raising the potential for freezing rain.  Accumulating ice is
expected across portions of eastern Upstate New York and central
to northern New England.  Significant ice accumulations are
possible, especially for portions of northern New York and central
to southern Vermont and southern New Hampshire.  WPC probabilities
indicate a significant threat for ice accumulations of 0.10 inch
or more across these areas.  While widespread heavy snow
accumulations are not expected, thermal profiles suggest mainly
snow across northern New Hampshire -- supporting low probabilities
for 4 inches or more, primarily for the Presidential Range.


...Western U.S...
Days 1-3...
A steady stream of shortwaves and reinforcing shots of cold air
will continue to move into the base of a broad upper trough
centered over the western U.S., supporting additional rounds of
heavy snow through the weekend.

For Saturday into early Sunday, additional accumulations of a foot
or more are likely for portions of the Oregon Cascades and Klamath
Mountains in southwestern Oregon and northwestern California. 
Locally heavy accumulations of a foot or more are also likely for
portions of the northern Nevada mountains into the northern and
central Rockies.  In just the next day alone, several additional
feet are likely for the Sierra where enhanced onshore flow and
upper forcing ahead of an approaching shortwave will support
impressive snowfall rates shifting south along the favored terrain
beginning later today and continuing into the overnight.  Across
the Northwest, snow levels will continue to drop -- supporting
accumulating snows across the lowland area of western Washington
and Oregon, with locally heavy accumulations possible in the
coastal ranges.

Heavy snows are expected to continue through Sunday into early
Monday across the southern Cascades and Klamath Mountains, where
additional accumulations of 1-2 feet are likely.  The Sierra will
continue to see the heaviest amounts, with an additional 2-4 feet
likely for the northern and central Sierra.  Meanwhile, locally
heavy snows will also remain a threat farther east across the
higher elevations of the Great Basin into the northern and central
Rockies.

By Monday and continuing into early Tuesday, the threat for heavy
snow is expected to diminish across the Pacific Northwest and
northern Rockies as colder, but drier air spreads across the
region.  More heavy snows are likely for the Sierra however as an
amplifying shortwave moving through the base of the broader scale
trough swings across California into the Four Corners region late
Monday into early Tuesday.  In addition to heavy snows for the
Sierra, locally heavy snows are possible along the Transverse into
the northern Peninsular Ranges.  Locally heavy amounts are also
possible farther east as well, including the Kaibab Plateau and
the Mogollon Rim in northern and central Arizona, as well as the
Utah and western Colorado ranges.


~~~Key Messages for Western U.S. Heavy Snow Through Early Next
Week~~~

Periods of heavy snow will continue across many of the mountain
ranges through Tuesday. Snowfall accumulations are likely to reach
several feet, and may exceed 7 feet in the Sierra.

Falling snow levels may allow for locally moderate snow
accumulations near sea level across the Northwest, especially late
Saturday through Monday. The foothills and elevated valleys of
California and the Great Basin will also see light to moderate
snowfall this weekend and next week.

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

Extremely cold temperatures will overspread the Northwest early
next week, with some record cold possible especially over the
Pacific Northwest. Low temperatures below 0F will be common east
of the Cascades through the Northern Plains.


Pereira