Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Tue Dec 28 2021

Valid 12Z Tue Dec 28 2021 - 12Z Fri Dec 31 2021

...Upper Midwest to the Northeast...
Day 1...
A broad upper trough remains centered over the western half of the
U.S. this morning.  Several shortwaves translating through the
base of the trough are expected to move in tandem -- lifting
across the Plains, with a negatively-tilted trough positioned over
the northern Plains/Upper Midwest by this afternoon.  This is
expected to bring the next round of snow into the Upper Midwest
and northern Great Lakes.  Moisture is not as robust as the
previous system -- limiting the potential for widespread heavy
totals.  Low-to-mid level frontogenetic forcing is expected to
support a swath of relatively higher totals across north-central
Minnesota.  Meanwhile, low level easterly flow will once again
bolster totals along Minnesota's North Shore.  WPC probabilities
are 50 percent or greater for accumulations of 4-inches across
both areas.

Meanwhile, a west-east oriented baroclinic zone will be the focus
for mixed precipitation from the southern Great Lakes region into
the northern Allegheny Plateau.  Here also widespread heavy
amounts are not expected, however low-to-mid level frontogenetic
forcing along with favorable forcing along the right-entrance
region of a powerful upper jet may support a brief period of heavy
precipitation translating east from the Great Lakes in the
afternoon to northern Pennsylvania and the Southern Tier of New
York during the evening/overnight.  HREF guidance suggest rates
may briefly reach 1-2 in/hr, supporting a window of rapidly
accumulating snow before changing over to mixed precipitation. 
For most areas only an inch or two of snow is expected, however
WPC probabilities suggest pockets of locally heavier accumulations
can be expected, especially along the western New
York-Pennsylvania border.

...Western U.S...
Days 1-3...
While remaining unsettled, Tuesday is expected to be relatively
quieter day along the West Coast -- with widespread light snow
accumulations expected, but with only very localized heavy amounts
expected across the Pacific Northwest and California.  A greater
threat for heavier amounts is expected farther east -- with a
well-defined shortwave supporting the potential for heavier
amounts across northern and central Arizona along the higher
terrain and then east of the Four Corners over the southwestern
Colorado and north-central New Mexico ranges.

Meanwhile, the series of shortwaves diving south into the base of
the broader scale trough is expected to continue, with an
amplifying positively-tilted trough developing across California
Wednesday into Thursday.  This will support yet another round of
heavy snows spreading south along the Sierra and into the
Transverse and Peninsular ranges.  For the southern Sierra and the
Transverse and northern Peninsular ranges, accumulations of 1-2
feet, with locally heavier amounts are expected.  In addition to
the heavy amounts across California, guidance shows low-to-mid
level frontogenesis enhancing the threat for locally heavy amounts
farther east into the southern Nevada and southwestern Utah
mountains.

This trough is expected to swing east across Southern California,
raising the threat for additional heavy snows across the Southwest
and the central to southern Rockies on Thursday into early Friday.
 Meanwhile, yet another shortwave is expected to begin impacting
the western U.S. on Thursday.  This system is forecast to take a
more inland track across the Northwest, bringing a threat for
heavy snows back into the Washington and Oregon Cascades, as well
as the Blue Mountains beginning Thursday and continuing into
Friday.


~~~Key Messages for Western U.S. Heavy Snow and Cold Through The
Rest of This Week~~~
Mountain snow...
Heavy mountain snow continues across the Southwest through the
Southern Rockies through tonight. Periodic moderate snows can be
expected down the Cascades and Sierra Nevada for the rest of this
week.

Lowland snow...
Sea level snow levels persist across the Pacific Northwest through
Thursday, so any precipitation will be snow which could be
moderate at times tonight in western Oregon and on Thursday in
western Washington.

Widespread impacts to travel and infrastructure...
Travel will remain discouraged especially along mountain passes
where long duration closures may be slow to end given the
continued cold and snow. Avalanche warnings remain in effect for
the Sierra Nevada, Northern through Central Rockies, and Wasatch.

Dangerous temperatures...
Particularly cold air will continue across the Northwest through
midweek. Low temperatures below 0F remain common from the Cascades
eastward through the Northern Plains through Thursday.

Pereira