Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
427 PM EST Thu Dec 12 2019

Valid 00Z Fri Dec 13 2019 - 00Z Mon Dec 16 2019


Day 1...

...Western U.S....
Mountain snows will continue, with additional heavy accumulations
possible for portions of the Cascades and northern and central
Rockies.  Mid-level energy embedded within progressive westerly
flow will support additional periods of organized heavier
precipitation and lower snow levels across the Northwest and
Rockies Thursday night and Friday.  Along the Washington and
Oregon Cascades and the Rockies from northern Idaho and western
Montana to northern Utah and Colorado -- WPC guidance shows high
probabilities for additional accumulations of 8-inches or more
across the higher terrain.

...Southern and Central Appalachians/Piedmont...
A cold air wedge associated with a 1040mb centered near Long
Island is forecast to persist through the overnight into early
Friday as precipitation associated with an upstream upper trough
spreads north across the region.  This will support a wintry mix
and a potentially icy morning commute for portions of the southern
and central Appalachians and adjacent Piedmont.  Model handling of
the low level cold air is always a source of uncertainty, however
recent guidance, including the general consensus of the
high-resolution models, showed greater potential for a brief
period of snow and ice pellets than previously shown.  Measurable
freezing rain still remains likely from western North Carolina to
central Pennsylvania Thursday night into Friday.  As the high
continues to move offshore, expect most areas to transition to all
rain Friday afternoon.

Days 1-2...

...Northern Plains to the Upper Great Lakes...
A weak shortwave trough is expected to produce generally light
amounts across North Dakota and Minnesota as it moves across the
region Thursday night and Friday.  Some locally heavy lake-effect
snow showers may develop by late Friday and continue into the
overnight along the western shores of Lake Superior.  As a weak
wave tracks to the south, easterly flow developing over Lake
Superior is expected to support lake effect snow showers, with
heavy amounts possible along the North Shore in northeast
Minnesota.

Day 2...

...Western U.S....
Mid-level energy will continue to amplify the flow in the west,
with snow levels continuing to drop behind a baroclinic zone
settling south into the Sierra, Great Basin and central Rockies. 
Low-to-mid level frontogenesis will help support a swath of
locally heavy mountain snows extending from the Sierra to the
Colorado Rockies.

Day 3...

...Western U.S. into the Central Plains and Mid Mississippi
Valley...
Models show a well-defined mid-level shortwave dropping southeast
into California Saturday night.  This system is expected to move
east across California into the Great Basin and Southwest on
Sunday.  Moderate to locally heavy snows are expected across
southern Utah and northern Arizona to the central Rockies, with
heavy accumulations likely for portions of the San Juan and Sangre
de Cristo ranges late Saturday and Sunday.  By late Sunday, models
show a swath of generally light snows extending east across the
central Plains into the mid Mississippi valley.

Pereira