Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
437 AM EST Mon Feb 18 2019

Valid 12Z Mon Feb 18 2019 - 12Z Thu Feb 21 2019


...Southwest to the Southern Rockies...

Shortwave energy diving south along the West Coast will continue
to amplify an upper trough centered over the southwestern U.S.
Monday into early Tuesday.  Favorable dynamics aloft along with
low to mid level frontogenesis and upslope flow are expected to
support moderate to heavy snows along the Mogollon Rim into the
White Mountains of eastern Arizona, as well as the southern
Colorado and northern New Mexico ranges.  Despite limited
moisture, the models show a period of enhanced divergence aloft
helping to support heavy snowfall accumulations across portions of
southwestern Colorado and northern New Mexico.  WPC Day 1
probabilities (ending 12Z Tuesday) indicate a Slight Risk or
greater for accumulations of a foot or more for areas that include
portions of the San Juan and Sangre de Cristo mountains.

...Central Plains to the Northern Great Lakes...

The previously noted trough over the Southwest on Day 1 is
forecast to shift northeastward Days 2 and 3 as energy in the base
of the trough lifts out of the Southwest into the central Plains
by early Wednesday, before phasing with a more northerly wave over
the Upper Midwest Wednesday night into Thursday.  The associated
mid to upper level forcing is expected to support a broad area of
light to moderate snows spreading north and east from the central
High Plains to the lower Missouri and mid Mississippi valley late
Tuesday into early Wednesday.  Heaviest snow accumulations through
Day 2 (ending 12Z Wednesday) are expected to center near the lower
Missouri and mid Mississippi valleys, with WPC probabilities
indicating a High Risk (70 percent or greater) for accumulations
of 4-inches or more centered over southwest Iowa into northern
Missouri.  With the system continuing to move progressively to the
north, snows will shift into the upper Mississippi valley and
northern Great Lakes region on Wednesday, before tapering off
early Thursday as the low lifts into eastern Canada.  Highest
probabilities for significant snows in the Day 3 period (ending
12Z Thursday) center from the Minnesota-Wisconsin border to the
U.P. of Michigan, with a Moderate Risk or greater for 4-inches
indicated by the WPC probabilities.

...Southern to Central Appalachians and Mid Atlantic...

Models continue to show a strong CAD signature developing over the
East by late Tuesday as confluent flow aloft and a strong surface
high move east from the upper Midwest into the Northeast.  This
will set the stage for a significant icing event for portions of
the southern and central Appalachians and the adjacent Piedmont as
strong low level theta-e advection and favorable mid to upper
level dynamics ahead of the trough to the west support a
blossoming area of precipitation falling into the shallow cold
air.  Overnight guidance has trended a little colder, indicating
more widespread ice accumulations with some heavier amounts along
the southern to central Appalachians into the Piedmont.  Heaviest
ice accumulations through Day 2 are expected to fall across
western North Carolina, with WPC probabilities maintaining a
Moderate Risk for amounts of 0.25 inch or more.  The threat for
significant icing is forecast to shift farther north on Wednesday,
with WPC probabilities showing a Moderate Risk covering large
portions of eastern West Virginia, western Virginia and western
Maryland on Day 3.  Before changing over to mixed precipitation,
overnight models showed a period of moderate to heavy snow
supported by strong upper divergence along with low to mid level
frontogenesis centered over the central Mid Atlantic region
Wednesday morning.  WPC Day 3 probabilities indicate a Moderate
Risk for accumulations of 4-inches or more extending across
south-central Pennsylvania and from the eastern West Virginia
Panhandle and western Maryland to the Baltimore-Washington metro
region.  By Wednesday night, with the high sliding offshore and
warm air surging north, the overnight models offered little
indication for widespread heavy snow and ice accumulations farther
to the north, with only a Slight Risk for accumulations of
4-inches or more across portions of Upstate New York and northern
New England.

...Pacific Northwest to the Northern Rockies and Great Basin...

Models show an upper trough returning to the West as shortwave
ridging gives way to a shortwave diving south into the Pacific
Northwest Wednesday morning before continuing farther to the south
along the West Coast, with a closed low developing over Northern
California and the Great Basin early Thursday.  Mountain snows
will return to the region, initially to the Pacific Northwest and
northern Rockies on Day 2, with more expansive coverage extending
farther south and east into the Great Basin on Day 3.  For the two
day period, WPC probabilities for accumulations of a foot or more
are highest over the Oregon Cascades and the southeast Washington
and northeast Oregon ranges. 

Pereira