Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Fri Dec 21 2018

Valid 12Z Fri Dec 21 2018 - 12Z Mon Dec 24 2018


...Southern Appalachians to the Lower Great Lakes...

A full-latitude trough centered from the Great Lakes to the Gulf
of Mexico this morning is forecast to lift east-northeast across
the eastern U.S. Friday into early Saturday.  Surface low pressure
centered over the Southeast is expected to track to the north,
east of the southern to central Appalachians on Friday, followed
by a secondary push of colder air that will support a transition
from rain to snow as energy on the backside of the upper trough,
along with strong northwesterly flow interacts with residual
moisture behind the departing system.  Forecast certainty remains
limited due to model spread with respect to the timing and depth
of the cold air, as well as available moisture; however, several
guidance members continue to show significant snow accumulations
along the spine of the central and southern Appalachians late
Friday into early Saturday.  WPC Day 1 probabilities indicate a
Moderate Risk or greater for accumulations of 4 inches or more for
portions of the region. 

As the low continues to track to the north, rain changing to snow
is expected across the upper Ohio valley into the interior
Northeast Friday night.  Minor snow accumulations are expected for
most areas, however as the low continues to track further to the
north into eastern Canada, a period of west-northwesterly flow may
support additional snow showers, raising the potential for locally
heavier amounts in the higher terrain in the lee of lakes Erie and
Ontario.

....Pacific Northwest to the Northern and Central Rockies...

Overnight models showed little potential for heavy snow
accumulations through the Day 1 period, with mainly light amounts
associated with an amplified but progressive shortwave trough
moving across the northern Rockies on Friday, followed by a more
sheared shortwave diving east-southeast from the Northwest into
the central Rockies Friday night.

Probabilities for heavy snow increase once again across the
Olympics and along the Cascades on Day 2 as another well-defined
shortwave moves into the region Saturday night.  WPC probabilities
show the potential for localized accumulations of a foot or more
across the region.  As the heavy snow threat extends into the
northern Rockies, persistent onshore flow with embedded mid-level
energy will continue the threat along the Olympics and Cascades
into Day 3.

The probability of significant icing (0.25 inch or more) is less
than 10 percent.

Pereira