Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 AM EST Sat Dec 22 2018

Valid 12Z Sat Dec 22 2018 - 12Z Tue Dec 25 2018


...Western U.S....

Probabilities for heavy snow are expected to increase once again
across the Olympics and along the  Washington and Oregon Cascades
as a deepening low approaches British Columbia with a strong
trailing front moving into the Pacific Northwest late Saturday. 
Snows levels are forecast to rise ahead of this system, with WPC
Day 1 probabilities (ending 12Z Sunday) indicating that the
greatest threat for heavy amounts will generally be for areas
above 4000 ft.

Snow levels are expected to decrease Sunday night into Monday as
mid-level energy moves onshore, flattening the flow across the
Northwest.  This energy will support additional periods of snow
from the Olympics and Cascades to the northern Rockies, with
locally heavy accumulations possible across the region during the
Day 2 period.

A more amplified pattern is expected to ensue across the West late
Monday into early Tuesday as a vigorous shortwave moving off of
the top of a strong ridge over the Pacific dives southeastward,
carving out a deep trough across California Tuesday morning.  This
will bring the focus for heavier snows further south along the
southern Cascades into the Sierra and into the higher terrain of
the Great Basin.

...Upper Midwest to the Northeast...

A shortwave trough moving into the northern Plains and upper
Midwest this morning is expected to move into the upper Great
Lakes on Sunday, producing light amounts across northern Minnesota
into northern Michigan.  Meanwhile energy emanating from the
Northwest will dig to its south and eject east ahead of it out
across the northern Mid Atlantic early Monday, with
west-northwesterly flow supporting lake-enhanced and upslope snow
showers from the central Appalachians and upper Ohio valley into
eastern Pennsylvania and New York.  Overall, amounts with these
system are expected to be mainly light, with WPC probabilities
showing little threat for widespread amounts exceeding 4-inches.

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

Pereira