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

Valid 00Z Sat Dec 22 2018 - 00Z Tue Dec 25 2018

...Southern Appalachians to the Lower Great Lakes...
Days 1-3...

Full latitude trough aligned near the U.S. East Coast will lift
northeast through Saturday as a surface low moves into southeast
Canada. A surge of cold air will follow behind this system as
strong CAA develops on northwest winds. Guidance is in good
agreement that as the column cool, precipitation will transition
to snow tonight and then persist along the spine of the
Appalachians as orographic ascent taps still high 1000-500mb RH to
produce periods of moderate snow through Day 1. Forecast snow
accumulations have increased due to higher model QPF and
increasing SLRs, and WPC probabilities show a high risk for 4
inches of snow, with a slight risk for 8 inches in the terrain of
eastern WV. Drier air will eventually advect into the region from
the W/NW, bringing an end to the heavy snow on Day 2.

As the surface low continues to track to the north, rain changing
to snow is expected across the upper Ohio valley into the interior
Northeast tonight.  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 where WPC probabilities are moderate for 4 or more inches
of accumulation.

A second impulse will drop into the Ohio Valley late on Day 3,
potentially bringing some additional light snow to the
Appalachians and Great Lakes, but current WPC probabilities are
less than 30 percent for 4 inches of snowfall.


...Pacific Northwest through the Central Rockies...
Days 1-3...

A shortwave will dig through the Central Rockies and into the
Central Plains Saturday into Sunday, producing moderate snow from
the Mountains of Utah east towards the KS/CO line. This system is
progressive and moisture is limited, so snowfall amounts are
forecast to be only a few inches, with WPC probabilities for 4
inches only above 40 percent in the highest terrain of Colorado.

A much more robust system will move into the Pacific Northwest
late on Saturday and then slowly push eastward with moisture
spilling into the rest of the Mountain West Sunday and into
Monday. Snow levels will rise on SW flow ahead of the upper low
and associated surface feature, but then crash back to 3000 feet
or less Monday. Significant moisture spilling from the Olympic
Range all the way into the Colorado Rockies on Day 3, will be
tapped by robust ascent due to jet level diffluence and increasing
westerly 700mb flow. This will produce heavy snow in the
mountains, with the heaviest accumulations likely in the Cascades
of Washington and Oregon where WPC probabilities are high for 8
inches of snow. Across the remainder of the Mountain West ranges,
a high risk exists for 4 inches of snow above 5000 feet.


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


Weiss