Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Tue Dec 18 2018

Valid 00Z Wed Dec 19 2018 - 00Z Sat Dec 22 2018

...Northwest...
Days 1-3...

Two distinct cold fronts will bring heavy precipitation to the
mountain west through the remainder of the week. The first,
accompanied by a shortwave and 140kt Pacific Jet, will continue to
push onshore tonight spreading moisture as far inland as Colorado.
The strongest lift and heaviest snow is likely in the Washington
Cascades, but significant snow is also likely from the Oregon
Cascades through the Bitterroots and northern Rockies. In these
ranges, WPC probabilities are high for 8 inches of snow, with more
than 12 inches possible in Washington. Snow levels will be as low
as 2000-3000 feet inland from the Cascades, so even elevated
valleys in MT and ID could receive a few inches of snowfall
accumulation.

Expansive mid-level ridge then blossoms across the western CONUS
late on Day 1 into Day 2, as the next major low lifts into British
Columbia during Thursday. The cold front associated with this low
will reach the NW coast Thursday night, followed by another strong
Pacific jet maximum. Snow levels ahead of this feature will rise
considerably such that on day 2 only moderate snowfall is
expected, and confined to the highest terrain of the Washington
Cascades and the northern Rockies. By day 3, cold air flooding
into the region will again lower snow levels to near 2000 feet,
which when combined with continued moist advection from the
Pacific will lead to widespread snow from northern California
through Wyoming and points north. Heavy snow is likely as ascent
is driven by jet level diffluence, and WPC probabilities are
moderate for 8 inches of snow across most of the terrain above
6000 feet, with lesser amounts likely down to 3000 feet.


...Southern Appalachians...
Day 3...

Anomalously deep upper trough with 500mb heights below -5 standard
deviations will dive towards the Gulf Coast Thursday spawning
surface cyclogenesis over the Southeast. Deep southerly flow ahead
of this feature will drive warm moist advection into the region,
with precipitation likely becoming heavy late on Thursday.
Initially, all precipitation will be rain, but as the surface low
lifts northeast towards the Mid-Atlantic Friday, cold advection
will commence in its wake. The column is forecast to cool enough
that heavy precipitation rates due to modest frontogenesis and
mid-level deformation on the west side of the mid-level low will
allow rain to change to snow in the terrain from SW NC into
eastern WV. Snow levels will drop to less than 2000 feet, and a
few inches of snow are possible above that level. WPC
probabilities show a slight risk for 4 inches of snow, with the
highest chances in NW NC and the highest terrain of WV where
upslope W/NW winds will enhance snowfall.


The probability for significant icing (0.25 inches or more) is
less than 10 percent.


Weiss