Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 PM EST Sun Dec 09 2018

Valid 00Z Mon Dec 10 2018 - 00Z Thu Dec 13 2018


...Southern Appalachians and Southern Mid-Atlantic...
Day 1...

Surface low pressure drifts east from Cape Hatteras tonight as a
SW jet increases offshore tonight through Monday. The trowal will
be suppressed south from VA into NC by the low level ridge with
precipitation rates dropping this evening through tonight. A
wintry mix can be expected in this activity with low WPC
probabilities for 2 inches from central VA to central NC with high
probabilities in the mountains of western NC.

As additional shortwaves drop southeast into the longwave trough
on day 2 and day 3, the potential exists for further light wintry
precipitation. Currently, moisture appears to be too sparse for
significant accumulation, but this will need to be monitored as
the light precipitation and marginal temperatures may result in
additional snow and freezing rain/drizzle over the southeast into
Tuesday.


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

Two distinct yet potent shortwave troughs will advect into the
Pacific Northwest; one tonight and the second late Tuesday. These
will bring Pacific moisture and mountain snows to the region. The
first will reach the northern Rockies Monday night, but lose the
connection to Pacific moisture over the Intermountain West with no
heavy snow expected east of the Cascades. WPC probabilities are
moderate to high for 8 inches, in the Washington Cascades for Day
1, but low to moderate for 4 inches over ID.

A stronger and shortwave with stronger jet dynamics and more
significant Pacific moisture will stream into WA/OR Monday night
through Tuesday. High 1000-500mb RH will spread across WA/OR and
into the northern Rockies producing snowfall across elevations as
low as 2000 feet. 850-700mb flow will become aligned perpendicular
to the Cascades, which combined with intense moist advection and
jet level diffluence will produce up to 2 feet of snow in the
Washington Cascades, with 12 inches likely in the higher terrain
of the Oregon Cascades. Further east, heavy snow is also likely in
the Northern Rockies and Bitterroots where WPC probabilities are
high for 8 inches. With snow levels down to 2-3 kft, the valleys
also have the potential to see a few inches of accumulation as
well.

Persistent subfreezing air pooled east of the Cascades in WA/OR
will continue to result in pockets of freezing rain through Day 2.
Moderate probabilities for a tenth inch of ice are present along
the Columbia River east of the Gap for Monday and in the lee of
the Cascades in WA Tuesday.


Jackson