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Winter Weather Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0839Z Dec 10, 2018)
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 AM EST Mon Dec 10 2018

Valid 12Z Mon Dec 10 2018 - 12Z Thu Dec 13 2018

...Southern Appalachians to the Southeast Coastal Plain...
Day 1...

Surface low pressure will move away from the Outer Banks of NC
beneath an initial shortwave, while two additional vorticity lobes
drop into the longwave trough from the northwest. The third and
final shortwave, which is the most potent, will swing across the
Gulf Coast and off the coast of GA, spawning weak surface
cyclogenesis tonight. While this surface feature will contribute
little to any wintry precip, a strengthening upper jet will
provide ascent within the left exit region, which will combine
with increasing 1000-500mb RH to produce the potential for light
to moderate snow showers through this afternoon. Weak
frontogenesis and modest deformation in the vicinity of the 700mb
trough axis will provide mesoscale support for lift, which may be
strong enough within the increasingly saturated DGZ to produce a
swath of an inch or two of snow, despite marginal low-level
temperatures. WPC probabilities are low, less than 20 percent, for
4 inches across the area, but 1 inch, with isolated amounts up to
2 inches, are possible. This is most likely in the terrain from NE
GA into SW NC, as well as in central NC where ascent within the
deformation axis is subtly more robust.

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

A series of shortwave with associated Pacific Jet energy and
enhanced moisture will advect into Washington/Oregon through the
forecast period. The first, and weakest, will move onshore this
morning accompanied by a surface cold front. This feature is
progressive and moisture is quick to spill across the Cascades and
into the ranges of ID/MT, so snow is likely in the mountains from
the Olympics of Washington through the Tetons of Wyoming. The
highest snow totals are likely in the Cascades where higher
moisture, better jet diffluence, and stronger upslope flow due to
700mb winds nearly orthogonal to the mountains, combine to produce
heavy snow, and WPC probabilities are moderate to high for 4
inches of snow, with lesser probabilities downstream to the south
and east from there.

A stronger and shortwave with stronger jet dynamics and more
significant Pacific moisture will stream into WA/OR beginning
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 as noted by
PWAT anomalies approaching +2 standard deviations above the mean,
and jet level diffluence will likely produce more than 2 feet of
snow in the Washington Cascades, with 8-12 inches likely in the
higher terrain of the Oregon Cascades and Bitterroots of Idaho.
Although the best moisture will begin to shift east by day 3,
continued snowfall in the higher terrain will make event totals of
more than 4 feet likely in the Cascades of Washington, with 1-2
feet probable across the high terrain of ID/OR. 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.
Low 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.