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

Valid 12Z Sun Dec 09 2018 - 12Z Wed Dec 12 2018


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

Surface low pressure developing along the Gulf Coast beneath an
upper trough will transfer to a renewed low pressure off the GA
coast this morning. This low will then lift northeast away from
the NC coast as a series of shortwaves dig through the longwave
trough across the eastern CONUS. Heavy precipitation has already
begun across this area, and will continue to blossom and spread
northward through Sunday.

Precipitation initially is being driven by warm advection and
intense isentropic lift as a wedge of high pressure becomes
reinforced by diabatic cooling of precipitation down the eastern
edge of the Appalachians. Very dry air within the high pressure
will initially inhibit precipitation from spreading quickly
northward, but as the column continually saturates moisture will
spread as far north as near Washington D.C. As this occurs, a warm
nose will advect NNW on S/SE 850-700mb flow, becoming more from
the E into VA. This will changeover precipitation to freezing rain
across upstate SC, central NC, and into S-Central VA. In this
stripe, WPC probabilities are high for 0.25 inches of accretion
and a few locations may approach 0.4 inches.

Further north and west, heavy snow will persist and expand as
strongly sloped 925-700mb frontogenesis lifts northward and then
stalls across southern VA. This will drive intense ascent into an
E-W oriented TROWAL, further enhancing snowfall potential.
Additionally, guidance suggests a region of -EPV atop the best
frontogenesis, which combined with low-level theta-e lapse rates
less than 0 suggests the potential for thunder snow and snowfall
rates approaching 2"/hr. Despite SLR's likely below climatological
norms, QPF of 1-2" supports snowfall of 12 to 18 inches from the
northern NC mountains into much of SW and S-Central VA, with
isolated higher totals possible in persistent bands. There has
been a consistent shift northward in the snow forecasts tonight as
well, and WPC probabilities have shifted such that a slight risk
now exists for 4 inches as far north as the southern Panhandle of
WV eastward to extreme southern Maryland. There is likely to be a
very sharp north-south gradient in snowfall due to the persistent
frontogenesis to the south, and the dry high pressure to the north.

As additional shortwaves drop southeast into the longwave trough
on day 2 and day 3, the potential exists for further light
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, bringing increased Pacific Moisture and
mountain snows to the region. The first will lift onshore
Washington state along with a surface cold front this afternoon,
and then shift eastward into Monday. A wave of low pressure will
likely develop along this front and drop southeast into Utah
Tuesday, with moisture spilling over into the mountains of
ID/MT/UT on day 2. Weakening height falls and a reduction in RH
will cause snow amounts east of the Cascades to be much less than
expected in Washington. WPC probabilities are high for 4 inches,
and moderate for 8 inches, in the Washington Cascades, but feature
just a slight risk for 4 inches elsewhere.

A stronger and shortwave with stronger jet dynamics and more
significant Pacific moisture will stream into WA/OR on day 3. 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.

Additionally, subfreezing air pooled east of the Cascades in WA/OR
and should result in pockets of freezing rain near the Columbia
Gorge Sunday when low-level easterly winds drain cold air through
the gap underneath the mid level trough axis which should result
in some precipitation. Moderate probabilities for a tenth inch of
ice are present just east of Portland. Some additional light
freezing rain is possible in this same area again on day 3.


Weiss