Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 PM EST Sat Dec 08 2018

Valid 00Z Sun Dec 09 2018 - 00Z Wed Dec 12 2018


...Southern Appalachians and Southern Mid-Atlantic...
Days 1-2...

A developing southern stream low will phase with northern stream
energy Sunday and allow the surface low to shift northeast up the
Carolina Coast Sunday into Monday. Further energy into the trough
will keep precipitation lingering over the southeastern US through
Tuesday though the bulk of wintry precipitation is tonight into
Sunday night.

Shortwave digging through Texas will eject northeast while
weakening through Sunday. A secondary impulse will follow quickly
on its heels as the mean trough persists across the eastern CONUS,
with a third vorticity maxima digging across the Southeast late on
day 3, enhancing the longwave trough. The initial surface low is
over LA this afternoon and will shift into FL tonight. The
secondary low pressure development is expected off the GA coast
late tonight, lifting northeast into Monday. As this occurs,
1036mb surface high pressure centered over the OH Valley wedges
down the coast east of the Appalachians as confluence persists
aloft. This cold high will prevent significant warm advection from
eroding the sub-freezing surface temperatures and a
snow/sleet/freezing rain event is likely from NE GA northward into
WV and east towards the coast.

Precipitation has overspread the southeastern US on isentropic
lift with some pockets of wintry precip in the southern
Appalachians. This precipitation falling into the wedge will
diabatically enhance the high pressure, cooling the column. This
combined with intensifying frontogenesis lifting northward will
allow precipitation north of the current precip shield to be all
snow initially, with a few inches of snow expected. However, as
warm advection occurs north of the surface low, 850mb temperatures
climb to above 0C well inland, and the model trend continues to be
for a warmer solution. As surface temperatures remain cold, a
changeover to freezing rain and sleet is likely, with significant
accretions forecast in from NE GA through the upstate of SC,
central NC, and into extreme southern VA. WPC probabilities for
0.25 inches of accretion remain moderate for extreme NE GA/SW NC,
as well as the northern Piedmont of NC into southern VA.

Farther west and north, persistent frontogenesis as well as
increasing mid-level deformation will support periods of banded
heavy snowfall. With the slightly warmer model solutions, SLRs are
likely to be reduced and a subtle decrease in snowfall
accumulations has been noted in the guidance. There still remains
a good signal for 12 inches of snow in the NC mountains and into
extreme southern VA, with WPC probabilities high for 6 inches
outside of the higher terrain.

The 12Z QPF guidance preference was for the NAM/ECMWF which has
trended north a bit and could result in heavy snow farther north
than currently forecast.

The persistent trough 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 2-3...

A potent shortwave trough axis will move into the Pac NW Sunday
night with a cold front moving onshore in WA/OR/far northern CA
late Sunday, reaching the northern Rockies Monday. This system is
progressive with precipitation rates quickly decreasing east of
the Cascades. Heavy snow is expected in the Cascades and Olympics
with NBM snow elevations around 4000ft Sunday, lowering to 2000ft
Sunday night behind the front. WPC probabilities for greater than
six inches are limited to above about 6000ft for Days 1 and 2.

Subfreezing air is pooled east of the Cascades in WA/OR and should
result in pockets of freezing rain near the Cascades Sunday, then
along the Columbia Gorge Monday when cold air draining through the
gap is underneath the mid level trough axis which should result in
some precipitation/freezing rain. Moderate probabilities for a
tenth inch of ice are present just east of Portland.

A second shortwave trough approaches from the Gulf of Alaska on
Tuesday with a plume of high moisture content reaching western WA
Tuesday morning. NBM snow elevations lift to 4000 to 5000ft in the
warm air with heavy snow limited to the higher WA Cascades and
Olympics where moderate probabilities for 12 inches are present.

Jackson