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

Valid 12Z Sat Dec 08 2018 - 12Z Tue Dec 11 2018


...Ozarks through the Southern Appalachians...
Days 1-3...

A complex series of shortwaves and associated surface features
will produce a significant winter storm, with snow and ice
spreading eastward from Arkansas to Georgia, northeast into
Virginia.

Shortwave digging through Texas this morning 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 shortwave will spawn cyclogenesis along the Gulf Coast
this afternoon, with secondary low pressure development likely off
the GA coast and then lifting northeast on Sunday. As this occurs,
surface high pressure centered over the OH Valley will begin to
wedge 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.

As isentropic lift increases this morning, precipitation will
overspread the region from the south and a period of light mixed
precipitation will occur in the terrain. This precipitation
falling into the wedge will diabatically enhance the high
pressure, causing temperatures to cool. This cooling of the column
combined with intensifying frontogenesis lifting northward will
allow precipitation to become 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 has been 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. Here, WPC probabilities for 0.25 inches of accretion
have increased to 50% for extreme NE GA/SW NC, as well as the
northern Piedmont of NC. Further 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
Mtns and into extreme southern VA, with WPC probabilities high for
4 inches outside of the higher terrain.

Further west into the Ozarks of Arkansas, warm and moist advection
will increase column moisture, and height falls associated with
the initial upper trough will produce ascent capable of producing
moderate snow in the terrain where temperatures will remain below
freezing. A few inches of snow is possible from NW AR into far NW
TN, where WPC probabilities show a slight risk for 4 inches of
snow.

On Day 3, the final shortwave with potent PVA will swing through
the region. Although snowfall accumulations of more than 1 inch
are unlikely Monday night, snow showers are possible across GA and
SC in response to steep lapse rates and increased RH beneath the
upper trough.


...Pacific Northwest...
Days 2-3...

A potent shortwave will move onshore Washington state, pushing a
cold front inland across the coastal ranges and towards the
northern Rockies. This system is progressive but Pacific moisture
does accompany the feature as it tracks inland due to warm
advection on W/SW flow ahead of the trough axis. Height falls and
jet level diffluence will produce lift across the west, while
upslope enhancement on westerly 700mb flow will produce heavy snow
in the Cascades and Olympics, with lesser snows likely towards the
northern Rockies as moisture spills eastward. WPC probabilities
are highest for 4 inches or more in the Cascades of Washington,
where there exists a multi-model signal for 6 inches or more above
7000 feet. Probabilities for 4 inches fall to show a slight risk
elsewhere into the Oregon Cascades and into the ID/MT ranges by
day 3 as the upper trough and surface front swing eastward.

Weiss