Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EST Thu Dec 06 2018

Valid 00Z Fri Dec 07 2018 - 00Z Mon Dec 10 2018


...Southern Rockies to the Ozark Region...
Days 1-2...

There remains some model signal for a significant winter weather
event initially over the higher terrain of northern and eastern
Arizona into the southern Rockies on Day 1 (ending 00z Sat),
before translating east into the southern Plains on Day 2 (ending
00z Sun).  However, as the associated upper trough moves across
the Rockies, the general trend of the 12z models was more
suppressed with lesser snow and ice amounts from eastern New
Mexico across the Texas Panhandle and Oklahoma into the Ozark
region.  There still remains a fairly good signal for moderate to
heavy snow amounts across the southern Texas Panhandle region into
southwestern Oklahoma, with a WPC probabilities showing a Slight
Risk for amounts of 8-inches or more, but as a whole the 12z
models trended downward, especially for areas further to the
north.  A wintry mix resulting in some light to moderate icing
amounts is still expected along the southern periphery of the
heavier snow band.  While probabilities for significant icing
increased across northwestern Arkansas, the models showed lesser
potential for significant ice accumulations elsewhere.


...Tennessee and Lower Ohio Valley to the Southern Appalachians...
Day 3...

While plenty of differences in the details remain, the 12z models
supported increasing confidence in a significant winter storm for
the southern Appalachians into the Piedmont beginning late Sat and
continuing into Sun.  As the associated shortwave trough moves
east into the lower Mississippi valley, models show a wintry mix
with snow and ice accumulations continuing east from the Ozark
region eastward into the Tennessee and lower Ohio valley Saturday.
 Meanwhile, confluent flow downstream across the Mid Atlantic
region will maintain high pressure and a cold air wedge east of
the mountains.  Deep southwesterly flow ahead of the trough,
supporting strong moisture advection into an area of strong upward
ascent will encourage heavy precipitation, with thermal profiles
supporting snow across the southern Appalachians into the western
Piedmont, with freezing rain further south and east.  WPC Day 3
(ending 00z Mon) probabilities show Slight to Moderate risks for
snow amounts of a foot or more across a good portion of
southwestern Virginia, western North Carolina into Upstate South
Carolina and far eastern Kentucky and Tennessee, with some
overlapping areas of significant ice probabilities extending
further east across the Carolinas and south into northern Georgia.


While confidence in the finer details is limited, WPC preferred a
solution more in line with the GFS and ECMWF, with less weight
given to the NAM which was along the southern edge of the guidance
with the low track and northern extent of the precipitation shield
late in the period.

Pereira