Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Fri Dec 07 2018

Valid 12Z Fri Dec 07 2018 - 12Z Mon Dec 10 2018


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

Guidance this morning has trended downward in its winter weather
potential across the Southern Plains and into the Ozarks due to a
more southern evolution of a surface low pressure as well as
stronger/drier high pressure to the north. This continues the
trend noted from this afternoon, but WPC probabilities are still
moderate for 4 inches of snow in the Panhandle of Texas, as well
as the high terrain of the Ozarks in Arkansas. This snowfall will
be in response to a shortwave and associated height falls moving
eastward from New Mexico into Arkansas late today through Saturday
night. Moisture will increase on southerly flow ahead of this
impulse, which will transport moisture into the Ozarks and the
Texas Panhandle, but total forcing is modest. Across Texas, weak
deformation NW of the mid-level circulation will produce a band of
snow which will accumulate Saturday morning before rapid dry
advection brings an end to the precipitation. In the Ozarks,
modest frontogenesis lifting northward will produce snow in the
high terrain, but a sharp gradient is likely near the MO border
due to dry air to the north.

Additionally, light to moderate freezing rain accretion is
possible across portions of Oklahoma, eastward through northern AR
and into TN. As WAA persists, 850mb temps climb above 0C while
surface temps remain sub-freezing in response to strong surface
high pressure to the north. Although significant (0.25) inches of
accretion is not expected, WPC probabilities show a moderate risk
for 0.1 inches across northern AR, with lesser probabilities
elsewhere.


...Southern Appalachians...
Days 2-3...

Confidence increasing in a significant winter weather event, but
details still remain murky as far as p-type and accumulations. A
shortwave moving across the southern tier will eject northeast off
the Carolina coasts Sunday while a second impulse digs into the
mean trough behind it. At the surface, low pressure moving along
the Gulf Coast will re-organize off the GA coast and then lift
northeast into early Monday. North of these features, mid-level
confluence will assist in asserting dry surface high pressure, and
between these an area of heavy wintry precipitation is likely.

Guidance has trended southward this morning in response to a
stronger and drier high pressure to the north wedging down along
the eastern side of the Appalachians. SW flow aloft will transport
increasing moisture on WAA and intense isentropic upglide, causing
precipitation to overspread the region from south to north early
Saturday, and persisting through Monday morning. Initially,
precipitation may all be freezing rain/sleet, but intensifying
frontogenesis will cause dynamic cooling of the column changing
the precip over to snow in the terrain of SC/NC and points north
towards WV. A prolonged period of heavy snow is likely in this
area, and WPC probabilities show a high risk for 8 inches of snow,
with a moderate risk for 12 or more. South and east of the maximum
area of snow, a sharp gradient is likely where the 850-700mb warm
nose greater than 0C lifts northward. The mean flow is easterly,
which is not as robust as a more typical SE wind to erode the
wedge to permit warm nose advection into the area. Additionally,
diabatic cooling due to precipitation falling into the wedge will
likely reinforce or intensify that feature, causing an increase in
the pressure gradient, and a southward push of colder air due to
isallobaric acceleration. This suggests heavy snow is likely into
the upstate of SC, and eastward towards the Triangle of NC, as
well as approaching the Coastal Plain of SE VA late on Day 3.
Here, WPC probabilities are moderate for 4 inches. Well north,
into WV and KY, confidence is lower due to the southern trend in
guidance and the dry air which will be tough to overcome, so WPC
probabilities are lower for heavy snowfall.

There will also likely be a stripe of moderate freezing rain
accretion south and east of the heaviest snow. As the warm nose
lifts northward, surface temps will struggle to rise above
freezing, the result of which will be sleet/freezing rain. There
may also be a period of time on Sunday when the DGZ dries out,
changing the precip from snow to freezing rain for a time even in
the mountains. Although confidence is low in the exact placement
of the heaviest freezing rain, WPC probabilities do show a 30-50%
chance of 0.25 inches of accretion across portions of NE GA,
upstate SC, and western NC.


...Pacific Northwest...
Day 3...

A shortwave moving into the Pacific Northwest late on Day 3 will
push a cold front onshore Washington/Oregon. SW moist flow ahead
of this feature will increase column relative humidity, which will
be squeezed out as snow in the terrain as height falls and forced
upslope wind provide ascent. Snow levels will initially be low,
but will rise to around 4000 feet, with the highest accumulations
likely in the Cascades of Washington above 6000 feet. In this
area, WPC probabilities are high for 4 inches and moderate for 8
inches. Elsewhere along the Cascades of Oregon, the Olympics of
Washington, and the northern California ranges, a few inches of
snow are possible.


Weiss