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

Valid 12Z Thu Dec 06 2018 - 12Z Sun Dec 09 2018


...Southern Rockies to the Ozark Region...
Days 1-3...

Guidance continues to signal a developing winter storm, bringing
heavy snow to portions of the Southwest and southern Rockies late
Thursday into Friday, followed by heavy snow and significant icing
spreading east from the southern High Plains to the Ozark region
Friday and Saturday.  Models continue to show the
positively-tilted upper low/trough currently centered along the
California coast dropping south today before swinging inland
across Southern California and the Baja peninsula on Friday.  Deep
southwesterly flow ahead of the trough is expected to support
increasing snow chances along the Mogollon Rim northwestward into
the southern Utah, southwestern Colorado and northern New Mexico
ranges, with WPC probabilities highlighting the potential for
locally heavy amounts across these areas, including across the San
Juans where a Slight Risk for amounts of 8-inches or more is
indicated. 

Meanwhile, increasing low level warm air/moisture advection with
favorable dynamics aloft will support a blossoming area of
precipitation further east across the High Plains by late Friday. 
Confluent flow aloft with cold high pressure at the surface will
support subfreezing boundary layer temperatures - raising the
threat for an ice event from eastern New Mexico across the Texas
Panhandle into central Oklahoma.  Significant ice accumulations
are possible, and WPC probabilities for 0.25 inches or more have
increased to 40 percent /Moderate Risk/ from the Panhandle of
Texas through central Oklahoma, and into the Ozarks of Arkansas.

Late Friday into Saturday, the potential for heavy snow is
expected to increase across the region as the upper trough moves
across the Rockies into the High Plains and dynamic cooling within
a developing deformation zone promotes freezing rain changing to
sleet and snow along the northern extent of the precipitation
shield.  WPC probabilities show two distinct high risk areas for 4
inches of snow. The first is along the Texas Panhandle along and
just NW of the Caprock Escarpment where upslope enhancement and
slightly cooler temperatures will combine with favorable jet
dynamics and modest frontogenesis to produce a band of heavier
snowfall. The second maximum is likely along the Ozark Range of
Arkansas where, again, upslope enhancement on E/SE winds will
combine with a prolonged period of mid-level deformation to
produce heavy snow. WPC probabilities indicate a moderate risk for
8 inches of snow in these areas. However, there will likely be low
SLRs and potentially mixing at times, at least initially, with
freezing rain and sleet which may keep these snowfall totals down.
Still, outside of the cold 00Z NAM and the fast/north 00Z CMC, the
ensemble means of the GEFS and ECEns are in good agreement and
favored heavily for this forecast package.


...Great Lakes...
Days 1-2...

A broad trough across the Great Lakes combined with multiple cold
frontal passages will produce a favorable environment for heavy
lake effect snow. Cold advection on increasing westerly winds
behind today's cold front will produce lake effect snow on the
east/southeast side of the lakes. The heaviest snow is likely
downwind of Lake Erie, and especially Lake Ontario, where
sfc-700mb flow becomes unidirectional and instability atop the
Lakes reaches 600-900 J/kg with an EL as high as 12 kft. Steep
lapse rates and forcing into a low DGZ supports intense snow
rates, and the potential for thunder snow, especially off Lake
Ontario through the first half of day 1 before directional shear
subtly increases. As this occurs, the intense single bands of Lake
Effect will ease to a more widespread but less intense snow bands.
WPC probabilities are high for 4 inches east of Lake Erie and Lake
Ontario, with up to 12 inches possible in the Tug Hill Plateau.
Lesser amounts are likely south of Lake Superior and east of Lake
Michigan.


...Southern Appalachians...
Day 3...

Developing low pressure along the Gulf Coast will strengthen and
push eastward to be near the GA/SC coast at the end of day 3. This
low will develop in response to a southern stream shortwave moving
through the Arklatex and into the TN Valley, as well as diffluence
within the right entrance region to an upper jet streaking across
New England. Guidance is in reasonably good agreement in the
evolution and placement of this surface low, with the NAM a strong
and very cold outlier, while the CMC is too fast with the phasing
of northern stream energy, and is pulling the surface low too far
north too quickly. A blend of the GEFS and ECEns mean is
preferred, which suggests precipitation spreading across the
Southeast during Saturday. The guidance has overall slowed precip
onset due to dry air within a strong wedge of high pressure which
remains in place as a result of robust mid-level confluence. E/SE
low and mid-level winds spread northward atop the surface wedge,
isentropic lift will increase, and precipitation will gradually
saturate the column. This will result in wet-bulb cooling of the
thermal profile and snow/sleet/freezing rain is all likely to
develop Saturday night, with snow in the higher terrain and
freezing rain/sleet elsewhere. Despite model differences in timing
and p-type, there is a strong multi-model signal for more than 4
inches of snow in the Southern Appalachians before 12Z/Sunday and
this is where WPC probabilities are highest for accumulating snow.
Some uncertainty exists into how far north precipitation will
spread on Day 3 due to dry air inhibition, and there is likely to
be a sharp north gradient to snowfall. Additionally, a period of
freezing rain and sleet is likely in portions of the mountains and
foothills, and WPC probabilities have increased to 20 percent for
0.25 inches of accretion across portions of the area.

Weiss