Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EST Wed Dec 05 2018

Valid 00Z Thu Dec 06 2018 - 00Z Sun Dec 09 2018


...Southwest and Southern Rockies to the Ozark Region...

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 through Thursday 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 to Moderate Risk for
amounts of 8-inches or more is indicated on Day 2 (ending 00Z
Saturday). 

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 icy onset from eastern New Mexico across the Texas
Panhandle into southwest Oklahoma.  Significant ice accumulations
are possible, with WPC Day probabilities indicating at least a
Slight Risk for accumulations of 0.25 inch or more from the New
Mexico-Texas Panhandle border to southwestern Oklahoma. 

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 Day 3 probabilities (ending 00Z Sunday) show a broad
Slight Risk area for snow accumulations of 8-inches or more from
the Texas Panhandle region eastward to the southern Ozark region. 
Heaviest amounts through late Sunday are expected to center across
portions of the Texas Panhandle region into southwest Oklahoma,
where favorable upper jet forcing along with low-mid level
frontogenesis may support banded precipitation, resulting in
heavier accumulations late Saturday into early Sunday.  Meanwhile
along the southern edge of the heavier snow band, an icy
transition zone will exist, with models showing the potential for
significant ice accumulations from the southern Texas Panhandle
region into central Oklahoma, as well as across portions of the
northwestern and central Arkansas.

Overall, the 12Z models showed good agreement with the larger
scale features into late Friday.  However, from late Friday
onward, model spread increased, including a number of the more
typical biases.  Given the model trends, WPC preferred a slower
solution with heavier snow accumulations than the operational GFS
(which is along the leading edge of the deterministic guidance). 
While the slower timing of the NAM was more preferred, it is also
one of the coldest and most amplified solutions and therefore
suspect its heavier snow amounts are overdone on Day 3.

Pereira