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

Valid 12Z Wed Dec 05 2018 - 12Z Sat Dec 08 2018


...Southern Plains to lower MS Valley...

...Significant to Major freezing rain event possible across
portions of northwest TX and southwest OK...

...The models indicate the storm system crossing the Southern
Rockies early Friday moves into the Southern Plains Friday night
into Sat.  700 mb warm/moisture advection causes precipitation to
develop, with snow developing where deeper cold air exists, which
is more likely the further north you go in the TX panhandle and
eastern NM.  The continuing mid level warm advection changes snow
over to sleet and freezing rain further south in southeast NM and
west TX. The surface high over the central Plains to mid MS Valley
and then a sfc ridge in western OK to the TX panhandle reinforces
surface cold air.  This sets up the possibility of extended period
of sleet and freezing rain over parts of western Oklahoma and
western Texas on Friday/Friday night.

The WPC probabilities show a moderate risk of significant icing
(0.25 inches) over the southern TX panhandle to western and
central OK, with the NAM and SREF mean showing potential for half
an inch liquid equivalent.  A moderate to high risk of 4 inches of
snow was shown over eastern NM to the TX panhandle and adjacent
western OK. The risks lower the further east in OK and AR due to
both QPF gradually decreasing further downstream plus potential
for mixed precipitation types in central to northern OK and
northern AR.

For this set of forecasts, the faster GFS was given less weight
than the slower NAM/UKMET/ECMWF forecasts, given the long term
bias of the GFS to move closed lows too quickly.
 

...CA Sierra Nevada Mountains to the Southern Great Basin and
Southern Rockies...

The models continue to show an upper low dropping southeast off
the California coast Wednesday.  Moisture transport occurs along
with bands of 700 mb convergence move inland into the CA Sierra
Nevada. This supports several inches of snow through Thursday,
with the maxima shifting south in tandem with the offshore
circulation and bands of 700 mb convergence during the day on
Wednesday.
WPC probabilities show a Slight Risk for accumulations of 4-inches.

On Thu the 700 mb trough moves onshore into the southwest, with
moisture transport and bands of 700 mb convergence shifting out of
the southern CA Sierra Nevada range and the ranges of southern NV,
northern and AZ into the four corners region and southwest CO by
late Thursday and Friday.

Snow develops in the NM mountains Friday and a forming closed 700
mb low brings low level convergence and ascent across the
mountains of NM late Thu night through Fri. A few models forecast
locally heavy snow both in the NM Rockies and east on the high
Plains.


...Great Lakes Days 1-3...

Lake effect snows pick up in the lee of Lake Superior into the
Keweenaw Peninsula of the western UP of MI as a cold front
progresses across the region today, with Lake Effect snows ramping
up to the lee of Lake Ontario and Lake Erie Day 2 as the cold
front continues east across the lower lakes.  Several inches of
snow are possible centered on the Tug Hill east of Lake Ontario
and the GFS shows high layer relative humidity and a local maxima
of lift in favored upslope areas. The models show snow extending
further inland into the western Adirondacks, where several inches
of snow are possible Thu-Thu night.
On day 3 (Fri) the overall trend is towards lower amounts as a low
level ridge builds into the lower Lakes, with weakening low level
flow and drying aloft.


...Southern NJ Day 1...
A period of snow is expected on Wed during the day as a 700 mb
wave crosses southern NJ with low level convergence along an
inverted trough. Some of the high res guidance and Canadian
regional GEM shows a quarter to half inch liquid equivalent in the
form of snow with global guidance and 03z SREF Mean a little less.
 With the 700 mb wave moving off the coast this evening, snow
coverage and intensity should wane. 

The probability of significant icing (0.25 inches or more) is less
than 10 percent days 1-2.

Petersen