Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 AM EST Sat Nov 28 2020

Valid 12Z Sat Nov 28 2020 - 12Z Tue Dec 01 2020

Days 1 to 3...

...NM/TX Day 1...
An upper low bringing light snowfall to portions of NM is forecast
to cross the TX Panhandle early today before departing tot he
east.  Moisture wrapping around the low is causing rain showers to
develop over the TX panhandle. Cooling aloft allows for a mixture
and then a possible brief period of light snow before tapering
this afternoon as the low departs.
Most models have about one to two tenths liquid equivalent, with
the 00z NAM showing higher amounts of both liquid equivalent and
resultant snow in the TX panhandle. A swath of an inch or so seems
plausible in the TX panhandle, with a lower chance of a narrow
axis of 2-3" if rates get high enough underneath the upper low.

...Pacific Northwest/Northern Rockies...
Snow is expected to develop late Sunday night in the WA Olympic
Mountains and continue Monday across the higher elevations of the
WA Cascades, then extending south into the OR Cascades, and inland
into the northern Rockies. This system on Monday is a progressive
but pretty dynamic system...with a strong shortwave bringing the
potential for a quick shot of moderate to heavy snow to the higher
elevations (mainly above 4000 feet). The short duration with most
progs showing a 6 hr period of peak snow fall in the Cascades
(with drying aloft afterward) should provide a ceiling to
potential snow amounts. The latest snowfall probabilities depict a
40-60% chance of 6"+ of snow Monday.

...Ohio Valley...
An area of low pressure will track from the Gulf Coast northeast
into WV by 12z Monday and NY State by 00z Tue, followed by Quebec
Tue morning. We should begin to see some phasing of the southern
and northern streams on Monday resulting in a deepening of the low
pressure system. This should support a deformation band on the
northwest side of the low, and an expanding area of lighter
snows/snow showers further west underneath the developing
mid/upper level low. Thus we should end up with accumulating
snowfall across portions of the OH Valley by late Sunday night
into the day Monday.

Models are beginning to cluster on the low track, but QPF
differences remain from model to model, so the snowfall
accumulation remains uncertain. Boundary layer temperatures start
off warm, with cooling as the low strengthens and advects cooler
air into the region, changing rain to snow.  Ground temperatures
remain warm, so it will likely take decent snowfall intensity to
get accumulations on road surfaces, with accumulations on grass
surface initially.

The highest probabilities for accumulating snowfall through 12z
Tuesday will be from eastern KY into eastern IN, western OH, and
eastern lower MI.  An area of 4-6 inches of snow appears likely in
northwest Ohio, and near Lake Huron in southeast MI.
Another area of 4-6 inches of snow is possible with lake effect
bands of snow streaming off Lake Michigan into northwest IN.  Long
cross-lake MI trajectories on north winds and steep lapse rates
combine with lee shore convergence and ascent sfc-850 mb to
produce bands of snow showers Monday in northwest IN. The NAM and
Canadian global/regional GEM show potential for several inches of
snow, and are given more weight than the lighter global models.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen