Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 AM EST Sun Dec 02 2018

Valid 12Z Sun Dec 02 2018 - 12Z Wed Dec 05 2018

...Western Great Lakes and adjacent Upper MS Valley...
Day 1...

The cyclone in the upper MS Valley this morning is forecast to
lift northeastward into the Great Lakes today. Mid-level
deformation will persist northwest of the 700mb low, combining
with moisture advection to produce snow in northern WI across the
U.P. of MI. In the UP, lake effect snow off Lake Superior supports
locally higher amounts, so several inches is expected near the
lake in the eastern UP of MI, where upper divergence maxima are
forecast early today. WPC probabilities for 4 inches are highest
in the U.P. of Michigan in the areas where Lake enhancement and
lake effect snows occur by Lake Superior.

...Western U.S...

...Day 1 Southwest US...

On Day 1 (Sun through Sun night), a potent mid-level trough and
associated upper Pacific jet maximum will move southeast towards
the Four Corners region.  This feature will advect enhanced
moistured noted by an increase in layer sfc-500mb RH, with height
falls and jet level diffluence combining with convergence
downstream fro the 700 mb wave to produce ascent.  The heaviest
snow on day 1 is expected from the ranges of Utah and into
southern CO and northern New Mexico, where the longest duration of
synoptic ascent is expected.  In these ranges, WPC probabilities
show a high risk for 4 inches of snow, with a moderate risk for 8
inches, especially above 7000 feet.

...Day 2/3 CA into southwest OR...
On Day 2/early Day 3 Monday night into Tuesday, a deep low
pressure is progged to approach the California coast spreading
moisture onshore.  However, enough forcing and moisture is likely
to reach the high terrain of the California mountains late in the
forecast period, and WPC probabilities for day 2 show a slight
risk for 4 inches of snow in the northern Sierra, the Trinities
and Siskiyous of California, as well as the southern Cascades of
Oregon. On Day 3 (Tue), the probabilities become moderate as the
low pressure drifts closer to the coast, with more widespread and
longer duration precipitation.

...Upstate New York and Northern New England...

Day 1...

Deep SW flow will flood into New England. The combination of warm
advection and enhanced isentropic lift will cause precipitation to
overspread the region today. Initially, the entire column is below
freezing across much of northern New England. However, the warm
advection will change the snow over to sleet/freezing rain. 
Accumulating snow is likely in western ME before the p-type
transition occurs. WPC probabilities are moderate for 4 inches of
snow in northwest ME.

As the warm air floods overhead, surface temperatures will be slow
to warm as the wedge of high pressure retreats.  This suggests a
period of freezing rain is likely, especially in the terrain of
upstate NY and northern NH and northern ME.  There remains a
multi-model signal for 0.1-.025 inches of accretion in northern
ME, with WPC probabilities near 20 percent for 0.25 inches.

Day 2...
The approach of the low-mid level front and arrival of colder air
provides a period of light snow. The cold front will be
progressive and moisture is somewhat limited, but a few inches of
snow are possible where W/NW winds enhance snow potential due to
upslope in the mountains of western ME, northern NH and VT, as
well as downwind of Lakes Ontario and Erie.

The probability of significant icing is less than 10 percent days
2 and 3.

Petersen