Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Sat Jan 05 2019

Valid 12Z Sat Jan 05 2019 - 12Z Tue Jan 08 2019


...New England...
Days 1-3...

Surface low pressure moving off the Southern New England coast
today will interact with a northern stream shortwave digging
through southeast Canada. This will draw moisture northward into
interior New England. Temperatures will be marginal, but some
light snow and/or freezing rain is possible from central New
England through eastern Maine. The best chance of snow will be
along the Maine coast where surface convergence and lift within
the TROWAL may be sufficient to wring out some light snow in a
column characterized by moistening 1000-500mb RH. WPC
probabilities for 4 inches of snow are less than 10 percent
however, but an inch or two is possible near downeast Maine.
Additionally, a stripe of light freezing rain can be expected
northeast from the Catskills through this morning. WPC
probabilities are low to moderate for a tenth inch of ice over
southern VT/NH though precip will have to overcome dry air first
limiting the potential there.

Tonight into Sunday, flow will become W/NW behind the departing
low pressure. This will provide a favorable environment for Lake
Effect snow into Upstate New York off of Lake Ontario, as well as
produce upslope snow across the mountains of northern VT, NH, and
ME. Accumulations are expected to be light however, with WPC
probabilities moderate for 2 inches.

More significant snow is possible Monday night as WAA develops
associated with a warm front lifting northeast through New
England. This warm front will extend from an occluded front and
low pressure in southern Canada. Very dry air ahead of this
feature will be tough to overcome, but strong WAA will eventually
saturate the column to produce snow across upstate New York and
through central and northern New England. The retreating high
pressure will allow the column to slowly warm, but WPC
probabilities are moderate for 4 inches in the terrain of the
Adirondacks eastward into the Green and White mountains.


...Western U.S....
Days 1-3...

Deep low pressure will move onshore Oregon Saturday preceding a
full latitude trough which will move eastward and push a cold
front through the Mountain West. Snow levels rise ahead of this
feature, but will fall behind the associated cold front.
Significant moisture within a Pacific Jet maximum will advect
onshore, and intense lift within the left exit region of the jet
max combined with increasing 700mb flow will produce heavy snow
from the Sierra Nevada to the OR Cascades. The heaviest snow is
likely in the Sierras where low-level winds will be orthogonal to
the ranges to produce efficient upslope enhancement, and WPC
probabilities are high for 12 inches of accumulation on both days
1 and 2 from the Sierras into the ranges of northern CA. More than
4 feet of snow is possible above 6000 feet in the Sierras by the
end of the forecast period.

As the trough shifts eastward during Sunday, moisture and
associated heavy snow will spread into the rest of the Mountain
West, with heavy snow likely as far south as the Mogollon Rim of
AZ, and as far north as the Canadian Rockies. Snow levels will
fall to as low as 4000 feet in AZ/NM, and less than 2000 feet from
Utah and points north. WPC probabilities show a moderate risk for
8 inches in the highest terrain of the Four Corners states, with
widespread high probabilities for 4 inches elsewhere.


...Upper Midwest...
Days 2-3...

An upper trough moving out of the northern Rockies Sunday night
will drive a weak surface low towards the Great Lakes through
Monday. Ahead of this feature, robust WAA will drive moisture and
ascent into a marginally cold air mass. Snowfall is likely north
of the 800mb warm nose, with freezing rain expected in the
transition zone between rain and snow. The system is transient so
duration of moderate precipitation should be limited. However,
briefly heavy snow is likely from far northeast ND through the
northern L.P. of Michigan, and WPC probabilities are moderate to
high for 4 inches of accumulation. The greatest risk for heavy
snow will be along the north shore of Lake Superior within the
Arrowhead of Minnesota, where upslope enhancement is expected, and
WPC probabilities show a slight risk for 8 inches of snow. South
of the heaviest snow, the aforementioned warm nose will climb
above 0C while surface temps hover below freezing. This will
create a period of freezing rain, with the greatest accretion
expected across Wisconsin and into the L.P., where WPC
probabilities are moderate for more than 0.1 inches.


Weiss