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

Valid 00Z Sun Jan 06 2019 - 00Z Wed Jan 09 2019


...Northeast...
Days 1 and 3...

A southern stream low off the Mid-Atlantic coast this afternoon
will shift northeast and off Nova Scotia through tonight. A
northern stream shortwave trough over Ontario will approach the
southern stream low through Sunday. Northwest flow from the
northern stream trough over the Northeast CONUS will bring some
snow to higher upslope areas and some lake enhancement for the Tug
Hill Plateau tonight into Sunday. WPC probabilities for two inches
are moderately high for Day 1 over upstate NY and northern VT.

More significant snow is possible Monday night as WAA develops
associated with a warm front lifting northeast through New England
ahead of a longwave trough. 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
Day 3 WPC probabilities are moderate for 4 inches in the terrain
of the Adirondacks eastward into the Green and White mountains and
interior Maine.


...Western U.S....
Days 1-2...

Deep low pressure will continue to shift north along the OR coast
and reach Vancouver Island tonight ahead of a deep, long-wave
trough that crosses the entire West Coast tonight. An associated
cold front will move into the Mountain West Sunday. 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 Day 1
WPC probabilities are moderately high for 18 inches across the
Sierra Nevada. More than 4 feet of snow is possible above 6000
feet in the Sierras through the forecast period.

As the trough shifts east 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. Day 1 WPC probabilities are moderate for 8 inches in the
highest terrain of the Four Corners states, with widespread high
probabilities for 4 inches elsewhere.

A deep low approaches the west coast Tuesday with heavy snow
expected to start late Tuesday.


...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 Day 2 WPC probabilities are
moderate to high for 6 inches of snow. 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 in
the southeasterly low level flow. Day 2 WPC probabilities are high
for 8 inches of snow in the Arrowhead. 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 north-central
Wisconsin and across north central portions of the L.P., where WPC
probabilities are moderately high for more than 0.1 inches.


Jackson