Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 PM EDT Fri Oct 26 2018

Valid 00Z Sat Oct 27 2018 - 00Z Tue Oct 30 2018

Days 1 through 3...

...New England and Upstate New York...
Low pressure will lift northeast along the coast and into
southeast Canada this weekend while strengthening. As this system
moves up the coast, precipitation will lift northward and blossom
in response to intensifying isentropic lift, as well as diffluence
within the left exit region of an upper jet and in the vicinity of
the increasingly negative tilt of the mid-level low. Widespread
moderate stratiform precipitation is expected across much of New
England and New York, with wintry weather likely on the northern
edge.

Initially, temperatures are cold enough across the high terrain of
Upstate New York and northern New England that snow will
overspread elevations above 1500 feet. A few inches of snow are
likely in the Adirondacks of New York as well as the mountains of
northern New England and WPC probabilities show a moderate risk
for 4 inches of snow across this area late Saturday into Sunday.
Some light accumulations are also possible across the Catskills in
New York, but WPC probabilities are less than 10 percent for 4
inches of snow there.

More concerning is the increasing freezing rain threat across
northern New England. As warm air advection occurs between 850mb
and 700mb, the column will warm aloft, while cool ageostrophic
flow at the surface will entrench sub-freezing temperatures at
least into Saturday night. Forecast profiles show an extended
duration of light freezing rain, and WPC probabilities continue to
increase for significant accretion in northern New Hampshire and
Maine, with a moderate risk for 0.25 inches across this area. The
elevated sheltered valleys are most likely to experience moderate
accretions. During Sunday, enough warm air is expected to erode
the cold surface high pressure to change all precipitation over to
rain.


...Cascades and Northern Rockies...
A series of impulses will move onshore, with significant moisture
and Pacific jet energy creating widespread high elevation snows
across the mountains of Washington and Oregon, especially Sunday
and Monday. High column relative humidity and strong lift due to
upper level diffluence and vorticity advection will create
significant accumulation above 5000 feet. WPC probabilities are
high for 4 inches of snow on Day 2 in the northern Cascades, with
over 8 inches possible. By Day 3, moisture spreads eastward into
the Rockies while continuing in the Cascades down to Oregon. WPC
probabilities for 4 inches are moderate across many of the
northwest mountains from the Absaroka Range west to the Olympics.
Upslope enhancement on westerly flow is likely in the Cascades,
and 8 inches of snow is possible above 7000 feet.


The probability for significant icing (0.25 inches or more) is
less than 10 percent all 3 days outside of New England.

Weiss