Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 PM EST Mon Nov 19 2018

Valid 00Z Tue Nov 20 2018 - 00Z Fri Nov 23 2018

...New England and New York...
Days 1-2...

Coupled upper jet structure will combine with approaching height
falls associated with a northern stream shortwave to spawn surface
cyclogenesis over the Mid-Atlantic region. This surface low will
then traverse northeast to near Cape Cod, through the Gulf of
Maine, and then into Nova Scotia by the end of day 1. Warm and
moist advection ahead of this trough will increase 1000-500mb
relative humidity, which will be wrung out as a mixture of
precipitation across upstate New York and through New England.
Surface temperatures will be marginal, and rain is likely across
most of Southern New England. However, from the Catskills of New
York, through the Berkshires of Massachusetts, and points NE from
there, snow is likely to be the predominant precipitation type. A
band of robust 925-850mb frontogenesis will drive periods of
moderate snow, focused likely from western Mass, through the
Worcester Hills, into southern New Hampshire, and through eastern
Maine. This matches most of the high-res guidance which shows a
maximum in QPF along the MA/NH border into coastal Maine. In this
area, there is a moderate risk for 4 inches of snow, with more
than 6 inches possible in a small area across the terrain of
northern MA and southern NH.

Points north of this band will see more gently sloped
frontogenesis through 600mb, but lesser available moisture for
snow accumulation, so despite colder temperatures accumulations
are expected to be less. Just a few inches are possible, except in
the highest terrain of the White Mountains of NH.

The low will pull away Tuesday night into Wednesday /day 2/ with
subtle theta-e ridging lifting into Downeast Maine early in the
period associated with the best mid-level deformation. This may
enhance snowfall locally, but WPC probabilities are low for more
than 2 inches of additional snowfall in Maine.

Additionally on day 2, potential exists for briefly heavy snow
squalls across northern New England and upstate New York
associated with the arctic cold front and weak clipper type low
pressure. Briefly intense low-level frontogenesis and modest
SBCape may combine with sfc-2km theta-e lapse rates less than 0,
favorable for brief snow squalls. While accumulations within these
squalls will not be significant, heavy rates may produce isolated
1 inch amounts in a short period of time on top of the snow that
falls Tuesday.


...Great Lakes...
Days 1-2...

Periods of heavy lake effect snow are likely Tuesday and Wednesday
on the E/SE side of all of the Great Lakes. A broad trough across
the Great Lakes will be reinforced by a shortwave on Tuesday,
followed by a more potent impulse on Wednesday. Both of these will
bring cold advection across the Lakes with sfc-700mb flow becoming
increasingly unidirectional. Light lake effect snow is possible
across the U.P. of Michigan on day 1, but otherwise the
significant lake effect is expected Wednesday downwind of Lake
Huron, Erie, and Ontario. On Wednesday, as the second impulse
pushes a weak surface low and arctic cold front eastward, strong
CAA will produce steep lapse rates and more than 500 J/kg of CAPE
atop the lakes. Unidirectional westerly flow and an inversion more
than 11 kft above the surface suggests the potential for heavy
lake effect snow east of Lake Erie and Ontario, with the heaviest
amounts likely east of Ontario where a dual fetch from Lake Huron
and upslope enhancement into the Tug hill Plateau will produce
snow amounts in excess of 4 inches, with probabilities over 30
percent for 6 inches. Southeast of Lake Erie, 4 inches of snow is
possible, with 1-2 inches likely downwind of the remainder of the
Lakes.


...Mountain West...
Days 2-3...

A deep trough and surface cold front will push across the west
coast Wednesday into Thursday. This front will transport
significant Pacific Moisture into the West Coast on the leading
edge of a weak atmospheric river. PWAT anomalies are forecast to
climb to 2-2.5 standard deviations above the mean by Thursday.
Lift through this moisture due to jet level diffluence and
mid-level height falls will produce increasing snow across the
coastal ranges before spilling over into the interior west
mountains late on day 3. Heavy snowfall is likely from the
Cascades southward through the Sierra Nevada, and the focused
moist advection will likely produce over 12 inches of snow in the
Sierra Nevada above 10000 feet. Snow levels will initially be
high, 7000-8000 feet, but will fall through Thursday to as low as
3000 feet in the Northern Cascades, and 5000 feet elsewhere across
the west. WPC probabilities are high for more than 8 inches of
snow only in the Sierra, but widespread 4 or more inches is likely
across the mountains from the Olympics in Washington, east to the
Northern Rockies, and south to the Wasatch of Utah.


The probability of significant ice (0.25-inch or more) is less
than 10 percent.

Weiss