Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Sat Nov 23 2019

Valid 00Z Sun Nov 24 2019 - 00Z Wed Nov 27 2019


Days 1-3...

...Central Appalachians to the Northeast...
A 700 mb low and mid level deformation/frontogenesis axis crosses
PA tonight and a closed low crosses southeast NY and southern new
England Sun.  Cooling as the low passes allows a transition from
rain to mixed precip types and then snow across northern OH and PA
and then a period of snow across the Catskills/Helderbergs and
Southern Green Mountains/White Mountains.  Several inches are
possible. In the precip type transition zone, a period of light
freezing rain is possible in northern PA to the Catskills, upper
Hudson Valley, Berkshires, southern Green, and White Mountains. 
These area generally show measurable to a tenth of an inch of
icing accumulation on exposed surfaces. 

While marginal boundary layer temperatures are expected to help
keep snow accumulations in check in most valley areas, strong
upper forcing along with low-to-mid level frontogenesis have
raised the probabilities for significant snow accumulations across
the higher elevations of The Green and White Mountains. By early
Monday as the low tracks progressively northeast into the Canadian
Maritimes, expect snows to diminish from west to east across
northern New England.

The GFS with its further offshore sfc cyclone track was given less
weighting. While the NAM cyclone track was close to the
ECMWF/ECMWF Mean/Canadian/Ukmet, it had higher QPF than the
majority of models and high res windows, so was also given less
weighting.  This is also supported by the NAM high snow bias in
its long term verification.  

...Pacific Northwest and Northern Rockies...
A shortwave trough diving southeast into the Northwest is forecast
to bring colder air and the potential for several inches of snow
to some of the higher peaks of the northern Cascades and northern
Rockies on Sunday.  This will be followed by a second shortwave
digging farther south, amplifying as it moves out of the Pacific
northwest across the northern Rockies and Great Basin.  Falling
heights lead to colder temperatures and lower snow levels across
the northwest. Beginning Monday and continuing into early Tuesday,
a prolonged period of strong onshore flow will help support heavy
snows across the Olympics and Cascades, as well as the Blue
Mountains in northeastern Oregon and far southeastern Washington.

On Day 3, as the upper jet drops south from OR into northern CA,
ascent in the left exit region of the upper jet combines with
cooling temps aloft to lead to a threat for heavy snow extending
south form the OR Cascades south into the ranges of northwest CA.
The ECMWF, NAM, GFS, and UKMET differ on the intensity of the low
level cyclone as it approaches northwest CA or southwest OR.
Several inches of possible is expected, with potential up to a
foot in higher elevations, where longer duration of snow is
expected.

..Central Rockies and High Plains...
The upper trough amplifying over the Great Basin on Monday is
expected to translate east across the central Rockies and central
high Plains Tuesday.  This will likely produce some locally heavy
snow amounts across Colorado and Wyoming before extending into
western Nebraska. 

Low pressure in Co favors low-mid level frontogenesis in northeast
Co across southeast WY and western NE.  Higher totals are where
return easterly component upslope winds will likely favor enhanced
lift in the foothills and high Plains after departing the front
range.
There is potential for a foot of snow in parts of southeast WY,
including the Snowy and Laramie ranges.  Model timing and
amplitude differences with the 700-50 mb wave/possible embedded
closed low leads to differences in snow totals, with the NAM a bit
less following faster movement of the low level cyclone.  The 12z
GFS and UKMET had more snow owing to slower forward progression of
the system.

The probability of significant icing (0.25-inch) is less than 10
percent.

Petersen