Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Mon Jan 07 2019

Valid 00Z Tue Jan 08 2019 - 00Z Fri Jan 11 2019


...Great Lakes across northern PA/upstate New York and New
England...
Days 1-3...

The wave causing snow and mixed winter precip types across the
upper Great Lakes moves east across Northern New York and northern
New England through Tuesday.  Low-mid level warm advection and
convergence associated with a warm front lifting northeast will
result in  snow, sleet, and freezing rain as the warming results
in a precip type transition. 
Moderate WPC probabilities exist for 4 inches of snow from the
Adirondacks of NY eastward across the Green and White Mountains
and terrain of western Maine.  Farther southwest, moderate WPC
probabilities for a tenth inch ice exist over the the Tug Hill
Plateau, portions of the Adirondacks, and down in north-central
PA.  

On Tuesday, a surface low is forecast to develop near Lake Ontario
and move into northern New York and New England, with a triple
point low developing near the southern Maine coast around Wed
morning.  Another round of snow driven by low-mid level warm
advection and convergence crosses northern New York and northern
New England, followed by the cessation of warm advection as the
mid level circulation arrives.  With the arrival of the
circulation, cold air advection begins and allows the precip to
stay as snow in northern Maine. The heavier snow is expected over
Maine due to longer duration of snow. The event winds down as the
low moves off the Maine coast later Wed.

Post frontal lake effect snow is likely in a cold advection
pattern starting Tues evening with low level convergence in the
lee of lakes Superior and northern Lake Michigan. On Wed,
continuing low level convergence and cross-lake flow supports
additional accumulating snow in the UP of MI and northwest lower
MI, and developing off Lakes Erie and Ontario. Relatively higher
amounts are expected with longer duration snows in the western UP
of MI.

On Thu, moist northwest flow with bands of low level convergence
favors snow showers in the lee of Lake Erie in northwest
PA/adjacent southwest NY, off the southeast corner of Lake
Ontario, and in windward terrain of the Tug Hill and Western
Adirondacks/northern Green Mountains. Activity winds down off Lake
Superior as a low level ridge builds in from the upper MS Valley,
advecting drier air into the region and weakening near shore
convergence.


...Northern California/Pacific Northwest/Northern Rockies...
Days 1-3...

A slow moving, deep area of low pressure drifting northeast across
the Pacific will support deep southerly flow advecting warm and
moist air into northern CA and the Pacific Northwest.  As a
result, snow levels will climb.  Heavy snow is likely at higher
elevations of the Shastas/Sisykiyous and OR/WA Cascades. The
models are hitting windward terrain in the Shastas hard with
potential for a couple of feet of snow at higher elevations over
this period.  Secondary maxima are expected over the northern WA
Cascades, where a foot or two of snow is expected. Since the
system is slow to move east, lower amounts are expected downstream
in the northern Rockies, with several inches likely from the OR
Blue Mountains to the ranges of ID and northwest MT.

There is an increasing threat for freezing rain across portions of
the the Cascades of Washington and Oregon and lee areas on
Tuesday. Cold surface temperatures will be reinforced by easterly
winds banking against the Cascades, which will support freezing
rain as warm air advection aloft produces widespread
precipitation. The current model suite is in good agreement for
significant ice accretion in or east of the Cascades, and WPC
probabilities show a possibility of a quarter inch of icing. Light
icing may also occur in the Columbia River Gorge Tuesday into
Wednesday. The uncertainty revolves around multiple precip types
and duration of each type.

The probability of a quarter inch of icing is less than 10 percent
on day 3.

Petersen