Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 AM EST Tue Jan 08 2019

Valid 12Z Tue Jan 08 2019 - 12Z Fri Jan 11 2019


...Great Lakes, Northern Appalachians, through New England...
Days 1-2...

Light WAA snow will push through northern New England this morning
producing a few inches of snowfall, mostly across Maine. A brief
break in precipitation will follow much of today, before a
secondary wave of low pressure develops and moves across central
New England this evening into Wednesday. This low will deepens as
it approaches the Gulf of Maine in response to a closing mid-level
circulation and height falls, producing robust ascent within a
deformation zone in the vicinity of the 700mb low tracking across
central New England.

The guidance has trended a bit northward/warmer overnight which
creates a challenging accumulation forecast. Ahead of the
mid-level low, WAA will again increase across much of the area
spreading precipitation northeastward. Much of this will be rain
or sleet/freezing rain as 850mb temperatures climb above 0C.
However, as the mid-level low shifts eastward, CAA will develop in
response to both mid-level NW flow and low-level isallobaric flow
towards the rapidly deepening surface low. This will allow cold
air to collapse back to the southeast and change precipitation
over to snow from Upstate New York through Maine. The heaviest
snowfall is likely in Maine due to longer duration of cold air and
the enhancement due to a developing TROWAL and theta-e advection
on Wednesday. In northern Maine, WPC probabilities show a moderate
risk for 8 inches through Wednesday, with high probabilities for 4
inches across the rest of the high terrain from the Adirondacks
and points east. South and southwest of this region, WPC
probabilities show a slight risk for one-tenth of an inch of ice
accretion before CAA cools the column to support all snow.

As the flow turns NW behind the system, upslope flow will increase
into the mountains along the Canadian border, as well as into the
Appalachians centered over West Virginia. A lowering DGZ, moisture
connection from the Great Lakes, and increasing SLR's support
several inches of snow in the terrain due to upslope enhancement,
with 8 inches possible in the mountains of WV.

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. Although fetch
across the Lakes is not ideal for single intense band development,
several bands and increasing instability/rising EL supports
several inches of snow downwind of the Lakes. The highest amounts
are likely downwind of Lake Erie and Ontario both due to channeled
connection from Superior/Huron, as well as upslope enhancement
just east of these Lakes.


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

A slow moving deep low pressure will drift northeast across the
Pacific and into British Columbia through Wednesday, resulting in
deep southerly flow across the western CONUS. This southerly flow
will bring warm and moist air into California and the Pacific
Northwest, with moisture eventually spilling over into the Rockies
and adjacent ranges as a piece of the associated Pacific Jet moves
east on Thursday. The highly amplified flow across the west will
result in rising snow levels on the warm advection, confining the
heaviest snow to elevations above 5000 ft before snow levels crash
late on D3. With snow levels elevated and flow predominantly from
the S/SW, the heaviest snow accumulations are expected across the
high terrain of the northern Sierra, Washington Cascades, and most
notably the Siskiyous/Shastas which will be more perpendicular to
the flow. In these ranges, WPC probabilities are high for 8 inches
both today and Wednesday. Lesser amounts are likely into the
ranges from the Northern Rockies southward into Colorado as the
moisture and upper diffluence providing ascent shift eastward on
D3.

Additionally, there remains a strong signal for significant
freezing rain in the Cascades of Washington and Northern Oregon,
and areas to the east just in the Lee of these ranges. Cold
surface temperatures caused by easterly winds banking against the
Cascades will cause precipitation due to WAA/southerly flow aloft
to fall as freezing rain. Some uncertainty still exists due to the
potential for multiple precipitation types and periodic episodes
of freezing rain, but guidance is in good agreement that
significant freezing rain will occur. WPC probabilities for 0.1
inches remain around 50% from the Columbia River Gorge and points
north towards the Canadian border, with low probabilities for 0.25
inches.



Weiss