Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 PM EST Mon Jan 06 2020

Valid 00Z Tue Jan 07 2020 - 00Z Fri Jan 10 2020


Days 1-3...

...Pacific Northwest to the Northern Rockies...
Confluent flow has led to channeling of moisture and lift today
across the ranges of the Pacific northwest, with the moisture
spreading inland through the northern Rockies the rest of today
into tonight.  The persistence of the moisture and lift leads to
continuing snow at higher elevations of the WA Cascades into Tue.
1-2 feet of snow are expected in the upslope regions of the ranges
of northern ID into northwest MT, and also the WA Cascades, with
even higher amounts in the tallest mountains.

Another shortwave will move onshore Tuesday, bringing continued
precipitation but with snow levels decreasing as the front crosses
the OR Cascades, so snow will develop in the OR Cascades and then
spread inland across the Blue Mountains and then the WY Tetons.

The heaviest snowfall is likely in the WA/OR Cascades, as WPC
probabilities are moderate to high for 12 inches. On Wed evening
to Thu, the next wave in the series of 700 mb shortwaves comes
ashore in the northwest and steadily progresses southeast into
northern CA and the Great Basin.  Snow will likely continue in
WA/OR Wed night and taper on Thu as drier air aloft advects into
the region, while becoming more widespread in northern CA and
inland to the ranges of northern UT/western WY. The highest
probabilities for 8 inches are in the OR Cascades Day 3. 

...Central Appalachians to the Northeast...
A surface cyclone moves across the Central Appalachians Tue
morning and off the Mid-Atlantic coast Tuesday evening, continuing
east of New England Wednesday.
As the low approaches, low-mid level warm advection and
frontogenesis produce lift, with accumulating snow in the central
Appalachians mostly in the 3 to 5 inch range in the mountains,
lower in valley areas. Further east,  this lift could produce wet
bulb cooling and a change from rain to snow in the I-95 corridor
of the Mid-Atlantic, and into southeastern New England.

Tue evening, as the low deepens and moves south of Long Island, a
modest deformation band may try to pivot snowfall back into
NJ/Long Island/southern New England, but this ends by Wed morning
as the storm moves further offshore.

...Great Lakes...
Cross Lake Ontario flow with 20-30 kt bndry layer winds produce
impressive heat and moisture fluxes to support continued showers
forming and coming into the Tug Hill and western Adirondacks this
evening into the early overnight hours.  Snow shower
coverage/intensity wanes Tue as a low-mid level ridge approaches
from the west, with mid level drying and sinking motion.

The next cold front passes through the upper Lakes Tue, with post
frontal cold advection and cross Lake Superior flow leading to
development of lake effect snow showers in leeshore areas in the
Keweenaw Peninsula of MI. On Tue evening, the northwest flow
develops downstream into western lower MI, with areas around
Traverse Bay receiving locally higher amounts due to a fetch
across Superior and northern lake MI, plus a longer cross Lake MI
fetch.

A low level ridge in the upper MS Valley approaches the upper Lake
Wed, so activity begins to dwindle as the airmass dries. This the
shift focuses in the deeper moisture crossing the lower lakes.

On Thu, return southeast flow produces warm advection across Lake
Superior, with onshore flow in the arrowhead of MN.  The
combination of leeshore convergence in the arrowhead plus upslope
flow should produce a period of lake enhanced snows. As the warm
front continues east, the warm advection snows shift into the UP
of MI by 00z Fri.

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

Petersen