Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 PM EST Sat Jan 18 2020

Valid 00Z Sun Jan 19 2020 - 00Z Wed Jan 22 2020


...Great Lakes/Northeast...
Warm advection and moisture advection on the nose of a 700 mb jet
forecast to be near 50 kt as it crosses southern VT, southern NH
and Maine combines to produce snow across VT, NH and Maine.
Snow get enhanced in downeast Maine along the mid level
deformation zone associated with developing low pressure near the
coast tonight.  The event winds down Sunday as the low and ascent
depart.

In addition to the larger scale forcing, orographic enhancement is
expected to bolster amounts in the Green Mountains to the White
Mountains and ranges of western Maine.

The initial low level warm front combines with southerly flow this
evening to produce snow from the Tug Hill region eastward into the
southern Adirondacks. The flow veers across Lake Ontario tonight
to become westerly by morning.  The moisture convergence along the
eastern shore line and upslope flow favors heavy snow in the Tug
Hill to the western Adirondacks. Around a foot of snow is possible
tonight into Sunday in the Tug Hill. The flow veers with time
Sunday afternoon, shifting the band south to the southern shore of
Lake Ontario.

The west southwest flow developing across Lake Erie leads to
leeshore convergence in southwest NY and northwest PA, with a
prolonged period of moderate to heavy snow tonight into early
Sunday.  The flow veering late Sunday allows bands to shift and
develops further south along the lake into northeast Ohio.  Where
bands persist, clusters of 8-12 inches of snow are expected in
southwest NY and northwest PA in counties adjacent to Lake Erie.  

Strongly confluent flow off Lake Superior leads to well defined
moisture convergence over western lake Superior with a vertical
velocity maxima in the western UP of MI leading to a burst of
heavy snow tonight.  The lift spreads into the central UP, where
several inches are also expected into early Sunday where winds
become north Sun morning.  Activity tapers later Sunday as drier
air aloft moves across the lake.

Boundary layer west winds tonight favors snow showers across
southwest lower MI.  Once the sfc trough passes later Sun, the
flow veers and snow showers focus further south in southwest lower
MI and develop Sunday evening in northern IN. Drier air aloft
cross lake on Mon should lead to a decrease in snow shower
coverage/intensity.  

...Pacific Northwest/California...
Snow coverage and amounts tonight into Monday should be minimal
the next few days as the mid level flow back and the region has
only light QPF.
On Tue, a large, slow moving circulation over the northeast
Pacific drifts east, driving a cold front towards the Pacific
northwest and northern CA.  A band of enhanced moisture and lift
precedes the front, with precip breaking out in a band of low-mid
level convergence and moving inland into he WA/OR Cascades and the
Shasta/Siskiyou ranges of northern CA.  Locally heavy mountain
snows are possible at higher elevations, as southerly flow ahead
of the front warms the region. Snow elevations gradually drop as
the front arrives Tue.

The probability of greater than 0.25 inches of freezing rain is
less than 10 percent.

Petersen