Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 AM EST Sun Jan 20 2019

Valid 12Z Sun Jan 20 2019 - 12Z Wed Jan 23 2019


...Great Lakes and Northeast...

A southern stream shortwave and its associated surface cyclone
will continue to track to the east-northeast from the upper Ohio
valley and northern Mid-Atlantic region to the northern New
England coast on Sunday.  An northern stream trough amplifying to
its west over the Great Lakes will support strong low to mid level
frontogenesis and strong divergence along the entrance region of
an strengthening upper jet.  This will continue to support heavy
snows north of the low track from northern New York into northern
New England early in the period, with WPC probabilities indicating
a High Risk (greater than 70 percent chance) for accumulations of
12-inches or more across northern Maine.  Further to the south,
snow will change over to a wintry mix, with significant ice
accumulations possible from central Massachusetts northeastward
into southern New Hampshire and along the Maine coast. 
Synoptically driven snows are expected to diminish as the low
lifts into Atlantic Canada Sunday night.  However, as the northern
steam trough continues to amplify, with a closed center developing
over New York Monday morning, strong northerly to northwesterly
flow on the backside of the system across the Great Lakes will
support lake effect snows.  These may produce some locally heavy
totals across portions of central New York, as well as parts of
northwest Indiana and the U.P. of Michigan.

...Western U.S....

Heavy snows are expected to return to portions of the West as an
upper ridge gives way to a pair of shortwave troughs moving
onshore.  A low amplitude shortwave moving onshore this morning is
expected to support some locally heavy mountains snows as it moves
from Oregon into the northern Rockies on Sunday.  Meanwhile, a
more amplified upstream trough is forecast to track into northern
California late Sunday into early Monday, supporting very heavy
snows across the northern Sierra, where WPC Day 1 probabilities
show a High Risk for accumulations of a foot or more.  This second
trough will continue to amplify as it moves further east on
Monday, supporting locally heavy mountain snows across Utah into
Wyoming and Colorado.

...Northern and central Plains, upper Mississippi valley and upper
Great Lakes...

The leading wave moving into the Northwest on Sunday will advance
into the northern Plains on Monday, supporting a swath of light to
moderate snows that will develop ahead of the wave and along a low
level front pushing southeast from the northern High Plains into
the central Plains.  Snows will begin to become better organized
further to the southeast as the second trough continues to amplify
and shift east into the central Rockies/High Plains Tuesday
morning.  Low pressure developing east of the Rockies will track
northeast into the mid Mississippi/lower Missouri valley regions
on Tuesday.  Low to mid level frontogenesis and favorable upper
forcing will support some light to moderate snows north of the low
track Tuesday into early Wednesday.

Pereira