Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
449 PM EST Sun Feb 10 2019

Valid 00Z Mon Feb 11 2019 - 00Z Thu Feb 14 2019


...Western U.S....

Models show a pair of systems impacting the Northwest through the
Day 1 period.  The initial system - an upper low dropping into
southern British Columbia, will provide a brief period of snow as
its associated surface wave tracks east across Washington Sunday
night into Monday.  With ample cold air remaining in place, this
will include additional lowland snows for western Washington. 
Even as this system moves quickly to the east, unsettled weather
will continue as the next system - an upper low dropping southeast
from the Gulf of Alaska approaches the coast on Monday.  Warm
advection precipitation ahead of the low will contribute to
additional snows, with Day 1 totals (ending 00Z Mon) likely to
exceed a foot along the southern Washington and northern Oregon
Cascades.  Locally heavy amounts are expected farther to the east
across the northeast Oregon into the southern Idaho ranges.

The upper low is expected to slow near Vancouver Island, while
additional shortwave energy dropping to the west contributes to an
elongated positively tilted trough extending from British Columbia
into the eastern Pacific on Tuesday.  The associated axis of deep
onshore flow will shift slowly to the south, setting up a
prolonged period of heavy snow along the southern Washington and
Oregon Cascades Tuesday into Wednesday, where event totals may
exceed several feet.

Models show the upper trough pushing inland on Wednesday, shifting
the axis of deeper inflow and heavy snow potential farther south
into Northern California. Heavy snows accumulations are likely in
the higher elevations of the coastal ranges in Northwest
California as well as along the Trinity Mountains and California
Cascades.

...Midwest...

An upper trough currently centered over the West is forecast to
move east into the Plains by late Monday.  Shortwave energy in the
base of the trough is expected to assume a negative tilt as it
moves from the Plains toward the Mississippi valley on Tuesday. 
As the associated surface low amplifies and tracks northeast from
the mid Mississippi valley toward the Great Lakes, low to mid
level forcing northwest of the low should be sufficient for a
blossoming area of moderate to locally snowfall across the upper
Mississippi valley into the upper Great Lakes on Tuesday.  WPC
probabilities have maintained a moderate risk for 8-inches or more
during the Day 2 period (ending across 00Z Wed) across portions of
southeast Minnesota and northeast Iowa and southern to central
Wisconsin.  Farther south and east, a wintry mix will contribute
to light snow and ice accumulations, with WPC probabilities
indicating a Moderate Risk for ice accumulations of 0.25 inch or
more across southern Lower Michigan during the Day 2 period. 
Snows will continue across the upper Great Lakes into Wednesday,
before diminishing late in the day as the low tracks east of the
region.

...Northern Mid Atlantic and Northeast...

Warm advection precipitation associated with a weak wave will
spread east from the Ohio valley into the Mid Atlantic region late
Sunday into Monday.  With surface temperatures currently near
freezing and dewpoints in the teens and single digits in most
places, a wintry mix is expected to develop Sunday evening and
overnight from the central Appalachians into the Mid Atlantic. 
Warm air advection aloft will support sleet and freezing rain from
the central Appalachians into the central Mid Atlantic, with the
best chance for significant accumulating snows confined mainly to
southern Pennsylvania.

This initial wave is expected to move off the Mid Atlantic coast
on Monday, leaving behind a baroclinic zone extending back into
the central U.S.  By late Monday models show a well-defined cold
air damming signature in the East as strong high pressure shifts
east into southeast Canada.  This will be fortified by a weak wave
developing along the front and lifting north along the Mid
Atlantic coast.  As the low in the Midwest continues to track
east, this will set the stage for additional snow and ice
accumulations across the northern Mid Atlantic region on Tuesday. 
Heaviest ice accumulations are expected to center over western
Pennsylvania and western Maryland, with WPC Day 2 probabilities
indicating a Moderate Risk for ice accumulations of 0.25-inch or
more.  Farther to the north light to moderate snows will lift
north across the Northeast, before changing over to a wintry mix
across much of New York and southern New England late in the day
on Tuesday.  Tuesday night into Wednesday, precipitation will
continue to lift to the north as the coastal wave becomes the
primary surface feature and tracks north.  While most areas of the
northeast will see some transition to mixed precipitation, some
parts of far northern New York and interior northern New England
may remain mostly snow.  These areas may see snow accumulations of
a foot or more by late Wednesday.

Pereira