Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EST Sun Nov 25 2018

Valid 12Z Sun Nov 25 2018 - 12Z Wed Nov 28 2018


...Central Plains to the Great Lakes...
Days 1-2...

A winter will continue to develop over the central U.S., producing
a swath of significant snowfall extending from southwestern
Nebraska and northeastern Kansas to Lower Michigan Sunday into
early Monday.  Snowfall will accompanied by strong winds,
producing blizzard conditions for some locations. 

A well-defined shortwave trough and its associated surface cyclone
will continue to track east across across the central Plains into
the lower Missouri valley on Sunday.  Low to mid level
frontogenesis, along with left-exit region upper jet forcing is
expected to support moderate to heavy snowfall rates northwest of
the low, producing a swath of moderate to heavy accumulations from
southeast Nebraska and northeast Kansas to northern Illinois on
Sunday.  Strong frontogenetic forcing is expected to support a
narrow stripe of heavy accumulations, with dry air to the north
producing a very tight gradient.  The general trend in the
overnight models was toward a slightly slower solution, further
raising the probabilities for heavy amounts across the region. 
Overnight guidance also continued to move into better agreement
with the larger scale features, however some detail differences
remain, limiting forecast confidence in the exact placement of
heavy amounts. 

Overnight Sunday into early Monday, moderate to heavy snow will
begin extend into Lower Michigan.  Consensus of the overnight
models was to shift heavier accumulations further south across
Lower Michigan, with weakening dynamics supporting lesser totals
as the system moves east of Lake Michigan Monday morning.  By late
Monday with the low moving across Lake Huron into eastern Ontario,
models show only some light snows on the backside of the system,
along with some lake effect snow showers remaining across Michigan.

...Northeast...
Days 2-3...

As the Great Lakes system lifts into eastern Ontario and its
trailing cold front reaches the eastern U.S., models continue to
show trailing mid level energy moving through the base of the
upper trough across the Ohio valley into the Mid Atlantic late
Tuesday.  This will support the development of a surface wave
along the cold front that will continue to deepen as the shortwave
assumes a negative tilt and lifts to the north along the Northeast
coast.  Ample moisture with strong upper forcing, along with low
to mid level frontogenesis will raise the potential for moderate
to heavy precipitation rates.  Forecast detail differences with
respect to the low track and the rain/snow line are limiting
forecast confidence across this region.  However, do expect the
intrusion of warmer maritime air to hamper the potential for heavy
snow accumulations along the northern New England coast.  Further
to the north however, enough cold air should remain in place for a
mostly snow event, especially across the mountains, with WPC
probabilities showing a significant chance for storm total amounts
exceeding a foot across northern Maine.

...Pacific Northwest to the Northern Rockies...
Days 2-3...

Models continue to show a good signal for accumulating ice within
the mountain passes and valleys of the northern Washington
Cascades on Monday.  While southwesterly winds ahead of a
approaching shortwave warm the low levels, lingering cold air
supported by easterly boundary layer winds is expected to produce
some freezing rains, with WPC probabilities continuing to show the
potential for some significant ice accumulations within the
region.  On Tuesday into early Wednesday, ridging in the West is
expected to give way to the previously noted shortwave, with
models showing largely zonal flow across the northwestern U.S. by
the period's end.  High snow levels and limited moisture are
expected to hamper the potential for widespread heavy amounts. 
However some stronger forcing may produce at least some locally
heavy accumulations across the higher peaks of the northeast
Oregon into the central Idaho ranges, as shown by the WPC Day 3
probabilities.

Pereira