Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
437 AM EST Fri Jan 18 2019

Valid 12Z Fri Jan 18 2019 - 12Z Mon Jan 21 2019


...South Dakota to the upper and mid Mississippi valley...

Models continue to show a stripe of light to moderate snows
developing along the track of a 700mb wave, axis of low to mid
level frontogenesis and upper divergence shifting east  from South
Dakota into the Mississippi valley on Friday.  Overnight models
remained consistent with previous runs with respect to the axis of
heavier amounts, however the overall consensus was toward a faster
solution.  Meanwhile to the south, models continue to show a
developing deformation zone to the northwest of a deepening
surface cyclone moving east of the Rockies across Oklahoma and
into Arkansas Friday night.  Rain changing to snow is expected to
result in some light to moderate snow accumulations across
portions of the mid Mississippi valley by Saturday morning.

...Ohio Valley to the northern Mid Atlantic and Northeast...

Models show the previously noted surface wave and associated
deformation band continuing to produce light to moderate snows as
they track further to the northeast along the Ohio valley on
Saturday.  As the low moves toward the Mid-Atlantic, phasing mid
level energy is expected to support significant surface
intensification, with the models continue to show a strong signal
for heavy snows developing north of the low track.  Strong upper
divergence within a coupled jet region, along with low-mid level
frontogenesis continues to support a strong signal for heavy
amounts extending from the upper Ohio valley to northern New
England.  WPC Day 2 probabilities (ending 12Z Sun) show a High
Risk (greater than 70 percent chance) for amounts exceeding
8-inches from northeast Ohio to southern New Hampshire, with WPC
probabilities also indicating the strong likelihood that some
areas within this region will see amounts of a foot or more.  An
icy transition zone is expected further to the south, with
guidance continuing to signal some potential for significant ice
accumulations along the southern periphery of the heavier snow
axis from the Ohio valley to southern New England on Day 2.  With
the low deepening rapidly along the northern New England coast,
heavy snows will continue to shift north across northern New
England, with WPC Day 3 probabilities (ending 12Z Mon) showing a
High Risk for additional accumulations of 8-inches or more
centered across eastern Maine.

...California, Pacific Northwest, Great Basin, Central to Northern
Rockies...

Warm advection precipitation ahead of a wave tracking northeast
across the eastern Pacific toward British Columbia is expected to
produce some locally heavy accumulations across portions of the
northern Cascades and northeast Oregon ranges Friday into early
Saturday.  Locally heavy amounts are expected to extend further to
the east into the central Idaho and western Wyoming ranges along a
low to mid level frontal band dropping southeast across the region
Saturday into early Sunday.  By late Sunday as the upper ridge
begins to give way to a well-defined shortwave trough, the
potential for heavy snows will become more widespread, with
locally heavy snows becoming more likely from the Cascades and
northern Sierra to the northern Rockies.

Pereira