Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 PM EST Sat Dec 29 2018

Valid 00Z Sun Dec 30 2018 - 00Z Wed Jan 02 2019


...Pacific Northwest through the Southern Rockies and Southwest...
Days 1-3...

A shortwave and associated Pacific Jet will drop southward along
the West Coast before shifting inland towards the Great Basin.
Beneath these features, a surface cold front will push southeast
into the Rockies, while a wave of low pressure drops towards the
Northern Plains. The combination of these features will support
heavy snow across the terrain from the Washington Cascades east
and south through the Northern Rockies and into NW Wyoming. WPC
probabilities are high for 8 inches of snow, and more than 12 is
possible, in the Cascades, Bitterroots, and Northern Rockies.
Elsewhere across the northwest, accumulating snow will occur as
low as 1000-2000 feet as snow levels fall behind the front, and
WPC probabilities are high for 4 inches across much of the area.

This system will then progress slowly southward through the Great
Basin and towards the Four Corners and reinforce a deep but
positively tilted longwave trough. As this occurs, the
aforementioned surface cold front will drop through the Plains and
into the Southwest. This will spread snowfall across the Central
Rockies on day 2, but the heaviest snow is likely once again
across the Southern Rockies and southwest into New Mexico. Here,
the best combination of jet level diffluence, low-level
convergence and moist advection, and upslope enhancement will
combine to produce heavy snowfall from Southern Colorado through
much of New Mexico and into eastern Arizona. The heaviest snowfall
is likely in the southern Sangre De Cristos, as well as the
mountains in western New Mexico/SE Arizona, where WPC
probabilities are high for 8 inches and more than 12 inches is
possible. Heavy snow is likely at the lower elevations as well,
and WPC probabilities feature a high risk for 4 inches across much
of New Mexico.


...Central and Northern Plains, Northern Mississippi Valley, Great
Lakes,and New England...
Days 2-3...

Complex split flow pattern creates a challenging forecast for the
central part of the CONUS to start 2019. A northern stream
shortwave will eject eastward from MT during day 2 aiding in
ascent produced by a potent jet maximum moving across southern
Canada. This will likely produce a swath of snow where the best
upper divergence, height falls, and low level convergence along a
surface cold front combine. This feature is likely to lift quickly
eastward and is somewhat moisture starved, so total snow
accumulations are likely to be modest, with WPC probabilities
30-50% for 4 inches from near the Black Hills of SD, northeast
through the area around Lake Superior.

Further south, a southern stream shortwave lifting out of the
Southwest will move northeast while inducing surface cyclogenesis
near the Gulf Coast. This surface wave will move quickly to the
northeast into the eastern Great Lakes by the end of Day 2, and
robust ascent due to warm advection and forced synoptic lift
beneath a jet maximum will produce widespread precipitation across
the east. Much of the area will be too warm for snow. However, it
is likely that on the northern and northwest side of the
precipitation shield, enough cold air will be present either
initially (north/northeast) or advect in (northwest) to produce
freezing rain or snow. The best chance for freezing rain will be
across the Southern Great Lakes, but accretions are likely to be
minimal. Further northwest from Kansas northeast into Lower
Michigan, enough cold air behind the front combined with lift
within a deformation zone may produce a few inches of snow
accumulation, and WPC probabilities are low to moderate for 4
inches. There remains considerable spread in the guidance into how
much phasing of the northern and southern streams will occur, with
the ECMWF producing more snow further north due to more
interaction of the two streams than much of the other guidance. A
general compromise was used for this iteration of the forecast, so
while some snow accumulation seems likely, the large spread
prevents higher probabilities for accumulation.

Further northeast, as the southern stream impulse and surface low
lift further northeast, interaction with the northern wave is
expected. This will produce more widespread precipitation from the
Mid-Atlantic into New England. The column will initially be cold
enough for snow across northern New York and New England where
several inches of snow are likely. Lesser amounts are forecast
further south and in the lower terrain, with a few inches of snow
possible before the transition to freezing rain, and eventually
rain, occurs across most of the area by Tuesday morning.


The probability for significant freezing rain (0.25 inches or
more) is less than 10 percent.


Weiss