Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 AM EST Tue Dec 25 2018

Valid 12Z Tue Dec 25 2018 - 12Z Fri Dec 28 2018


...Western U.S....

A well-defined shortwave trough will continue to amplify, digging
southeast across California into the Desert Southwest on
Christmas.  Sufficient moisture, along with strong vertical ascent
will support locally heavy snow accumulations across the higher
elevations of the Southwest into the central Rockies, including
the San Juans where WPC probabilities continue to indicate the
potential for amounts of a foot or more during the Day 1 period. 

Meanwhile, upper level energy and an associated 700mb low
remaining further to the north are expected to produce generally
light snow accumulations across the northern Great Basin into
western Wyoming and southern Montana on Christmas into Christmas
night.

On Wednesday probabilities for significant snows are expected to
increase across the Olympics and along the Cascades as the next
shortwave system drops southeast into the region.  This system is
expected to dive to the south as well, maintaining a deep trough
across the Southwest while supporting another round of high
elevation snows across Southwest into the central and southern
late Thursday into early Friday.

...Central U.S....

Models continue to show a strong post-Christmas storm developing
across the central U.S. into the upper Midwest on Wednesday. 
Driven east by the system moving into the Northwest on Wednesday,
the deep upper trough traversing the Southwest is expected to lift
out into the Plains, with a closed low developing over the central
Plains Wednesday night into Thursday morning.  Precipitation will
spread well out ahead of the low.  Driven by mid level energy and
low level warm air advection, precipitation is expected to spread
northeast from the central Plains into the upper Midwest on
Wednesday, beginning as a period of snow across portions of the
upper Midwest.  Meanwhile, northern stream energy emerging from
the northern Rockies will support snows further to the west,
producing generally light accumulations across the Dakotas. 

By Wednesday night, as the upper low begins to close off and
deepen, rain changing to snow can be expected in the associated
deformation zone setting up from the High Plains of eastern
Colorado northeastward across central Nebraska into eastern South
Dakota.  Widespread moderate to locally heavy snow accumulations
can be expected across this region, with WPC probabilities
indicating at least a Slight Risk for amounts of 8-inches or more
by Thursday morning.  The system will continue to track to the
north, maintaining a closed upper center as it moves into northern
Plains on Thursday, before beginning to shear out to the northeast
across Minnesota Thursday night.  Overnight models have moved into
better agreement with the evolution of the system as suggested by
the WPC probabilities which show a Moderate Risk for accumulations
of 8-inches or more from the South Dakota-Minnesota border
northeastward across the central Minnesota into the Arrowhead and
northwest Wisconsin and the western U.P. of Michigan. 

The probability of significant icing (0.25 inches or more) is less
than 10 percent.

Pereira