Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 AM EST Mon Dec 24 2018

Valid 12Z Mon Dec 24 2018 - 12Z Thu Dec 27 2018


...Western U.S....

Guidance continues to show a well-defined shortwave diving
southeast from the eastern Pacific into northern California on
Monday.  This will not only support significant mountain snows
from the southern Cascades to the northern Sierra, but also
further east along the southern Oregon border into northern
Nevada, where favorable upper jet forcing and low level
convergence is expected to support higher totals.  WPC
probabilities show the potential for localized amounts of a foot
or more across these areas.

This system will continue to dig to the south, carving out a deep
trough across southern California into the Desert Southwest late
Christmas into early Wednesday.  While limited moisture is
expected to help hamper the potential for widespread heavy
accumulations, strong upper forcing along with orographic ascent
will likely be sufficient for at least some locally heavy
accumulations from the higher elevations of the northern and
central Arizona and southern Utah into western New Mexico and
Colorado.

By late Wednesday as another shortwave trough dives southeast into
the region, the probabilities for significant snows are expected
to increase once again across the Olympics and northern Cascades.

...Central and Northern Plains...

Driven downstream by a shortwave trough emanating from the Gulf of
Alaska diving southeast toward the Pacific Northwest, models show
the system over the Southwest beginning to lift to the northeast,
assuming a negative tilt as it moves across the southern toward
the central Plains late Wednesday into early Thursday.  Model
spread begins to increase at this point, limiting forecast
confidence across the central Plains.  WPC preferred the slower
and more closed solution of the ECMWF, with a more defined
deformation zone and heavier snow totals centering from northeast
Colorado to southwest Minnesota during the Day 3 period.  This was
in contrast to the operational GFS, which was more progressive and
less amplified than the remaining model consensus.

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

Pereira