Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EST Wed Dec 26 2018

Valid 12Z Wed Dec 26 2018 - 12Z Sat Dec 29 2018


...Central and Southern High Plains to the Upper Midwest...

A well-defined southern stream shortwave will continue to deepen
as it moves further east across the southwestern U.S. on
Wednesday, with a closed upper center expected to develop as it
moves east of the southern Rockies and into the High Plains
Wednesday night.  Deepening cold air, along with strong upper
forcing and low-mid level frontogenesis will support rain changing
snow across eastern Colorado and northeast New Mexico late
Wednesday into early Thursday, with the overnight guidance showing
an increasing signal for locally heavy accumulations, especially
from southeast Colorado back into the Sangre de Cristo Mountains.

Meanwhile, mid level energy and low level warm advection will
support precipitation advancing northeast well out ahead low with
snow changing to rain for portions of the central Plains and upper
Midwest.  Further west, expect mostly snow ahead of a northern
stream wave advancing east from the northern Rockies into the
Dakotas Wednesday into early Thursday, with the overnight models
trending upward with amounts across portions of central into
northern North Dakota. 

As the southern stream low continues to deepen and lift to the
north, models continue to show rain changing to snow within the
associated deformation band that is expected to develop from
eastern Colorado into central Nebraska Wednesday night before
shifting further to the northeast across eastern South Dakota and
central to northern Minnesota on Thursday.  An upper jet couplet
and low-mid level frontogenesis is expected to support locally
heavy accumulations within this band.  Heaviest storm totals are
expected to center from southeast South Dakota to the Arrowhead of
Minnesota, where the 48-hour probability of snow accumulations of
8-inches or greater is 80 percent or more for the period ending
12Z Friday.    The overnight model consensus did shift a little
further to the east with the axis of heavier amounts, but overall
remained generally consistent with previous runs.

By early Friday, as the upper low begins to shear out to the
northeast ahead of an amplifying trough over central Canada, the
larger scale heavy snow threat is expected to diminish across the
upper Midwest.  However, cold air advection with northwesterly to
westerly flow is expected to support lake effect snow showers,
which may produce some additional significant amounts across parts
of the U.P. and northern Lower Michigan Friday night into Saturday.

...Southwest and Southern Rockies...

Models show a deep positively tilted trough developing across the
Southwest as another shortwave trough dives south into the region
on Friday.  Upslope flow along with low to mid level frontogenesis
is expected to support an increasing potential for heavy mountain
snows across portions of eastern Arizona and New Mexico. 
Overnight guidance showed a significant signal for heavy
accumulations across the Sandia-Manzano, Sacramento and southern
portions of the Sangre de Cristo mountains, with WPC probabilities
indicating a Moderate Risk for accumulations of 8-inches or more
during the Day 3 period.

Pereira