Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 PM EST Wed Feb 06 2019

Valid 00Z Thu Feb 07 2019 - 00Z Sun Feb 10 2019


...Central Plains and Lower Missouri Valley to the Upper Great
Lakes...

A deep upper trough extending from the northern Rockies into the
Southwest will continue to shift east, with southern stream energy
lifting out of the Southwest into the southern Plains on
Wednesday.  Models show the associated surface low developing and
moving east of the Colorado Rockies overnight, into the Ozark
region around daybreak.  As strong northeasterly flow advects and
maintains a shallow dome of cold air west of the low, warm air
advection aloft will support freezing rain.  While the NAM and
some of the hi-res members may be overdone with respect to QPF and
the eastward advance of the cold air, the overall trend of the
models has been colder and wetter -- resulting in an increase in
ice amounts, particularly across portions of eastern Kansas and
western Missouri.  However, in addition to model spread, forecast
confidence is limited by the model indication of elevated
instability, which may work to increase the potential for sleet,
while lowering freezing rain accumulations.  This has helped limit
WPC probabilities for 0.25-inch ice amounts to a Slight Risk from
the eastern Oklahoma/Kansas border northeastward to eastern Iowa.

As the upper trough assumes a negative tilt and lifts farther to
the northeast, models show a significant deepening the of the
surface low as it tracks into the Great Lakes region late
Thursday.  In addition to the upper level dynamics, low to mid
level frontogenesis will support a developing swath of moderate to
heavy snows northwest of the low, with the heaviest accumulations
expected to center across northern Wisconsin and the western Upper
Peninsula.  A wintry mix closer to the low is expected to result
in accumulating ice with a  Slight Risk for accumulations of the
0.25 inch or more extending from southern Wisconsin to northern
Lower Michigan.

...Northeast...

Low amplitude mid level energy and a weak surface wave emanating
from the Ohio valley will support the eastward advancement of
light to moderate precipitation across the Northeast Wednesday
evening.  Current observations show temperatures in the 20s to mid
30s, with dew points in the teens across much of northern New York
and central New England.  High pressure establishing to the north
over eastern Canada will help keep the cold air in place, with the
models showing a good cold air damming signature across northern
New England Wednesday evening.  With southwesterly flow/warm air
advection aloft, freezing rain will continue to spread east across
northern Upstate New York into northern and central New England. 
Significant ice accumulations are possible, with WPC probabilities
indicating a Slight Risk for accumulations of 0.25 inch or more
centering over the mountains of central New England.

...Western U.S....

A pair of features are expected to bring widespread snows to the
Pacific Northwest and northern California, including lower
elevations snows to western Washington, Oregon and the North Coast
of California late Friday into Saturday.  Models show an upper low
dropping southeast from the Gulf of Alaska, reaching Northwest
California on Friday.  As this system weakens and moves across the
region, snows are expected along the coastal ranges into the
southern Cascades and northern Sierra late Friday into early
Saturday.  Meanwhile, an upper low is forecast to develop over
British Columbia late Friday and deepen as it moves farther south
along the Pacific Northwest coast on Saturday.  This will support
low elevations snows across western Washington and Oregon Friday
night into early Saturday, with additional snows reaching into
Northwest California on Saturday.

Pereira