Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Sat Dec 15 2018

Valid 12Z Sat Dec 15 2018 - 12Z Tue Dec 18 2018


...Pacific Northwest to the Northern Rockies...
Shortwave ridging results in light to no precip over the Pac NW
into this afternoon. The trough from a deep low over the Gulf of
Alaska brings the next round of precip to the Pac NW late today.
However, associated tropically sourced moisture will have rather
high snow elevations (6000-7000ft) in the Coast Ranges and
Cascades/Sierra Nevada ahead of the mid-level trough axis which
does not reach the coast until late Sunday night. The greatest
moisture fetch is into far northern CA with the northern CA
Cascades having moderate probabilities for six inches of snow for
Day 2 while lower snow elevations in the northern WA Cascades
results in high probabilities for six inches.

Another trough quickly follows with an atmospheric river focused
on OR where snow elevations will be around 7000 ft, lower in
WA/northern CA. Lower snow levels north of the AR axis with plenty
of moisture and lift should result in rather heavy snow for the
Olympics and especially for the northern Cascades Monday night.


...Northeast U.S...
A southern stream low pressure system currently over MS will shift
northeast to the Mid-Atlantic coast through tonight. The
combination of high pressure/cold air to the north and onshore
flow north of the flow will create a wintry mix for the
northeastern US late tonight through Monday night. Surface low
pressure develops along the Mid-Atlantic coast Sunday and drifts
northeast near the coast through Sunday night before phasing with
a northern stream trough Monday allows rapid intensification of
the low as it pushes north over Nova Scotia. Snow, sleet, and
freezing rain are all possible in this complex low passage over
mainly interior sections northeast from PA. Bands of heavy snow
are possible in the trowal over Maine Monday.

Jackson