Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 AM EDT Tue Oct 15 2019

Valid 12Z Tue Oct 15 2019 - 12Z Fri Oct 18 2019

...Pacific Northwest...
Days 2-3...
A plume of moisture is directed into the Pac NW late Tuesday with
an upper trough pushing inland Wednesday night.  The subtropically
sourced moisture will have a high snow level, above 8000 ft.  This
limits heavy snow to the highest Cascade peaks into Wednesday.
Temperatures aloft fall with the approach of the upper trough with
snow elevations decreasing to around 5000ft under the trough over
WA and northern ID/MT Thursday. Periods of heavy snow are expected
late Wednesday into Thursday as successive waves of height falls
and corresponding moisture surges and peaks in 700 mb vertical
velocities occur for the high Cascade peaks.  The combined
strength of lift and multiple periods of lift within a long
duration low-mid level warm advection pattern result in long
duration snow, with 1-2 feet of snow expected as a result in the
higher elevations of the WA Cascades.


...Northeast...
Day 3...
A powerful cyclone is expected to develop over the Mid-Atlantic
coastal waters Wednesday and move into eastern New England while
rapidly intensifying. Cold air is delayed until Thursday with
sub-zero 850mb temps over interior New England and the Adirondacks
where the snow level reaches 4000ft by late Thursday.  The higher
elevations of the Adirondacks, Green and White Mountains have
potential to change from rain to snow after the cold air arrives,
with several inches possible due to slow movement of the upper
low.
Lower elevations are forecast to remain as mostly rain. The NAM
had the furthest west track of the model suite and was given less
weight as a result of good clustering among the ECMWF, UKMET, and
GFS cyclone tracks.

The probability of significant icing is less than 10 percent Days
1-3.
The probability of heavy snow is less than 10 percent Day 1.

Petersen