Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 AM EST Mon Nov 18 2019

Valid 12Z Mon Nov 18 2019 - 12Z Thu Nov 21 2019

...Northeast...
Days 1/2...
An occluded southern stream low currently off the Mid-Atlantic
coast will track north to Nova Scotia through this evening ahead
of a northern stream upper trough currently moving into the OH/TN
River Valleys. A swath of warmer air aloft pushing ahead of the
occluded low will allow most interior northeast precipitation to
be a wintry mix as the precip shield shifts across the area this
afternoon through tonight before becoming mostly snow Tuesday as
the low lifts north over the Canadian Maritimes. Expect
low-to-mid-level frontogenesis along with upper level forcing to
support an increase in precipitation rates northeast from
northeast PA tonight with freezing rain the dominant precipitation
type northeast from the Adirondacks with snow west of there and
rain south.

Day 1 WPC probabilities remain moderately high for ice accretion
above 1/4" over north-central Maine along with lower probabilities
along the Green Mountains and across northern New Hampshire. Snow
is mainly expected for far northern Maine with moderately high
probabilities for four inches.

...Northern Rockies/High Plains...
Days 2/3...
Complex interactions with a low currently shifting east toward WA
will send low pressure across the northern Rockies early Tuesday.
There is potent energy from this wave and lee side surface low
development is expected to enhance snow in an east to west stripe
along the north side of this low where upslope flow is enhanced.
However, there remains a fair degree of uncertainty to the track
of this low for Day 2. The 00Z GFS/NAM are farther south and have
the main swath of snow over MT stretching into ND, while the 00Z
ECMWF/CMC/UKMET are farther north, with the ECMWF/CMC  having the
resultant swath north in Canada. WPC QPF is a compromise of these
solutions with the axis of 1/4" to 1/2" QPF right along the
northern MT border which is fairly similar to the 00Z UKMET.
Regardless of where the plains low develops and tracks, heavy snow
can be expected for the northern Rockies of MT (particularly the
eastern slopes) where the Day 2/3 probability for 12 inches is low
to moderate.

...California/Great Basin/Central Rockies...
Days 2/3...
An upper low currently centered west of Baja California is
expected to weaken and eject northeast ahead of an upper low
closing off over California Tuesday night. As the ejecting wave
moves across the Southwest, snow levels will initially be high
(above 10,000ft), but lower through Day 3 to around . This will
limit the initial snow to only the highest elevations across
southwestern Utah into the southern Wasatch and the San Juans in
Colorado. Meanwhile, as the upstream low continues to drop south,
lowering snow levels on Day 3 will help support some locally heavy
accumulations along the southern Sierra Nevada and the highest
southern coastal ranges of California on Wednesday before shifting
to the Mogollon Rim area of AZ Wednesday night.

Jackson