Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Sun Jan 05 2020

Valid 12Z Sun Jan 05 2020 - 12Z Wed Jan 08 2020

...Pacific Northwest through the Northern Rockies...
Days 1-3...

A pair of shortwaves embedded in fast Pacific flow and jet energy
will bring periods of heavy snow to the terrain of the Northwest
and northern Rockies through Tuesday night. The next trough
arrives into the PacNW early today with a decent plume of Pacific
moisture. The following trough axis reaches the PacNW Tuesday, but
persistent fast flow across the Pacific produces even more
moisture in advance of this feature, with forcing increasing with
the associated height falls. Snow levels remain low (1000 to
2000ft) through Sunday night before rising to 4000 to 5000ft
Monday/Monday night in the next Pacific moisture plume.

Nearly continuous snow with periods of heavy snow is expected
through the three day period for the WA Cascades and Olympics,
with heavy snow spilling into the Rockies and ranges as far south
as WY/CO through day 2. WPC probabilities are high for 12 or more
inches for the WA Cascades/Olympics with 3 day totals likely to
exceed 5 ft in the highest terrain. Day 2 probabilities are
moderate for 12 or more inches over the highest ID/MT Rockies, and
3-day snowfall in these ranges will likely exceed 2 feet.


...Northeast...Central Appalachians...and Great Lakes...
Days 1-3...

A shortwave racing east-southeast from central Canada will combine
with LFQ jet level diffluence to spawn weak cyclogenesis which
will move across or just north of the Great Lakes today before
weakening and lifting away from Maine on Monday. Robust ascent
through height falls, diffluence, and PVA will produce ascent and
snowfall from far Northern Minnesota through Upstate New York.
This feature will be progressive and moisture will be somewhat
limited, but the intense lift will likely produce a swath of
moderate snow, and WPC probabilities for 4 inches are moderate to
high in the eastern U.P. of MI, NW lower Michigan, and ne PA. All
of these areas are locally higher due to Lake enhancement
snowfall. This LES will persist into Day 2 as the low pulls away,
producing heavy snow especially downwind of Lake Ontario and into
the Tug Hill Plateau where WPC probabilities for 6 inches are
moderate on Monday.

A southern stream impulse will eject from near the Ozarks, across
the Southern Appalachians, and then lift off the New England coast
Tuesday, while a northern stream wave digs out of Manitoba at the
same time. Guidance has been very scattered with both the
evolution of these features regarding any interaction, as well as
the track of the associated developing surface low. However,
increasing moisture on WAA into an environment which should be
cold enough for snow, has led to an increase in the potential for
moderate snowfall on D3 from SW Virginia, along the I-95 corridor,
and into Southern New England. WPC probabilities for 4 inches are
only above 30% in the mountains of WV, but there is potential for
heavier snow, especially into the northern Mid-Atlantic and
southern New England late in the forecast period. The I-95 cities
may also experience snow with this system, but attm probabilities
for 1" are confined to the urban corridor north of Philadelphia.

The probability of significant icing (0.25 inch or greater) is
less than 10 percent.

Weiss