Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EST Sat Jan 04 2020

Valid 12Z Sat Jan 04 2020 - 12Z Tue Jan 07 2020

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

Several shortwaves embedded in fast Pacific flow and jet energy
will bring periods of heavy snow to the terrain of the Northwest.
The first of these will come onshore this morning, pushing a cold
front inland beneath it. High PWAT air will be shunted onshore
orthogonally into the terrain, aiding ascent which will already be
robust through jet level diffluence and mid-level height falls. A
second shortwave trough is progged to follow quickly behind the
first, reaching the PacNW coast Sunday morning with additional
forcing and moisture. A third impulse is likely to approach the
coast near the end of D3, but strong confluent mid/upper level
flow ahead of this final feature will drive PWATs again to
anomalously high values, supportive of heavy snow in the terrain.

While snow may not be continuous through the period, especially
east of the Cascades, periods of heavy snow are likely from the
Olympics of Washington eastward, spilling over as far southeast as
the Tetons of WY, the northern Wasatch of UT, and into the
Colorado Rockies. WPC probabilities are high for 12 inches over
the 3 days from the Northern Rockies, into the Bitterroots, Blues,
and Tetons. The heaviest snow is likely in Washington, however,
where the strongest and most persistent overlap of forcing and
moisture occur. On each of the individual 3 days, WPC
probabilities for 12 inches are moderate to high, and total
snowfall may exceed 4-5 feet in the highest terrain.

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

Southern stream energy phasing with a northern stream shortwave
over the Mid-Atlantic will spawn surface low pressure development.
This surface low now has better model agreement that it will move
quickly E/NE south of Long Island, NY, near Cape Cod, and then out
to sea south of the Gulf of Maine. This rapid and suppressed track
is due to later evolution of a negative tilt to this phased
trough, and keeps the surface low relatively weak as it moves
eastward.

With this weaker solution, moisture is more limited and thus
snowfall accumulations are expected to be modest. WAA today will
cause precipitation to overspread upstate NY and New England, but
the column will initially be too warm for snow except in the
Adirondacks and northern New England. As the low pulls away
tonight, cold air will wrap back down towards the coast and precip
should transition from rain back to snow everywhere. Guidance
indicates that a modest fgen band may try to develop on the NW
side of the low near the SE Maine/SE NH/NE MA coast late, but
accumulations should be light, and WPC probabilities above 30% for
4 inches are confined to the Adirondacks and terrain of VT/NH/ME.

Meanwhile, strong northwesterly winds and the passage of the
northern stream shortwave will support snows along the central and
southern Appalachians and Cumberland Plateau -- with several
inches possible Saturday night, especially along the Allegheny
Mountains in West Virginia, where WPC guidance continues to show
higher probabilities for accumulations of 4-inches or more.
Isolated accumulations above 4 inches are also possible in the
Smoky Mountains along the NC/TN border.

Dry conditions across much of the Northeast during the day on
Sunday will be followed by the return of wintry weather as the
leading shortwave impacting the West over the weekend moves across
the Northeast Sunday night and Monday, and is accompanied by
modest LFQ jet streak diffluence.  A period of strong westerly
flow across Lake Ontario late Sunday into Monday is expected to
support lake enhancement, and WPC probabilities for 4 inches are
high only for the Tug Hill Plateau.

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

Weiss