Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Sun Dec 16 2018

Valid 00Z Mon Dec 17 2018 - 00Z Thu Dec 20 2018


...Sierra and Pacific Northwest to the Northern Rockies...

A progressive shortwave trough is forecast to move into the West,
delivering a brief period of snow with a potential for locally
heavy accumulations from Olympics, Cascades and Sierra to the
northern Rockies late Sunday into early Monday.  This system will
move quickly to the east, with precipitation ending across the
Sierra by late Monday.

Heavy precipitation will begin to redevelop across the Northwest
as the leading edge of moisture-laden fast westerly flow moves
into the region.  Snow levels are forecast to fluctuate,
increasing early Tuesday ahead of a mid-level shortwave nearing
the coast, before lowering later in the day as the wave moves
onshore.  WPC Day 2 probabilities (ending 00Z Wednesday) indicate
that heavy amounts are likely across portions of the Washington
Cascades - with a High Risk for amounts for accumulations of a
foot or more along the western slopes above 4000 ft.  WPC
probabilities also indicate there is significant potential for
locally heavy amounts across portions of the northeast Oregon
mountains as well as the central and northern Idaho ranges.

Snows are expected to continue into early Wednesday before waning
late in the day as a strong upper ridge begins to build along the
West Coast.  WPC probabilities indicate that two-day totals ending
late Wednesday will likely exceed two feet for portions of the
northern Cascades.

...Northeast U.S...

Southern stream low presently centered near the Mid-Atlantic coast
will continue to assume a negative tilt as it moves further north
Sunday evening.  This will foster the intensification of its
associated surface low as it tracks north along the Northeast
coast late Sunday into Monday.  Over the past 24 hours, model
consensus has trended a little bit slower and closer to the coast
with the track of low -- supporting heavier snow totals across
portions of northern New England, particularly across central into
eastern Maine, where WPC Day 1 probabilities now show a Moderate
Risk for amounts of 4-inches or more.  While significant ice
accumulations are not excepted, a wintry mix resulting in some
light icing is expected along the southern edge of the heavier
snow amounts.  This system is expected to track into Atlantic
Canada late Monday, with light snows wrapping back into eastern
Maine.

Pereira