Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Mon Dec 04 2023

Valid 12Z Mon Dec 04 2023 - 12Z Thu Dec 07 2023

...Northern New England... 
Day 1...

A continuous E-SE fetch of 925-850mb moisture will be funneled
into New England while an elongated and strung-out upper level
trough resides overhead. The moisture advection is largely due to
the lingering 850mb low in southern Ontario that is approaching
the St. Lawrence River Monday morning and the strong dome of high
pressure over eastern Canada. These factors are keeping modest
moisture around while temperatures remain sufficiently cold enough
to support snow. Periods of snow are forecast to persist this
morning across northern VT/NH and much of Maine with the lingering
upper trough lagging enough to keep light snow in the forecast
through Monday evening. By Tuesday morning, snow will have finally
dissipated and travel conditions will dramatically improve. Latest
WPC probabilities show low-to-moderate chances (30-50%) for most
of central Maine, but there remain moderate chances (50-60%) for 
snowfall totals >4" in the White Mountains.

...Central Appalachians...
Days 2-3...

An Alberta Clipper racing south and east through the Middle
Mississippi Valley Tuesday morning will initially produce light
snow in the central Appalachians Tuesday afternoon and into
Tuesday night. However, as a trailing upper level disturbances
over the TN Valley catches up to the Alberta Clipper, troughing
aloft will deepen and northwesterly 850mb flow will introduce
added moisture flux into the terrain. With temperatures aloft
cooling and a favorably wind regime for upslope enhancement, look
for snowfall rates to increase late Tuesday night and through
Wednesday morning. Snow should taper off by Wednesday evening as
the best forcing and moisture quickly moves off the Mid-Atlantic
coast. Snow accumulations will generally be observed above
2,000ft, but minor totals below 2,000ft are not out of the realm
of possibility. WPC probabilities for >4" of snowfall show
moderate chances (40-60%) along the windward slopes of
east-central WV's Potomac Highlands. The experimental
Probabilistic WSSI shows moderate chances (40-60%) for Minor
Impacts in east-central WV, meaning there could be some hazardous
travel conditions (snow covered roads and reduced visibilities) in
these areas Wednesday morning.

...Northwest...
Days 2-3...

As the next upper level trough approaches the Northwest late
Tuesday into Wednesday, Pacific moisture will stream over the
Olympics, Cascades, and into the Northern Rockies. Snow levels
will initially start out as high as 8,500ft in some cases Tuesday
afternoon, but as the cold front pushes through and 500-700mb
height falls ensue, snow levels will drop to as low as 3,500ft in
the Cascades and 6,000ft in the northern Rockies Wednesday
evening. NAEFS does show an anomalous IVT present with values
above the 90th climatological percentile from northern California
to the northern Rockies. However, the upper trough will be on the
progressive side and become less amplified as it tracks through
the Northwest. At this time, WPC probabilities for snowfall >6" is
highest in the peaks of the Cascades and in the Boise Mountains
where there are low chances (20-40% on average) in these ranges.
The setup bear watching, but this setup does not look to be nearly
as impactful snowfall-wise compared to the atmospheric river
events that occurred late last week.

Mullinax