Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EST Sat Dec 28 2019

Valid 00Z Sun Dec 29 2019 - 00Z Wed Jan 01 2020


...Central and Northern Great Plains/Upper Great Lakes...
Days 1-3...

...Major winter storm to produce widespread heavy snow and light
to moderate icing across portions of the northern Plains...

Surface low pressure deepening across Kansas will lift northeast
as a full latitude trough begins to close off across the Central
Plains. Deep layer ascent through coupled jet streaks and height
falls will allow this low to strengthen as it shifts into
Minnesota Sunday morning. As this occurs, the upper low will begin
to stack vertically, causing the low to stall and retrograde back
towards the Dakotas during the latter portion of D1, as it
occludes to a triple point near Michigan. The primary low will
eventually weaken during D2, as the Great Lakes low becomes
dominant. This feature will then meander slowly in the vicinity of
the Great Lakes as yet a third triple point low develops near NJ
during D3.

The prolonged forcing and slow moving system will produce an
extended period of precipitation across the Plains and Great
Lakes. Strong moist advection will wrap cyclonically around the
primary low within a deep and robust TROWAL, to combine with
intensifying deformation to produce a swath of heavy snowfall from
northern Nebraska into northern Minnesota. Snowfall rates may
reach 1"/hr a times, but in general will be of moderate intensity.
However, the long duration event as the low moves slowly will
cause this TROWAL/deformation axis to pivot almost in place,
producing a high probability for 12 inches of snow where this
occurs. This is most likely in eastern SD, where WPC probabilities
are more than 50% for 12 inches. High probabilities for 4 inches
cover an expansive area on D1 from northern NE all the way to the
Canada/Minnesota border.

During D2, the secondary low begins to strengthen, cutting off the
moisture tap into the primary low. Persistent snow will occur in
the eastern Dakotas, but the heaviest snow will gradually
transition into WI, the U.P. of Michigan, and the Arrowhead of MN
associated with the secondary low. In this area, WPC probabilities
are moderate to high for 6 inches. By D3, the primary energy
shifts well away from the area, but moderate snow will likely
continue, especially downwind on the southern shore of Lake
Superior and eastern shore of Lake Michigan where WPC
probabilities for additional 4 inches are high.

Storm total accumulations may exceed 18" in isolated locations
across the eastern Dakotas.

South and east of the primary snow axis, mixed precipitation is
likely in the form of freezing rain. The heaviest freezing rain is
likely in a narrow band from near the Buffalo Ridge northeast into
northern WI and the Arrowhead of MN. While some of this freezing
rain will be due to phase change as a warm nose tops the surface
cold layer, there will also be a region of drying DGZ into MN
causing p-type to change from snow to light freezing rain or
freezing drizzle.

 
...Northeast...
Days 2/3...

Cold air damming event is likely to produce significant freezing
rain beginning Sunday night. A warm front lifting northward from
low pressure near the Great Lakes will spread precipitation on WAA
into the Northeast. As this occurs, a cold high pressure will be
entrenched across SE Canada, and the column will initially be cold
enough for snow. After a brief period of snow, which may
accumulate to a few inches in central New England, a warm nose
will race northward causing p-type transition to sleet and
freezing rain, and eventually rain far southern portions.

Mid-level confluence to reinforce the surface high will initially
be strong, but eventually is expected to erode as shortwave
ridging approaches from the west. At the same time, surface low
development near NJ moving south of New England will intensify the
cold ageostrophic drainage to persist wet-bulb cooling effects
long enough that damaging freezing rain is becoming more likely.
Despite rain rates which could be moderate and should lead to
lower accretion efficiency, there is a high probability for
greater than 0.25" of accretion in the terrain of the Worcester
Hills, Litchfield Hills, Catskills, Southern Greens, and
Adirondacks. The highest accretion is likely in the Berkshires and
southern Adirondacks where WPC probabilities are high for 0.25",
and isolated amounts near 0.75" are possible.

Further northeast, continued dry/cold advection may allow precip
to change back to sleet/snow lowering the freezing rain potential
into southern VT/NH and northeast MA, but WPC probabilities remain
moderate to high for 0.1" of freezing rain in these areas, as well
as the Poconos of PA.

By day 3, the surface low will lift northeast and begin to deepen
in the Gulf of Maine. Moist advection ahead of this feature will
spread significant precipitation into eastern portions of New
England and much of Maine. Guidance continues to feature a large
spread in the track of this low, but it appears it will be close
enough to the coast that some changeover to rain is possible along
the coast of Maine. However, further inland and into the terrain
of NH and ME, WPC probabilities are moderate to high for 6 inches
of snowfall.


...Pacific Northwest across California...
Day 1-3...

A progressive upper trough moves southeast from the northeast
Pacific southeast along the coast of CA. Upper divergence maxima
are forecast along and in advance of the upper trough. The
combination of this forcing and moisture will bring a period of
snowfall along the Cascades of OR, and down through the Siskiyous
and Sierra Nevada mountains of CA, before eventually spreading
into the transverse ranges of southern CA late on D2 /Sunday night
into Monday/. This system is likely to remain progressive to the
south so the duration of heavy snow should be limited. However,
robust forcing and ample moisture is sufficient that WPC
probabilities for 6 inches or more are moderate to high across
most of the terrain. 

Weiss