Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Sun Dec 29 2019

Valid 12Z Sun Dec 29 2019 - 12Z Wed Jan 01 2020


...Central and Northern Great Plains/Upper Great Lakes...
Days 1-2...

...Major winter storm to produce widespread heavy snow across
portions of the northern Plains and light icing across parts of
the upper MS Valley...

A large area of precipitation with snow in ND and the western 2/3
of SD and NE persists today as the low slows down and stalls over
southwest MN, then drifts east overnight.
The prolonged forcing and slow moving system will produce an
extended period of precipitation across the northern Plains.
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 across eastern SD
and North Dakota, including adjacent northwest MN.
The event should wind down as the system weakens Monday. Event
totals of 12-18z inches are still expected over parts of eastern
ND to central SD.

During D2, the secondary low begins to strengthen. The heaviest
snow is expected in northern WI, the U.P. of Michigan, and the
Arrowhead of MN. WPC probabilities are moderate to high for 6
inches. Probabilities decrease further south in MN, WI and MI due
to the possibility of mixed precipitation types. The highest
probabilities for heavy snow are on the southern shore of Lake
Superior, where probabilities for 8 inches are moderate. The event
winds down as the low pressure departs for Canada Monday
night-early Tue.

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

...Significant Icing expected for the
Catskills/Berkshires/Litchfield Hills...

A cold air damming event is likely to produce significant freezing
rain beginning Sunday night and continuing into Monday in the
southern Adirondacks, Catskills of NY and into the Berkshires of
MA and Litchfield Hills of northwest CT, with a secondary max in
the Worcester Hills of MA. 

A warm front lifting northward from low pressure near the Great
Lakes will spread precipitation across the northeast. As this
occurs, a cold high pressure will be entrenched across SE Canada,
building south into New England.

Surface low development 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 Catskills,
Berkshires Southern Greens, and Adirondacks. The highest accretion
is likely in the Berkshires and southern Adirondacks where WPC
probabilities are high for 0.25", and moderate for 0.50".

Further northeast, continued dry/cold advection may allow precip
to change back to sleet/snow.  Thus, there is a bit lower freezing
rain potential in northern NY, southern VT/NH and southern ME, but
potential for accumulating snow and sleet.  Probabilities for 8
inches of snow are low.

On day 3, the surface low will lift northeast along the eastern
New England coast. Moist advection ahead of this feature will
spread significant precipitation into eastern portions of New
England, lasting longer in Maine. Guidance continues to indicate a
changeover to rain is possible in downeast Maine. However, further
inland and into the terrain of northern NH and northern ME, WPC
probabilities are low to moderate for 8 inches of snowfall.


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

A progressive upper trough moves southeast from the northeast
Pacific southeast along the coast or just offshore 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 snow should be
limited. The latest trends are for the low to be weak and a little
further offshore than prior forecasts.  This has reduced event
total potential for the ranges of southern CA.

The probability of 0.25 inches of icing on Day 3 (Tue) is less
than 10 percent.

Petersen