Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EST Mon Dec 30 2019

Valid 12Z Mon Dec 30 2019 - 12Z Thu Jan 02 2020


...Great Lakes/Upper MS Valley...

Days 1-3...

Widespread precipitation has developed across the eastern 2/3 of
WI and lower MI and is moving north into the UP of MI.  Most areas
are receiving rain, except snow in the UP of MI.  As the mid level
circulation approaches, temperatures slowly drop, allowing a
transition from
rain to snow as the day progresses in northern and central WI.
Across the U.P. and into northern WI, a burst of heavy snow is
looking more likely as the low intensifies and the deformation
band picks up in intensity. 
WPC probabilities for 8 inches are high in much of the western
U.P. and central UP, gradually decreasing further south in
northern WI and MI due to precipitation type uncertainties.

The low weakens and moves east into southeast Ontario on Tue,
bringing the heavy snow event to an end. A cold front progresses
across MI, with a period of post-frontal snow showers and light
accumulations downstream from Lake MI.  Cross Lake Erie low level
wind trajectories are likely to be accompanied by band of low
level convergence in near shore areas in southwest NY, with
changing wind directions limiting potential for heavy amounts, as
the band axis is likely to shift with time.

On Wed, the greater cross lake trajectories are over Lake Ontario
into the Tug Hill, so several inches are expected where the west
component of wind and high moisture level turns upslope into the
terrain. Less snow is expected upstream as a building low level
ridge dries out the column.

...Northeast...
Days 1-2...

...Significant icing likely in the Berkshires, Catskills,
Adirondacks, and southern Green Mountains...

Mid level warm advection occurring over surface cold air has led
to widespread mixed precipitation across NY and New England, with
a couple of waves of precip noted.
Warming centered near 700 mb maintains the threat of sleet across
northern NY, VT, and southern NH and southern ME today.  Interior
Maine should stay cold enough for an extended period of snow, so
several inches are likely.

A transition zone of mixed sleet and freezing rain persists in the
Green Mountains and Monadnocks of NH. Further south, shallower
cold air leads to pockets of continuing freezing rain from the
Berkshires to Litchfield Hills of northwest CT and Worcester Hills
of MA.

While rain rates should be sufficient that accretion efficiency is
limited due to runoff, QPF  today will still allow for freezing
rain accretion of an additional 0.25 inches from portions of the
Adirondacks, Greens, Berkshires, and Catskills.  The event winds
down as higher precip rates move further northeast into Northern
new England.

Low pressure moves northeast to near southeast MA by Tue morning
and then near the Maine/New Brunswick border Tue evening before
departing into the Canadian maritimes .
Snow focuses on low-mid level deformation over interior Maine,
winding down as the low departs into Canada Tue night. Several
inches of snow are expected in interior northern Maine.

Inland and into the terrain of northern NH and northern ME, WPC
probabilities are low to moderate for 8 inches of snowfall.


...Pacific Northwest through the Central Rockies...

Days 1-3...

Confluent flow setting up over the northern Pacific will drive
high PWATs into the northwest, with mid level moisture pooling and
convergence on the nose of the 700 mb jet/in the left exit region
of a 170 kt 300 mb jet to support onshore flow and precipitation
across the ranges of Wa/ID/northwest MT starting late Monday night
and continuing thru Wed. Heavy snow is likely in the northern WA
Cascades at higher elevations, with a few feet expected.

On Wed., veering flow towards the west-northwest allows the pool
of moisture and lift to extend southeast across the OR Cascades
inland through southern ID and western WY, followed by northern
NV, northern Ut, and then northwest CO, where lift is aided by
upper divergence maxima as a trough progresses through. Several
inches are expected across the impacted ranges, with higher
amounts expected due to longer duration snow in the WA Cascades
and Bitteroots of ID.

Probabilities are high for 12 inches in the highest terrain of the
Cascades and Bitterroots through day 2 and then in the WY Tetons
on Day 3, with lower probabilities forecast across the other
ranges.

The probability of 0.25 inches of icing is less than 10 percent
Days 2-3.

Petersen