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

Valid 00Z Mon Dec 30 2019 - 00Z Thu Jan 02 2020


...Northern Great Plains/Great Lakes...
Days 1-3...

Continuation of the major winter storm already underway will
produce heavy snow and moderate freezing rain across portions of
the Northern Plains and Great Lakes through Tuesday.

Primary low pressure has occluded over Minnesota, while a
secondary low is developing along a triple point near Illinois.
This secondary low will rapidly become the dominant feature and
intensify as it lifts north/northwest through Monday. As this
occurs, precipitation will gradually wind down across the Plains,
but increase in coverage and intensity in the Great Lakes. This
increase will be due to strong moist advection wrapping
cyclonically around the secondary low, and within an intensifying
secondary TROWAL. Across the eastern U.P. of MI, the warm nose
will overtop sub-freezing surface temps, and some drying within
the DGZ will both contribute to a period of freezing rain before
changing over to snow. In this area, WPC probabilities are
moderate for 0.1". Further west across the remaining U.P. and into
northern WI, a burst of exceptionally heavy snow is looking more
likely as the low intensifies and the deformation band overlaps
with the TROWAL. Modest frontogenesis pivoting around the low will
also lead to increased omega, and precip will quickly change from
rain to snow, with snow rates potentially exceeding 3"/hr at times
Monday morning. While the longevity of this intense snow should be
short, continued moderate snow is likely until the low eventually
pulls away to the east Tuesday. WPC probabilities for 8 inches are
high in much of the western U.P. and into northern WI, and 12-18"
are possible.

As the low pulls away, vorticity lobes and mid-level fgen will
rotate around the occluding secondary low, bringing periods of
heavy snow down through the remaining Great Lakes. This will be
accompanied by increasing CAA to produce a period of LES as well.
WPC probabilities for 4 inches are high along the eastern lake
shore of Lake Michigan on D2, with additional accumulations also
likely downwind of Lake Erie and Ontario into D3.

 
...Northeast...
Days 1-3...

...Significant to Damaging icing likely in the Berkshires,
Catskills, and Adirondacks...

A warm front lifting northward from a low pressure near the Great
Lakes will spread precipitation into the Northeast, overrunning a
cold dome of surface high pressure centered north of Maine. This
created a classic CAD signature, with mid-level confluence to
support the surface high only gradually reducing as shortwave
ridging approaches from the west. While precipitation may
initially start as snow/mix, and all snow further north, a warm
nose will rapidly lift northward on the strong WAA. However,
surface temps will likely remain below 0C as strong wet bulb
affects continue on the isallobaric drainage of dry dewpoints to
the north, especially in the terrain. This suggests a prolonged
and heavy freezing rain event in the terrain of NY, MA, CT, and VT.

While rain rates should be sufficient that accretion efficiency is
limited due to runoff, copious QPF during Sunday night and Monday
will still allow for freezing rain accretion which will likely
exceed 0.5", and may exceed 0.75" in isolated locations of the
Adirondacks, Catskills, and Berkshires. WPC probabilities for 0.5"
are 40-60% in the Adirondacks and Berkshires, with high
probabilities for 0.25" extending into the Catskills and even
portions of the southern Greens, Worcester Hills, and Litchfield
Hills. The warm nose may waver back to the south as surface low
pressure intensifies south of Long Island, and this could cause
p-type to transition to sleet at times, especially in VT and
northern MA, but otherwise a heavy damaging freezing rain event is
likely.

As the low strengthens off Long Island it is likely to move
northeast into the Gulf of Maine Tuesday into Wednesday. 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 through the Central Rockies...
Day 3...

Confluent flow setting up over the northern Pacific will drive
high PWATs into the west to support onshore flow and precipitation
across the northwest and diving through the Central Rockies.
Moisture increases across northern WA late Monday night with high
Relative Humidity and lift persisting through Tue across northern
WA and then extending inland across ID and northwest MT and then
digging towards UT/WY/CO Tuesday.  The longer duration overlap of
high humidity and lift combines with upslope flow to produce
likely heavy snow in the terrain from the WA Cascades through the
Northern Rockies, and southward into the ranges of NW WY, the
Wasatch of Utah, and the Rockies in Colorado. WPC probabilities
are high for 12 inches in the highest terrain of the Cascades and
Bitterroots, with lower accumulations expected across the other
ranges. Snow levels are expected to be generally 3000-4000ft
across much of the area Tuesday.


Weiss