Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
508 PM EST Fri Dec 16 2022

Valid 00Z Sat Dec 17 2022 - 00Z Tue Dec 20 2022

..Significant winter storm for the Northeast through Saturday with
lake effect snow through Sunday...

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

To the southeast of the Great Lakes upper low, a mid-level
shortwave beneath a 160kt jet streak has continued to spur
deepening of a coastal low now moving through the NY Bight. Strong
upper ridging near the Davis Strait will block the coastal system
from gaining too much latitude as it moves through the Gulf of
Maine early Saturday and eventually near the coast of Nova Scotia
Sunday. Strong easterly flow (impressive U wind anomalies <1st
percentile) into Maine will support upslope into the northern
Appalachians with heavy snow rates > 1"/hr overnight into Saturday
per the WPC Snowband Tool. Due to the stronger low-level flow,
terrain enhancements and shadowing will be quite prevalent, and
coupled with marginal temperatures (near to just above freezing in
many valleys but sub-freezing in the mountains), the snowfall will
be quite exaggerated this evening between the Hudson/Champlain/CT
River Valleys vs the Green/White Mountains, Berkshires,
Adirondacks, etc. However, as the system starts to draw in colder
air overnight, rain or a mix will change to snow from northwest to
southeast and into lower elevations. WPC probabilities of at least
8 inches of snow over the next 2 days will be high (>70%) over
interior Maine, and moderate (>40%) over the Green and White
Mountains as well as most of Maine away from the coastal plain.


...Great Lakes...
Days 1-3...

Upper low over the Great Lakes will continue to promote a large
area of light snow but gusty winds associated with the old parent
low of the Northern Plains blizzard. In addition, as it slowly
moves eastward across Michigan and into Ontario, westerly flow
will invigorate multi-band lake effect snow over western Lower
Michigan tonight/Saturday. West-southwest flow over the eastern
lakes will favor a single band over and just south of Buffalo
where 2"/hr snowfall rates are likely. On Sunday, the band will
shift a bit southward as the upper low passes by 80W in Canada.
Off Lake Ontario, single band into the Tug Hill will start farther
north (just south of Watertown) Sunday on W to WSW flow before
sinking southward by Monday as the flow becomes more W to WNW. WPC
probabilities for at least 12 inches of snow are high (>70%)
within these bands where three-day totals may exceed two to three
feet.


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

Upper low and trailing vort max on its western/southwester side
will settle into southwestern Canada and across the border into
WA/ID/northwestern MT. This will drive an arctic front southward
with much colder air into western Washington as an increasing
upper jet provides broad scale lift over the region, promoting
generally light to modest precipitation to the region. Westerly
flow will maximize upslope into the Cascades with additional lift
across the Idaho panhandle and into northwestern Montana where
more than six inches of snow is likely. Snow levels will drop to
the valley floor over much of western Washington with some light
accumulation possible in the lowlands.



Key Messages for December 9-17 Winter Storm:
-Heavy snowfall rates of 1-2 inches/hour spread from eastern
Upstate New York through central New England tonight with an
additional 6 to 12 inches of snow for portions of the Adirondacks,
Green, and White Mountains. These rates will occur across much of
interior Maine late tonight through Saturday morning with storm
totals there of 1 to 2 feet. Dangerous travel conditions and
scattered power outages are expected in this heavy, wet snow.

-Heavy lake-effect snow will develop tonight across parts of
western New York south from Buffalo, and the Tug Hill Plateau.
Major impacts due to heavy lake-effect snow will begin Saturday
and continue through Sunday across these areas.

Fracasso