Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
218 AM EST Fri Dec 6 2024

Valid 12Z Fri Dec 06 2024 - 12Z Mon Dec 09 2024

...Great Lakes and Northeast...
Days 1-2...

Departing area of low pressure over Atlantic Canada and modest
high pressure over the Mid-MS Valley will maintain cyclonic flow
across the Great Lakes, especially off Lakes Erie/Ontario, for the
next day or two. A shortwave within the broad trough over the
region will help back the flow enough to instigate another strong
single band off Ontario into the Tug Hill between Watertown and
Syracuse this afternoon/evening and overnight. Robust snowfall
rates of 1-3"/hr (per the HREF) will help deposit another 12-18"+
of snow over favored areas before waning on Saturday. Downwind of
Lake Erie, multi-band streamers will result in a larger area of
light to moderate snow with WPC probabilities of at least another
6" of snow >50% from Erie, PA northeastward to the Chautauqua Ridge
in NY.

Another shortwave rounding the base of the longwave trough will
slip through the Great Lakes late Sat into early Sun with generally
light snow for the U.P., northern Lower MI, into NYS and northern
New England. WPC probabilities for at least 4 inches of snow are
generally low (10-40%) except for areas with lake enhancement and
parts of the Adirondacks, Green, and White Mountains where
probabilities are moderate (40-70%).


...Pacific Northwest/Northern Rockies...
Days 2-3...

An upper low currently south of the Gulf of Alaska will move
eastward over the next day or so, aimed toward British Columbia.
Ahead of it, a plume of moisture will be directed into the Pacific
Northwest starting early Saturday. Snow levels will start quite
high (around 8000-9000ft at precip onset very early Saturday) but
fall to around pass level (~4000ft) late Saturday into the
overnight hours. East of the crest, cold air in place at the
surface with overrunning WAA-driven precip supports a wintry mix of
snow/sleet/freezing rain for eastern WA. Some icing is possible,
from a few hundredths to around a tenth of an inch.

A second surge in moisture will occur on Sunday associated with
the parent upper low as it opens up across the region. With snow
levels already around pass level, snow should accumulate on Sunday
with WPC probabilities for at least 6 inches of snow >50% above
about 3500ft or so.

Farther east, moisture will spread across northeastern OR and into
the northern Rockies from late Saturday through the end of the
period early Monday. Snow levels will lower through time, and
nearly all areas except the lowest valleys will change from rain to
snow. WPC probabilities for at least 8 inches of snow days 2-3 are
>50% above about 5000ft or so, mostly in the Lewis Range and
across northern ID, the Blue Mountains, and into northwestern WY.


...Northern Minnesota and northeastern North Dakota...
Day 3...

Eastern extension of the northern Rockies system will manifest
itself as another upper low forming by Sunday over southern
Saskatchewan. That will move east-southeastward into early Monday
with WAA-driven precipitation to its east from northeastern ND
eastward across northern MN. Ptype will be an issue with milder air
overtopping the colder boundary layer, especially where precip
arrives near/after sunset Sunday. NBM ptype uncertainty maximizes
between all 4 types in a zone along the U.S.-Canadian border though
the majority of probabilities are either rain or snow. Still,
cannot rule out some light icing but will have time to narrow the
forecast with time. WPC probabilities for at least 4 inches of snow
are moderate (40-70%) over northern MN.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Fracasso