Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Mon Dec 30 2024

Valid 12Z Mon Dec 30 2024 - 12Z Thu Jan 02 2025

...Pacific Northwest...
Days 1-3...

Onshore flow and a couple impulses of upper level energy will race
across the Pacific Northwest through the day today. While snow
levels will be dropping with cold advection, the amount of moisture
over the area will also lessen with time. Snow levels will drop to
between 2,000 and 6,000 ft from north to south, which may allow
for some light accumulations in the populated valleys. The jet
stream will shift southward with time, which will also gradually
lessen the forcing for snow across much of the higher elevations of
WA/OR/ID/MT from west to east through the day. Thus, the heaviest
snow for today will be over the next few hours of this morning,
with diminishing snow over most of the mountains by tonight.
Heavier snow may persist the longest near Glacier N.P. in
northwestern MT. Quiet weather (other than some light snow at the
highest peaks near Glacier N.P. and into Wyoming) will persist
through the day Tuesday.

The next front will move ashore and into the Cascades starting
Tuesday night. It will push across the Pacific Northwest through
Wednesday night. The heaviest snow through this period will be
through the WA and OR Cascades, where some of the highest peaks of
the southern WA and OR Cascades have an over 80% chance of seeing 4
inches or more of snow through Wednesday afternoon. Through the day
Wednesday snow will move inland, dropping additional higher
elevation shows into northern Idaho and northwest Montana.
Probabilities into ID and MT are between 30-60%, with the highest
probabilities near Glacier N.P.


...North-Central Plains...
Day 1...

A 500 mb shortwave trough riding the polar jet stream will combine
with the LFQ of a 140 kt jet to cause cyclogenesis across portions
of SD and NE today. North of the low center, an expanding
precipitation shield may feature localized bands of heavier snow,
especially over south-central SD through the day today. The surface
low will track southeastward with the progressive shortwave
trough, so any one location can expect a 6-12 hour window of snow
through tonight. The heaviest snow totals are likely in the Black
Hills today where WPC probabilities of snow totals over 4 inches
are over 80% and probabilities of totals over 8 inches are over
50%. This is largely due to the localized upslope expected on the
north facing slopes which will enhance snow totals and rates.
Across the Plains of SD, probabilities of 4 inches are between
30-50% with 10-30% chances of 8 inches or more. As the low moves
into the Midwest late tonight through Wednesday, a lack of colder
air further east will make the predominant precipitation type
become rain, so the wintry threat with this low will diminish
greatly late tonight.


...Northeast...
Days 2-3...

The low tracking over KS/MO today shifts east to the Mid-Atlantic
through Tuesday before shifting up the New England coast Tuesday
night. It's a progressive system, but with the surface low
translating to the coast around Long Island then strengthening as
it tracks northeastward, there is a threat for banding on the back
side of the low as colder air is pulled in from Canada. Very warm
(for this time of year) air is in place even into interior New
England presently. By Tuesday night, the approaching precipitation
from the southwest with colder air close behind may encounter some
lingering cold air in the sheltered valleys of northern NH and
western ME. Thus, the precipitation may begin as freezing rain
before changing over to snow Wednesday. WPC probabilities for 0.01"
of ice are between 40-60%, primarily Tuesday night, for northern NH
and much of western ME.

Marginal thermals should be overcome at higher elevations over the
Adirondacks and Green/White Mountains esp >1500ft. To the west,
colder air will rush in behind the system across the still mild
Great Lakes and support some lake-enhanced/effect snow downwind of
Erie/Ontario on NW flow Wednesday thru the end of this forecast 00Z
Thu. Sufficient moisture will also be present over the central
Appalachians (eastern WV into the Laurel Highlands) for some
upslope snow. WPC probabilities for at least 4 inches of snow are
moderate (30-60%) there on the west side of the system as it
retreats to Canada. Over New England, higher probabilities ~50-70%
exist over parts of the higher elevations >2500ft in the
Adirondacks, along the northern spine of the Green Mountains, and
into the White Mountains in NH into northwestern Maine where banded
snow is more likely as the low scoots across the Gulf of Maine
into eastern Maine Wednesday afternoon. Snow will continue into
interior New England through Wednesday night. Meanwhile
lake-effect
and upslope snow will keep light snow going through Wednesday night
from western New York and northeast Ohio through the mountains of
western Pennsylvania and West Virginia.

Wegman