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

Valid 00Z Tue Dec 31 2024 - 00Z Fri Jan 03 2025

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

Light snow remains possible across the northern Rockies through
tonight following an exiting shortwave progressing into the middle
Mississippi Valley. The next surge of moisture and mountain
snowfall is forecast to enter the Pacific Northwest to start Day 2
(Tuesday night into Wednesday) as a weak front moves ashore and
into the Cascades. This front is then anticipated to stall and
begin lifting northward across Oregon on Day 3 (Thursday) as a
warm front. The heaviest snow through this period will be
throughout the OR Cascades (generally above 4000ft), with some of
the highest peaks having 60-80% probabilities of seeing at least 8
inches of snow through Wednesday night. Low probabilities (10-20%)
for at least 8 inches also extend into the WA Cascades and inland
across the northern Rockies as moisture spreads inland, but overall
not expected to impact a large area or populated low elevations.


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

For the first part of the day 1 forecast period (tonight) locally
heavy snow is possible over portions of SD, NE,and western IA. A
500 mb shortwave trough riding the polar jet stream combined with
the LFQ of a 140 kt jet spawned cyclogenesis across portions of SD
and NE today. North of the low center, an expanding precipitation
shield features localized bands of heavier snow, which are expected
to continued and slide eastward tonight. The surface low will
track southeastward with the progressive shortwave trough, so any
one location can expect a 6-12 hour window of snow tonight. The
heaviest additional snow totals beginning 00z tonight are most
likely over far northeast NE, where WPC probabilities for at least
4 inches of snow are 20-30%. Snowbands probability tracker (SPT)
from the 12z HREF depicts the potential for 1-2"/ hr rates across
eastern NE and western IA through about 05z Tuesday, which could
help accumulate snowfall where low-level thermal profiles start out
above freezing. As the low moves into the Midwest on Tuesday, 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 until reaching the Lower Great Lakes by
Tuesday night.


...Great Lakes/Northeast/Central Appalachians...
Days 2-3...

The low tracking over KS/MO today shifts east to the Lower Great
Lakes on Tuesday before a secondary triple-point low becomes the
predominant surface feature on Wednesday as it lifts north from
the Mid-Atlantic to Maine. Lake-effect snow machine kicks off on
Wednesday as the deepening low exits into southeast Canada while
upslope flow into the central Appalachians also promotes the
potential for heavy snow into the elevated terrain of WV, western
MD, and southwest PA through Thursday. Overall the system is
progressive, 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. A trend for a stronger 850 mb
circulation tracking north through the Saint Lawrence River Valley
also prompts the potential for bands of heavy snow across northwest
NY and the elevated terrain of VT. Very warm (for this time of
year) air is in place even into interior New England presently,
which will initially lead to most precipitation starting as rain
outside of the highest terrain of the Adirondacks.

Marginal thermals should be rapidly be overcome at higher
elevations over the Adirondacks and Green/White Mountains esp
>1500ft Wednesday into Wednesday night. To the west, colder air
will rush in behind the system across the still mild and mostly
ice-free Great Lakes and support some lake- enhanced/effect snow
downwind of Erie/Ontario on NW flow Wednesday until more typical,
but prolonged lake-effect snow continues through Thursday.
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 total
snowfall through Thursday are high (80-90%) in the elevated
terrain of northern New England, including along the northern spine
of the Green Mountains, into the White Mountains in NH into
northwestern Maine, and the Adirondacks where banded snow is more
likely as the low scoots across the Gulf of Maine into eastern
Maine Wednesday afternoon. Probabilities for at least 8 inches of
total snowfall are high (70-80%) downwind of lakes Erie and
Ontario, with moderate probabilities (40-60%) of at least 4 inches
across the Laurel Highlands of southwest PA and terrain of western
MD and WV.


The probability of significant icing greater than 0.10" is less
than 10%.

Snell