Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 PM EST Mon Dec 9 2024

Valid 12Z Mon Dec 09 2024 - 12Z Thu Dec 12 2024

...CO Rockies/Sangre de Cristos...
Day 1...

The upper level disturbance responsible for the ongoing snowfall
throughout the region will linger through this afternoon and into
the evening hours. High pressure to the north aiding in modest
NErly upslope flow at low levels will also help to generate low-
level ascent into parts of the Front Range and on south along the
Sangre De Cristo. Latest Doppler Radar showed a potent snow band
diving south along an area of strong 700mb FGEN that is producing
1-3" of snowfall south of Castle Rock. This band has caused
hazardous travel conditions along I-25 as it tracks south through
central Colorado. This band will work through the Pueblo area soon
and then head for the Raton Mesa this evening. WPC probabilities
are high (>70%) for at least 4 inches of snow around the Palmer
Divide southward to Raton Pass. The highest and more remote
elevations could see localized amounts around 8-10" by the time the
snow concludes Tuesday morning. Lighter amounts are expected
through the I-25 corridor but could be heavier around Raton Pass.
The WSSI does depict Minor Impacts along I-25 from Pueblo on south
to
Raton, NM, suggesting winter driving conditions are expected and
caution should be exercised while driving.


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

A shortwave trough approaching the Mid-Atlantic is working in
tandem with a warm front to advect Gulf of Mexico moisture into the
Northeast. These atmospheric features are providing sources of
lift aloft to create a slug of precipitation that will fall in the
form of a wintry mix over the Northeast. In terms of snowfall,
sub-freezing temperatures through the depth of the atmospheric
columns will hold on the longest from the White mountains on north
and east through Maine this evening. These areas will ultimately
succumb to the burgeoning >0C warm nose at low-levels and
transition to an icy wintry mix later tonight. With high pressure
wedged between two areas of low pressure (one in Ontario and
another near the Massachusetts Capes) early Tuesday morning, cold
air damming (CAD) will remain entrenched over Maine and allow for a
combination of wintry mix and freezing drizzle to persist through
Tuesday night. WPC probabilities show moderate-to-high chances
(10-30%) for ice accumulations >0.1" in parts of the Adirondacks,
Green, and White Mountains. Most areas in northern New England
could see anywhere from a glaze to as much as 0.1" of ice through
Tuesday afternoon. In terms of snowfall, only the peaks of the
White Mountains sport at least low chances (10-30%) for >4" of
snowfall while minor amounts are possible in lower Maine.


...Central Appalachians...
Days 2-3...

As a sharp 500mb trough takes on a negative tilt on Wednesday,
strong divergence and height falls will result in falling snow
levels Wednesday afternoon. Periods of rain will transition over to
snow in the central Appalachians from the Laurel Highlands on south
to the central Appalachians of West Virginia at the same time low
level winds shifting out of the northwest support upslope
enhancement. Snow will be heaviest through early Thursday morning
at elevations at/above 2,000ft before tapering off later in the day
Thursday. WPC probabilities show moderate-to-high chances (40-70%)
for snowfall totals >4" from the Laurel Highlands and western
Garrett County, MD on south into east-central WV. Elevations above
3,000ft are on the higher side of those listed >4" probabilities
with low chances (10-30%) for localized amounts >6" through
Thursday morning.


...Northern Plains...
Day 2...

The leading edge of the encroaching Arctic air-mass will track into
the Northern Plains on Tuesday. Surface-based warming out in front
of the Arctic front and strong CAA aloft is allowing for lapse
rates in the 500-700mb layer to be >7.5C/km in some cases. There
will also be just enough low-level moisture for snow squalls to
develop. These squalls could form as far north and west as eastern
Montana and western North Dakota Tuesday morning then race south
and west into South Dakota Tuesday afternoon and the Missouri
Valley by Tuesday evening. Squalls could even continue to track
into parts of Iowa, northern Kansas, and northern Missouri Tuesday
night. Snow amounts will be light, but these squalls can lead to
rapid reductions and visibility. Blowing and drifting snow are
possible, along with accumulating snow on roadways where road
temperatures drop below freezing.


...Great Lakes...
Days 2-3...

The jet stream pattern over North America becomes highly amplified
as an expansive ridge along the west coast of North America helps
direct a highly anomalous upper level trough into the Upper
Midwest Wednesday evening. NAEFS shows 500mb heights over Wisconsin
that are below the 1st climatological percentile while the mean
250-500mb trough axis is negatively titled over the Great Lakes
Wednesday night. An arctic front linked to this potent upper
trough will back winds to northwesterly as bitter cold air advances
into the region (850mb temps -15 to -25C). This will reinvigorate
the
lake-effect snow (LES) machine Wednesday evening and into the
second half of the work-week. WPC probabilities of at least 12
inches of snow are high (>70%) northwest Michigan, from Erie
County, PA on northeast along I-90 into the southern Buffalo metro
area, and in the Tug Hill. Snowfall rates of 1-2"/hr (surpassing
3"/hr at times) are likely in the more intense bands. The WSSI-P
is showing moderate chance probabilities (40-60%) for Moderate
Impacts (disruptions to daily life; closures, hazardous driving
conditions, some detrimental impacts to infrastructure) between
Erie and the southern Buffalo suburbs on Thursday.


Mullinax