Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 PM EST Sat Dec 14 2024

Valid 00Z Sun Dec 15 2024 - 00Z Wed Dec 18 2024


...Western U.S...
Days 1-3...

A trio of storm systems will make for an active stretch of wintry
weather to much of the West through early next week.

An elongated 500mb trough tracking through the Northwest will
direct the IVT responsible for the heavy snow in the Sierra Nevada
today into the northern Rockies this evening. According to NAEFS,
the IVT is ~100-200 kg/m/s in parts of the Intermountain West by
00Z this evening, which is above the 90th climatological percentile
from Southern CA to as far north and east as Wyoming. As snow
winds down in the Sierra Nevada by tonight, this moisture source
will work in tandem with broad diffluent flow at 250-500mb aloft to
support periods of snow across the northern Rockies through Sunday
afternoon. Farther west, persistent onshore flow will result in
more mountain snow in the Olympics and Cascades before high
pressure builds briefly Sunday afternoon. Through 00Z Monday, WPC
PWPF shows moderate-to-high probabilities (50-70%) for snowfall
totals >6" in the WA/OR Cascades at/above 5,000ft. The Tetons and
Big Horns have the best odds (high chances, or >70%) for snowfall
totals >8" with the tallest peaks of the Tetons likely receiving
between 1-2 feet of snow. It is worth mentioning that portions of
the Absaroka, Little Belt, and Big Snowy Mountains in Montana also
have moderate-to-high chances (50-70%) for snowfall totals >6". The
WSSI shows Moderate Impacts in parts of the Tetons and Big Horns
through Sunday afternoon.

By Sunday night, the brief reprieve in the Pacific Northwest ends
as yet another Pacific storm system approaches the region. This is
due to a shortwave trough breaking off from its parent upper low
located in the northeast Pacific. Mountains snow from the Olympics
and Cascades on south to the Siskiyou and Trinity/Shasta begin
early Monday morning, peaking around late morning in intensity, and
snowfall rates diminishing to a degree by Monday afternoon. The
upper low will then make its way towards the region Monday night,
albeit weakening on approach. Still, a healthy plume of 850-700mb
moisture will allow for more heavy mountains snow in the Cascades
through Tuesday morning before snow levels rise above 5,000ft
Tuesday afternoon. The WSSI-P shows moderate probabilities
(40-60%) for Minor Impacts for most elevations below 4,000ft
through the duration of this multi-day event. The Oregon Cascades
will likely pick up anywhere between 1-3 feet of snow through
Tuesday afternoon.

These pair of disturbances will track east into the Northern
Rockies as well. The first arrives Monday afternoon as the
divergent left-exit region of a 250mb jet streak moves over head,
helping to maximize upper level ascent. In fact, with both Pacific
systems will help to co-locate that divergent left-exit region of
the 250mb jet streak through Tuesday afternoon. With sufficient
Pacific moisture aloft and some additional upslope enhancement as
well, this will be a snowy Monday and Tuesday from the Blue
Mountains on east through the Boise, Sawtooth, Bitterroots, Lewis
Range, Absaroka, and Teton Mountains. The Blue, Sawtooth, and
Tetons are the most likely mountain ranges to see snowfall totals
of 1-2 feet with localized amounts topping 2 feet through Tuesday
afternoon.

...Central Plains into the Upper Midwest...
Day 1...

An icy wintry mix on the northern flank of a closed 500mb low will
track through portions of the Midwest this afternoon and into the
Great Lakes tonight. Modest 850mb WAA and 290K isentropic glide via
southerly low level flow will overrun a boundary layer air-mass
that contains wet-bulb temperatures at or below freezing. While
some minor snow accumulations are possible in east-central
Wisconsin, this setup favors sleet/freezing rain the most. Once the
worst of the ice concludes over eastern Iowa this evening, freezing
rain will be most problematic tonight in Michigan where surface
temperatures are coldest thanks to residual snow pack,
particularly
in western Michigan following recent lake-effect snow. WPC
probabilities show low-to-moderate chances (30-50%) in parts of
western and northern Michigan through Sunday morning. A light
wintry mix is expected to linger across the northern half of
Michigan's Mitten through midday Sunday before finally concluding
Sunday evening. West-central Michigan does feature Minor Impact
potential on the WSSI due to Ice Accumulation tonight and into
Sunday morning.

...Central Appalachians...
Days 1-2...

The same closed low responsible for the icy conditions in the Great
Lakes will direct its plume of low-mid level moisture at the Mid-
Atlantic tonight and into Sunday. The Northeast and Mid-Atlantic
are both witnessing a record-breaking dome of high pressure
overhead today and into tonight that is anchoring a sub-freezing
air-mass through Sunday. Accumulating freezing rain is possible as
early as this evening in the Smokeys and Blue Ridge where easterly
low-level winds direct Atlantic moisture into these ranges, while
simultaneously fostering upslope enhancement of precipitation
rates. By Sunday morning, the plume of moisture arrives in the
Central Appalachians most areas starting off as snow, but as the
burgeoning 850-750mb warm nose arrives, should force snow to
changeover to an icy mix of sleet/freezing rain from parts of
western PA on east through the Laurel Highlands and Allegheny
Mountains Sunday afternoon. Freezing Rain will be common as far
south as the Potomac Highlands and even parts of the Blue Ridge in
northern Virginia. How long freezing rain lingers is dependent
upon how long cold-air damming can remain wedged into the nooks and
crannies of the valleys of the Mid-Atlantic and Northeast. Periods
of snow are likely farther north in New York State and interior
New England while ice is possible from the Susquehanna and Delaware
Valleys to parts of the Hudson Valley Sunday night. This could
make for an icy Monday AM commute for portions of northern NJ,
southern NY (excluding NYC) and southern New England.

Latest WPC probabilities suggest low-to-moderate chances (30-50%)
for ice accumulations >0.25" in parts of western MD and into the
Laurel Highlands. It is likely that an expansive swath of minor
ice accumulations envelope much of the interior portions of he
Mid-Atlantic with the Central Appalachians most favored for
hazardous ice. From the Smokeys of NC on north along the Blue Ridge
and into the Alleghenys, WPC probabilities show moderate-to-high
chances (50-70%) for >0.1" of ice. It is worth noting that WPC
probabilities for >0.01" of ice are high (>70%) from the Catoctins
and Parr's Ridge of northern MD through southeast PA and into
northern NJ. While unlikely to cause significant problems, these
accumulations could still result in slick travel conditions for
these areas Sunday night.


...Northern Plains...
Days 1-3...

The upper level shortwave trough responsible for the excessive
rainfall and heavy snow across northern California will race
through the Intermountain West tonight and into the Northern
Plains Sunday morning. Low pressure tracking east through eastern
Montana will see weak WAA at low-levels overrun a sub-freezing air-
mass that results in minor ice accumulations in northeast Montana
and far western Montana. As the storm tracks east through North
Dakota Sunday afternoon and into northern Minnesota by early Monday
morning, the storm will strengthen and snow will wrap around the
western flank of the storm system. Snowfall rates may be generally
light, but a tightening of the pressure gradient will cause
blustery winds to take shape by Monday morning. The WSSI-P shows
low-to-moderate chances (20-40%) for Minor Impacts along the
ND/northwest MN border along Canada due to a combination of Snow
Amount and Blowing Snow through Monday morning.

As this system departs, a quick moving wave of light snow may track
across South Dakota and into southern Minnesota on Tuesday. WPC
probabilities are not impressed with probabilities >10% for
snowfall totals >4" at the moment, but this setup will have some
banded precip potential along the northern flank of a warm front
and the region's location beneath the divergent left-exit quadrant
of an approaching 250mb jet streak. These setups have been known
to produce quick 1-3" swath of snowfall that could result in
locally hazardous travel conditions, so this will be monitored in
subsequent forecast cycles.


Mullinax