Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 PM EST Sat Feb 15 2025

Valid 12Z Sat Feb 15 2025 - 12Z Tue Feb 18 2025


...Corn Belt/Midwest/Great Lakes/Northeast...
Days 1-3...

A dynamic storm system, forecast to produce a myriad of weather
related hazards over the next 48 hours, will gather strength over
the Mid-South this afternoon and rapidly intensify as it tracks
northeast through the northeastern U.S. on Sunday. The current
areas of wintry weather affecting the Great Lakes and Northeast are
largely due to 850-700mb WAA and 300K isentropic ascent into an
air-mass that is sufficiently cold enough to produce snow from the
northern Great Lakes to the Northeast. A little farther south,
lingering CAD in place over the northern Mid-Atlantic will be ripe
for a wintry mix of sleet and freezing rain this afternoon and this
evening. By 00Z this evening, the surface low tracking into the
Ohio Valley will begin to deepen rapidly as the 500mb trough aloft
sharpens and the divergent right-entrance region of a >175kt 200mb
jet streak maximizes upper level ascent. 850-700mb WAA will
increase over the Northeast, as will an IVT along the East Coast
Sunday morning that exceeds an impressive 1,500 kg/m/s. According
to NAEFS, these values would surpass all observed IVT values for
this time of year according to the CFSR climatology (1979-2009).
Some of this moisture will wrap around the strengthening 850mb low
in the in the Ohio Valley Sunday morning and result in a period of
heavy snowfall from the Lower Ohio Valley on north and east through
eastern IN, northern OH, and western NY through Sunday afternoon.

Over the Northeast, a banana-shaped area of high pressure will act
to reinforce the CAD signature over New England and the northern
Appalachians. This means despite the growing >0C warm nose at low-
levels aloft, surface temperatures will remain below freezing long
enough to support a lengthy period of freezing rains. This is the
case primarily in the Poconos, Adirondacks, Berkshires, Worcester
Hills, Green and White Mountains, and across southern Maine. The
latter is particularly prone to significant ice accumulations given
surface temperatures will struggle to top the mid 20s through
Sunday evening. Some of the heavier rates may limit ice
accumulations to some extent, but given the exceptionally cold
surface-925mb temperatures in place, expect ice to still accumulate
at a healthy clip Sunday afternoon in New England. By 00Z Monday,
the storm is forecast to be in the low 970mb range with strong
winds bringing about additional hazards from the Great Lakes to the
Northeast. In the Great Lakes, strong CAA will support numerous
lake-effect snow (LES) showers from the Michigan U.P. to the
eastern Great Lakes. Farther south, NWrly flow with ~40kt 850 mb
winds (above the 90th climatological percentile via NAEFS) will
also produce upslope snow enhancement along the spine of the
Central Appalachians through Monday morning. Lastly, for trees and
power lines that have lingering heavy icing in the Northeast,
strong winds will only add further stress to trees and power lines
and could exacerbate the potential for downed trees and power
outages.

The worst impacts of this event are likely to be felt in the
Northeast where the combination of heavy snow, significant icing,
and strong winds will make for dangerous to even impossible travel
conditions, along with the growing concern for numerous power
outages and downed trees. WPC probabilities show moderate-to-high
chances (50-70%) for >0.25" of ice accumulation in the Catskills
and Berkshires. Similar probabilistic guidance exists for >8" of
snowfall in the Adirondacks, Green, and White Mountains, as well as
much of northern Maine. Farther west, there are moderate chances
(40-60%) for >6" of snow from eastern Michigan on south to
northeast IN and northwest OH. As the storm departs, LES will cause
locally heavy snowfall to occur along the Michigan U.P. and along
the Tug Hill in northern NY. The Tug Hill in particular has
moderate-to-high chances (50-70%) for snowfall totals >8" through
Monday night. With very cold temperatures pouring in in wake of the
storm, expect LES to continue over the Tug Hill into Tuesday with
any lingering snowpack sticking around through mid-week.


...Pacific Northwest/Rockies/Northern High Plains...
Days 1-3...

An active Pacific jet stream pattern will direct a stream of mid-
upper level moisture into the northwestern U.S. tonight, through
Sunday, and through Monday. A series of 500mb disturbances embedded
ejecting out of the base of a longwave upper trough in the
northeast Pacific will provide upper-level ascent from the Pacific
Northwest to the Northern Rockies through Monday as well.
Snow levels in the Olympics and WA Cascades will drop to as low as
2,500ft today but rise to above 3,000ft by Sunday. Snow will pick
up in intensity Saturday night and through Sunday in the OR
Cascades, as well as the Blue, Sawtooth, Bitterroot, Absaroka, and
Tetons. This active jet stream pattern will also direct an
anomalous IVT well inland across the central Rockies. NAEFS shows
>200 kg/m/s IVT stretching from the CA coast to the Front Range of
the Central Rockies Sunday afternoon and through Monday morning
(values
that are well above the 90th climatological percentile). This will
support periods of heavy snow in the Bear River, Uinta, Wasatch,
and Rockies of southern WY and western CO. Snow will peak in
intensity across these ranges on Monday and gradually taper off on
Tuesday as a pair of upper level troughs (one in eastern Montana,
the other in the Southwest) track south and east, allowing for more
subsidence and a reduction in moisture to ensue. As much as 1-2
feet of snow are expected across many of these aforementioned
mountain ranges, although the WA/OR Cascades, Blue, Bitterroot,
Teton, and CO/southern WY Rockies are the ranges that could see
localized totals approach 3 feet between this afternoon and Tuesday
afternoon.

Farther east, the active jet stream and persistent stream of
Pacific moisture will track across the High Plains of Montana and
into mountain ranges such as the Big Snowy, Little Belt, and Lewis
Range. However, adding to the heavy snow potential is the placement
of strong high pressure of southern Canada (NAEFS shows >1040mb
high with >97.5 climatological percentiles present) and low
pressure over the the Pacific Northwest will support SErly low-
level upslope flow into these ranges starting Saturday night and
lasting through early Tuesday. While precipitation amounts and
rates will be moderate for the most part, high SLRs of 16-19:1 will
make use of the available precipitation and produce dry/efficient
snowfall over the region. The forecast calls for anywhere from
8-14" in the central Montana, which includes Billings and Great
Falls. In the Big Snowy, Little Belt, Lewis Range, and even as
south as the Big Horns, 1-2 feet of snow is forecast with some
of the peaks seeing as much as 30" through Tuesday.


...Central Plains...
Days 2-3...

The same anomalous IVT that is aiding in the heavy mountain snow in
the Rockies will also accompany a period of 700mb WAA that allows
for a stripe of moderate-to-heavy snow from the Black Hills and
eastern WY through central NE through Monday morning. The band of
snow will stretch as far east as the IA/MO border through Monday
morning before a brief break in the moderate snowfall arrives
Monday afternoon. By Monday night, one of the shortwave troughs
entering the Central Rockies will gradually deepen while upper-
level diffluence increases at the nose of a 250mb jet streak
rounding the base of the trough. By Tuesday morning, that
aforementioned 250mb jet streak will co-locate it's divergent
left-exit region the divergent right-entrance region of the 250mb
jet over the eastern U.S.. Periods of heavy snow will break out to
the north of the mean 850-700mb FGEN axis over eastern OK and
northern AR on Tuesday, although not all guidance is in agreement
with a good consensus on both totals and where the heaviest
snowfall occurs. That said, the setup favors heavy snow and
potentially significant ice accumulations given the favorable
synoptic-scale lift and strong WAA aloft overrunning a very cold
air-mass that will be anchored by a frigid Canadian high pressure
system to the north.

At the moment, WPC probabilities paint moderate-to-high chances
(50-70%) for snowfall totals >6" from central and eastern KS to
south- central MO through Tuesday afternoon. There is the potential
for disruptive ice accumulations from eastern OK on east to the
Ozarks of northern AR and southern MO as well. WPC's Probabilistic
WSSI shows high chances (>70%) for Moderate Impacts (disruptions
to daily life; hazardous driving conditions and closures) in the
areas mentioned above for snowfall, although there could very well
be treacherous travel conditions from eastern OK to the Ozarks due
to accumulating ice. This forecast remains fluid, so continue to
check in for the latest forecast information from your local
weather forecast office.

Mullinax



...Winter Storm Key Messages are in effect. Please see current
 Key Messages below...

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