278
FOUS11 KWBC 071824
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
124 PM EST Fri Feb 7 2025

Valid 12Z Fri Feb 07 2025 - 12Z Mon Feb 10 2025

...Pacific Coast through the Northern/Central Rockies...
Days 1-2...

The end of this active wintry period is finally in sight, but some
lingering periods of heavy snowfall will continue through the first
half of the upcoming weekend. As the more potent of two upper level
troughs track east this evening, periods of heavy mountain snow are
anticipated from the Bitterroots, Tetons, Bear River, and Wasatch
ranges on east through the Big Horns on south to the central
Rockies of southern Wyoming and Colorado through tonight. This is
largely due to the strong upper-level ascent via jet streak
dynamics aloft paired with IVT values (200-300 kg/m/s) that are
above the 99th climatological percentile. Most of the snowfall will
have concluded by Saturday morning (excluding the central
Rockies). WPC probabilities show the Tetons, Wasatch, and central
Rockies having high chances (>70%) for receiving >4" of snowfall
through Saturday morning with the taller and more remote peaks of
the Colorado Rockies sporting low-to-moderate chances (30-50%) for
>8" of snow.

Farther west, another upper level trough will track through the
Pacific Northwest with snow levels below 500ft in many cases
tonight and into Saturday. This would allow for some light snow
accumulations along the I-5 corridor, while <10th climatological
percentile 500-700mb heights could support steeper lapse rates that
result in some snow squalls (particularly east of the Cascades)
during the day. This upper trough is generally moisture-starved,
however, which should limit snowfall amounts to lighter overall
totals. WPC probabilities show some portions of southwest WA and
far northwest OR along the windward sides of the Cascades that
sports moderate-to-high chances (40-70%) for snowfall totals >6".
Otherwise, most areas will likely witness snowfall totals of 1-4"
through Saturday afternoon with the lesser-end of that range most
likely for communities along I-5. This upper trough will produce
some light snow Saturday night across portions of the northern
Rockies, but totals will pale in comparison to the snowfall the
region has witnessed over the past 5-7 days. Minor accumulations
of generally 2-6" in the Blue, Bitterroot, and Lewis Ranges are
forecast through Saturday night before snow tapers off on Sunday.


...Northern Plains through the Great Lakes and Ohio Valley...
Days 1-2...

Fast moving but significant storm system will bring a stripe of
heavy snow to the Northern Plains and Upper Midwest, with mixed
precipitation including freezing rain impacting the Ohio Valley.

This system will spawn from a potent but positively tilted
shortwave emerging from the Northern Rockies this evening. This
shortwave will traverse quickly to the east as flow across the
CONUS remains predominantly flat and progressive, lifting across
the Northern Plains by Saturday morning, and into the Great Lakes
Saturday evening. Despite the modest amplitude of this feature,
which has trended flatter the past several runs, it will work in
tandem with the favorably placed left exit of a 150 kt jet streak
to produce a brief period of intense ascent from west to east.
Additionally, this jet streak will help surge IVT into the area,
and combine with increasing 285-295K isentropic ascent to produce
elevated PWs favorable for moderate to heavy snowfall.

The WAA from the south accompanying this isentropic ascent
downstream of the shortwave will help to deepen the DGZ, and the
SREF probabilities are above 90% for 100mb of depth, which could
support impressive snowfall rates approaching 1"/hr as progged by
the WPC prototype snowband tool. With the wave being generally flat
and fast, and the most impressive fgen lying at the top of the DGZ,
ascent within the best snow-growth region won`t maximize
efficiently for the most intense snow rates leading to heavy
snowfall accumulations. However, SLRs should still generally be
above climo in the cold column, and with rates around 1"/hr at
times in the translating fgen band, a stripe of heavy snow
exceeding 6" still appears likely from MN through WI as reflected
by WPC probabilities of 50-70% within this axis.

Farther to the southeast, especially after 12Z Saturday, the WAA
downstream of a developing surface low will push a warm nose into
the Upper Midwest and Ohio Valley. This will occur in conjunction
with the expanding precipitation shield, leading to a stripe of
mixed precipitation from Illinois through eastern Ohio. There is
high confidence in this mixed precipitation region, although some
uncertainty continues in the exact p-type transition. Still, the
mixture of sleet and freezing rain will create ice accumulations
that could exceed 0.1" as reflected by WPC probabilities across OH
and IN reaching 10-30%.


...Mid-Atlantic and Northeast...
Days 1-3...

A potent cold front will be offshore New England to start the
period, leaving pronounced CAA in its wake. 850mb temps falling to
around -15C within this CAA will help steepen lapse rates across
Lake Ontario (Lake Erie is now frozen) to support increasing
instability and rising inversion levels. Unidirectional W to NW
flow across Lake Ontario within this environment is supportive of a
single band of heavy lake effect snow (LES), focused along and
south of the Tug Hill Plateau and just north of the Finger Lakes
region. Snowfall rates of 1-2"/hr are likely within this band until
it erodes Saturday afternoon, leading to snowfall accumulations for
which WPC probabilities indicate have a 50-70% chance of exceeding
4 inches.

As the LES wanes, a much more significant system begins to approach
from the west as a potent shortwave embedded within generally zonal
flow across the CONUS races towards the region. By 18Z Saturday
/late D1/ this shortwave will be moving across WI/MI/IL,
accompanied by a 150kt upper jet streak. Some interaction of this
upper level ascent with a warm front draped across the Mid-
Mississippi Valley will help spawn a wave of low pressure in the
lower Ohio Valley. As this low begins to consolidate, downstream
WAA will intensify and manifest as 290-295K moist isentropic
ascent surging northeast reflected by mixing ratios exceeding
4g/kg. This will produce PWs that may briefly exceed the 97.5th
percentile of the CFSR climatology according to NAEFS, highest into
the Central Appalachians and Mid-Atlantic to start D2.

During this evolution, precipitation will rapidly expand as
southern stream and northern stream energy begins to interact
across the Mid-Atlantic states. This will provide increased ascent
across the region, and a plume of snow, sleet, and freezing rain
will overspread from the Ohio Valley and Central Appalachians
northeastward. The impressive accompanying WAA will push a warm
nose steadily northward through 06Z Sunday, so even though many
areas, especially north of the Mason Dixon line, will begin as a
burst of snow, p-type should transition pretty rapidly to sleet and
then freezing rain, eventually changing to rain Saturday night in
lower elevations south of I-80. Snowfall rates may be briefly
heavy before transition, but icing could be more problematic as
reflected by WPC probabilities for 0.1+" of ice that reach 50-90%
in the Central Appalachians D1, extending northeast at 30-50% D2 as
far as central PA and in the vicinity of Washington, D.C.

As the parent shortwave continues to track progressively to the
east, the accompanying forcing and moisture will also pivot with
it, and secondary low pressure development is likely off of the NJ
coast late Saturday night into Sunday morning. This low will become
dominant and intensify as zonal but modestly coupled jet structures
interact with a baroclinic zone offshore. As this low deepens and
moves almost due east south of New England, it will cross near the
Benchmark (40N/70W), leading to a substantial snow event for
Upstate NY, the Poconos, and at least Southern New England (SNE),
although there remains uncertainty into how far north (or south)
the heaviest snow will spread. As the low deepens and pulls away to
the east, a renewed surge of cold air both through the
ageostrophic flow into the low and the subsequent CAA in its wake
will limit the progress of the warm nose, while dry air to the
north noted in regional soundings inhibits PWs from aggressively
lifting towards Canada. In between, an axis of heavy snow is likely
as strong WAA produces impressive 850-600mb fgen, producing strong
ascent into the DGZ. The overlap of this strong fgen with some
theta-e lapse rates approaching 0C/km suggests banding potential
from Upstate NY into SNE, and with elevated SLRs, snowfall rates of
1-2"/hr appear likely. Total snowfall will be limited by the speed
of this system as it will only snow in most areas for 12-18 hours,
but this will result in heavy accumulations, and WPC probabilities
D2 for more than 6 inches of snow are high (>70%) from near Albany,
NY eastward into MA, southern VT, and southern NH. Locally 8-12" of
snow is possible in some areas.

As this low pulls away Sunday afternoon and drying occurs rapidly
in its wake, snowfall will wane quickly late D2. However, renewed
CAA behind the accompanying cold front will again setup an
environment favorable for LES E/SE of Lake Ontario. Shifting winds
(more N early, becoming more W late D3) will likely cause multi-
bands along and south of Lake Ontario into the Finger Lakes region,
before organizing into a more significant single band late in the
forecast period back into the Tug Hill Region. Confidence in the
exact location is low at this time, but WPC probabilities do
feature a moderate risk (30-50%) for 2+ inches of snow southeast of
Lake Ontario.


Weiss/Mullinax



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 Key Messages below...

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