FOUS11 KWBC 080753
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
253 AM EST Sat Feb 8 2025

Valid 12Z Sat Feb 08 2025 - 12Z Tue Feb 11 2025

...Pacific Northwest and Northern/Central Rockies... Day 1...

The end of this active wintry period is finally in sight across the
mountainous West, but some lingering periods of heavy snowfall
will continue today. An upper level trough will track through the
Pacific NW with snow levels starting below 500ft in many cases on
D1 before rising gradually. This would allow for some light snow
accumulations along the I-5 corridor early this morning, 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 are medium
(40-60%) for much of the Cascades for snowfall totals >6".
Otherwise, most areas will likely witness snowfall totals of 1-4"
through this afternoon with the lesser- end of that range most
likely for communities along I-5. This upper trough will also
produce some light snow tonight 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 today before snow tapers off on Sunday.
Additionally, continuing light to moderate snowfall is expected
early this morning across the CO Rockies as a cold front oozes
southward across the Rockies and contributes to favorable upslope
flow and low chances for an additional 6 inches of snow above 9000ft.

...Upper Midwest through the Great Lakes and Ohio Valley... Day 1...

Ongoing storm system will bring a stripe of heavy snow to the Upper
Midwest and Great Lakes region, with mixed precipitation including
freezing rain impacting the Ohio Valley.

Positively tilted shortwave crossing the northern Plains today is
the driving force for an east-west swath of snow D1 from central MN
and central WI through the northern L.P. of MI. This shortwave
will traverse quickly to the east as flow across the CONUS remains predominantly flat and progressive. Despite the modest amplitude
of this feature, 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
(around 700 mb), 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. WPC probabilities (beginning
12z Saturday) show a high chances (60-80%) of heavy snow exceeding
6" from central WI through the northern L.P. of MI.

Farther to the southeast the WAA downstream of a developing
surface low will push a warm nose into the Midwest and Ohio
Valley. This will occur in conjunction with the expanding
precipitation shield, leading to a stripe of mixed precipitation
from central IL 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 northern OH
and IN reaching 10-30%.

...Mid-Atlantic and Northeast... Days 1-2...

After a day of modest CAA and surface high pressure shifting
directly over the Northeast to start the D1 period, a significant
winter storm begins to approach from the west as a potent
shortwave embedded within generally zonal flow across the CONUS
races towards the region. By this afternoon the 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.

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 very 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 are likely to be
very impressive before transition from central/eastern PA to
northern NJ and Long Island. This is depicted well by the 00z HREF,
with non-zero chances for snowfall rates to exceed 2" per hour in
the 00z-06z Sun timeframe across from northern NJ to southern CT
and Long Island, including NYC. Icing could be just as problematic
in the Mid- Atlantic 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. northern suburbs. Probabilities for at least 0.25"
of ice reach 50-60% across western MD and the Laurel Highlands of
PA, where the greatest potential for tree damage and power outages exist.

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 still result in heavy accumulations. WPC
probabilities for more than 6 inches of snow are high (>70%) from
the Mohawk Valley of NY eastward into much of MA, southern VT,
southern NH, and the southeastern tip of ME. 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 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 high risk (70-90%) for 4+ inches of snow southeast of Lake Ontario.

...Central Rockies/Plains through the Ohio Valley and Central Appalachians... Day 3...

Next widespread wintry weather event to spread across the country
is expected to begin late D3 (Monday night) as shortwave energy at
the base of a longwave trough off the coast of southern CA gets
ejected eastward into the fast zonal flow dominating the upper
pattern across CONUS. In response, a strengthening 160kt jet streak
is forecast to stretch zonally from the southern Rockies to the
Northeast and shift eastward with time, placing areas from the
central Plains to the central Appalachians in the favorable right-
entrance region for enhanced upper divergence and lift. Current
WPC probabilities only go through 12z Tuesday, so values are low
(20-40%) for at least 2" of snow for the central Rockies, central
Plains, and central Appalachians, highest across the central
Rockies and central Plains. There is also likely to be a ribbon of
mixed ptype in the form of freezing rain/sleet, but not expected to
have large geographic coverage given weak relatively weak WAA and
not the coldest airmass in place at the surface. Current WPC
probabilities for at least 0.01" of freezing rain are low (10-30%)
and extend from north-central OK to the central/southern
Appalachians where CAD could elevate the freezing rain potential.

Snell/Weiss

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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

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