Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 AM EST Thu Feb 6 2025

Valid 12Z Thu Feb 06 2025 - 12Z Sun Feb 09 2025

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

...Winter Storm to impact the West as a potent low tracks across
California tonight, and the northern Rockies on Friday...

A stormy pattern across the West will finally come to an end late
this week as the upper level pattern resets to a much drier
pattern.
In the upper levels, the previous cutoff low centered near
Washington State will rejoin the main jet stream and open up into
a
potent trough. This upper level low has quite a bit of shortwave
energy circulating around it. This shortwave energy will support
cyclogenesis off the California coast. The surface low will track
northeastward over California and into Idaho, before transferring
its energy to a leeside low over Colorado. The upper level trough
will be weakening through late Friday night, however the surface
low
will tap into quite a bit of Pacific moisture which will be moving
into the Pacific Northwest. The low-turned-trough will start out
positively tilted but as it moves into the interior Pacific
Northwest, it will become more neutrally tilted as its energy
merges
with the jet stream it is rejoining.

NAEFS continues to show this system will have a significant
Pacific
feed of moisture with more than 500 kg/ms of Integrated Vapor
Transport, which still tops the 99th percentile. Snow levels will
rise ahead of the low but lower both behind it and to the north of
the track. NBM snow levels into Oregon and Idaho will be around
3,000 ft. Into the southern Sierras, snow levels will rise to
above
8,000 ft through the first half of Thursday night, then will drop
to
around 5,000 ft through midday Friday behind the cold front. For
the
northern Sierras, snow levels will stay between 6,000 and 8,000
ft.
WPC probabilities for the Sierras are low to moderate (30-50%) for
snow amounts above 24 inches. Into far northern California,
probabilities are moderate to high (50-70%) for snowfall amounts
of
over 12 inches. While snow levels will be lower, there will not be
nearly as much moisture available to convert to snowfall.

From late Thursday night into Friday morning, the snow will spread
northeastward into Idaho, Montana, and Wyoming. This will occur as
the upper level shortwave trough forcing the surface low causing
the
snow lifts to the northeast and continues weakening. Meanwhile, a
1030+ mb Arctic High will remain in place from Alberta and
Saskatchewan and into the Northern Plains. This high will supply
plentiful cold air to keep all of these areas as snow. Once again
however, plentiful moisture from the warm side of the system will
follow the cold conveyor belt into the northern Intermountain
West,
allowing for plenty of moisture for widespread snow. The greatest
snow in this area will be into the Sawtooths of Idaho, Bitterroots
of Montana and into the Wind River Range and Yellowstone/Teton
N.P.
areas of Wyoming. WPC probabilities in this area are moderate (40-
60%) for 12 inches or more of snow through Saturday morning.

WSSI values will get into the Extreme range (substantial
disruptions
to daily life and very difficult to impossible travel) for the
southern Sierras with the heaviest snow today into tonight.
Moderate
to major impacts (Disruptions to daily life and avoid travel) are
expected from the Klamath mountains of northern California into
southern Oregon as well as through the Sawtooth and Wind River
Ranges.

Wegman


...Northern Plains through the Great Lakes and Ohio Valley...
Days 1-3...

A fairly potent shortwave trough can be seen over the southern
Canadian Parries moving eastward with robust ascent focused within
the diffluent axis of the trough. Lift is currently strongest
across Northern MN into the Arrowhead north of Lake Superior.
Forecast is for the disturbance to continue advancing eastward with
the strongest forcing centered over Western Ontario into the U.P.
of Michigan by the morning. Additional snow accumulations >4" are
likely in-of the central and eastern side of the U.P. with the
latest WWD probs ranging between 50-80% within the corridor south
of Superior. Small section of elevated probs (40-70%) also exists
within the Keweenaw Peninsula as well, a combination of synoptic
based forcing with the potent shortwave, as well as some lingering
lake effect later this afternoon.

After a brief respite on D1 across the Northern Plains and Upper
Midwest, the progressive flow across much of the CONUS will surge
another winter storm system eastward into the Northern Plains to
begin mid-D2. This system is progged to be much more impressive as
a potent driving trough ejects from the Pacific Northwest and into
the Northern Plains late-Friday night into Saturday morning. The
overall mid- level pattern across the Northern Plains is progged to
remain generally flat, but rapid height falls and a potent
subtropical jet streak reaching 155kts will help push IVT
exceeding the 90th percentile from NAEFS into the region. At the
same time, strengthening WAA downstream of a low pressure moving
across the Central Plains will draw higher moisture poleward, and
the overlap of this WAA with the LER of the upper jet streak will
produce a stripe of impressive 850-600mb fgen, efficiently lifting
ascent into a deepening DGZ. This will result in a swath of snow
from the Dakotas through Wisconsin, with an embedded band of
heavier snow likely mainly intersecting the enhanced 700mb
frontogen being depicted within all viable deterministic as of 00z.
Spread has decreased among the deterministic with impressive probs
for >6" being depicted by the National Blend (70-90%). Consensus
among ensembles within the 00z GEFS/ECENS combo suggest high
confidence for this event and impactful snow is likely across the
Dakotas, especially northeastern SD and into Minnesota/Wisconsin.
WPC probabilities have remain elevated given the latest trends,
and now support a high risk (>70%) for more than 6 inches D2 over
northeast SD through south-central MN and central WI, surrounded
by a larger expanse of 40-60% from north-central SD through east-
central WI. High probs >70% continue into D3 with emphasis over
central WI over to the Lake Michigan shores, including the Green
Bay area to the southern portion of the Door Peninsula.

Farther to the southeast, the WAA will lift a warm nose more
robustly immediately downstream of the surface low into the Upper
Midwest and Ohio Valley. This will cause a stripe of mixed
precipitation to develop on D3 from Illinois through eastern Ohio,
and WPC probabilities increase once again, now featuring a 40-70%
chance for at least 0.01" of ice over central IL through much of
the northern half of OH.

Weiss/Kleebauer


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

Progressive moving shortwave analyzed over southern IL/LN will
continue moving to the east-northeast with several smaller mid-
level perturbations ejecting downstream of the mean trough. At the
surface, cyclogenesis in-of Cape Hatteras has transpired with the
expected low to progress rapidly to the northeast with modest
strengthening as it approaches LI and Southern New England by
later this morning. Further north, a formidable 1030mb high will
continue to shunting eastward with the previous cold air filtering
shutting off with prevailing easterlies likely occurring east of
the Piedmont creating a textbook case of WAA nosing through a very
shallow cold air mass. By morning, mixed precipitation will be the
dominant ptype over the Northern Mid Atlantic with the depth of the
cold air hanging on across New England and Upstate NY where snow
will remain the primary ptype through the front part of D1. Higher
elevations across New England will be the last areas to see any
changeover from snow to IP/ZR, or at least have significantly
degraded snow ratios due to the acceleration of warm air poleward
into the area as southwesterly flow within the 850-650mb layer
contributes to the mixed precipitation phases during the first half
of the period. Maturation of the surface cyclone off the coast of
New England should help maintain the cold air long enough within
the confines of interior ME and Northern New England where snow
will maintain the ptype through the period before the system moves
out into the North Atlantic during the back half of the forecast.
WPC probabilities for >4" remain elevated (40-70%) within those
higher terrain areas of the Tug Hill, Adirondacks, Green and White
Mtns., and the Southwest terrain over ME. Ice accumulations have
trended fairly low with some prospects of 0.01-0.1" of ice
accretion over much of PA through the Hudson Valley of NY and
Southern New England. These areas should remain outside warning
criteria, but there will still be some lingering hazards within
those locales that receive any ice accretion.

A sharp cold front will maneuver through the Northeastern U.S. by
D2 creating a locally enhanced zone of LES off Lake Ontario with
the primary fetch settled between flow between 270-290 deg.
creating a persistent band between Oswego up towards Pulaski and
areas downstream. The southern portion of the Tug Hill will be the
benefactor of the LES development with general persistence in the
band as it oscillates within that narrow corridor. Considering
delta-T's nearing 20C off the lake behind the strong CAA regime,
the band EL's between 15-20k ft will be sufficient for producing
rates between 1-2"/hr at peak intensity within the band offering a
narrow stripe of 6+" during the D2 time frame before the flow
settles and the band falls apart. WPC probs within the anticipated
axis is between 40-60% for >4" with a very small area of ~70% just
downwind of the Lake near Pulaski.

Finally by D3, a potent shortwave trough across the Plains and
Midwest will continue to advancement eastward with a surface low
spawning on the southern periphery of the trough axis across KS/MO,
quickly moving east-northeast during the D2 into D3 period. A
strong southwesterly WAA regime within the 850-600mb layer will
transpire once again, only this time will generate a much more
significant outcome of snowfall into New England. Very strong
850-700mb frontogenesis will accompany the setup with a deep enough
entrenched cold airmass over the Hudson Valley up through New
England to support a significant snowfall back over the region.
Surface low over the plains will move into the Ohio Valley with an
expected surface low transfer off the Delmarva coast creating a
prevailing east to northeast flow within the PBL that should create
enough of a barrier for the WAA pattern further south penetrating
much further than the NY state line across southeastern NY state.
This has been signaled among much of the latest deterministic with
a fair amount of ensemble support for this synoptic evolution. WPC
probabilities are very robust for >4" with 70+% encompassing all of
Western NY state through Central NY and the Hudson Valley. The
higher probs extend well east into all of Southern New England with
80% probs focused just to the west of the Boston Metro. Probs for
>8" are sufficiently within the 40-70% range across the Hudson
Valley with the epicenter of the highest probs located over the
Catskills and the Berkshires of western MA. Lower probs for at
least 4" also exist over a large area surrounding the above zones,
so there is a growing potential for an impactful snowfall occurring
during the D3 time frame which includes much of Saturday into
Sunday morning.

Ice potential will also exist further south as the CAD wedge over
the Mid-Atlantic will likely see repeating setup with WAA within
the PBL causing issues concerning ptype transition from frozen to
liquid hydrometeors. Similar to this past setup, areas northwest of
the fall line in the Mid-Atlantic within the Piedmont will have the
best chance for ice accretion after a short instance of SN/IP as
the storm enters mid-D3. Highest probs for >0.1" of ice accretion
still exists over the Laurel Highlands down into Western MD
(40-60%), the same areas that are currently contending with ice
this evening. It remains a fluid situation as guidance has not
settled on the timing of the mixed phase to predominantly liquid
ptype, but impacts are anticipated to be very similar to what is
currently occurring.

Kleebauer


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

<a href="https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png"><u>https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png</u></a>

<a href="https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png"><u>https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png</u></a>