Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 PM EST Thu Feb 6 2025
Valid 00Z Fri Feb 07 2025 - 00Z Mon Feb 10 2025
...Pacific Coast through the Northern Rockies...
Days 1-2...
...Winter Storm impacting the West as a potent low tracks into the
Pacific Northwest tonight and the northern Rockies by 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. A cutoff low centered near Washington State will rejoin
the polar jet and open up into a potent (but less anomalous)
trough. An associated surface low off California coast will move
ashore tonight, tracking northeastward into northern
California/Nevada, southwest Oregon, and into Idaho by Friday
morning (eventually transitioning to a leeside low over Colorado
this weekend). 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 it is fully absorbed into the polar jet.
Global model initializations (12z Thu runs) depict significant a
significant feed of moisture off the Pacific with more than 500
kg/ms of Integrated Vapor Transport (IVT), which still tops the
99th percentile (per NAEFS climatology). 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 are indicated to be
between 3-4k ft. Into the southern Sierras, snow levels will rise
to above 8k ft through the first half of Thursday night, then fall
to around ~5k ft through midday Friday behind the cold front. For
the northern Sierras, snow levels will stay between 6-8k ft for the
bulk of the snowfall, falling to between 3-5k ft by Friday morning
(as snowfall begins to taper off). WPC probabilities for the
Sierras are moderate to high (50-70%) for snow amounts above 24
inches through mid-day Friday. Into far northern California,
probabilities are moderate to high (50-70%) for snowfall amounts of
over 12 inches for the same time period. While snow levels will be
lower, there will not be nearly as much moisture available to
convert to snowfall (as the IVTs are focused farther south of the
front).
From late tonight into Friday morning, the snow will spread
northeastward into Idaho, Montana, and Wyoming as the shortwave and
associated surface low pivot to the northeast (while generally
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. Plentiful moisture from the warm side of the system will
follow the cold conveyor belt into the northern Intermountain West,
facilitating 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 to high (50-70%) for 12
inches or more of snow through late Saturday night.
WSSI values will get into the Extreme range (substantial
disruptions
to daily life, travel not advised) for the southern Sierras with
the heaviest snow through tonight. Moderate to major impacts
(considerable disruptions to daily life, avoid travel if possible)
are expected from the Klamath mountains of northern California
into southern Oregon as well as through the Sawtooth and Wind River
Ranges.
Churchill/Wegman
...Northern Plains through the Great Lakes and Ohio Valley...
Days 1-3...
A potent shortwave will eject from the interior Northwest Friday
evening and begin to race east within flat/progressive flow
entrenched across the CONUS. This shortwave trough will remain
positively tilted through its evolution across the region, but
have plenty of vorticity and accompanying PVA to produce impressive
omega. This will additionally be enhanced by a potent subtropical
jet streak exceeding 150kts which will both additionally enhance
deep layer lift but also enhance IVT (progged to exceed the 90th
percentile of the CFSR climatology into the Plains/Upper Midwest).
This moisture will be additionally enhanced by a surge of
WAA/isentropic ascent, especially along the 285-295K surfaces,
downstream of the parent shortwave. This WAA will manifest as well
as a stripe of enhanced fgen, especially within the 700-600mb
layer. This will additionally improve ascent, and there is likely
to be a region of intense omega driving heavy snowfall rates as the
system moves rapidly eastward. At the same time, the DGZ is progged
to deepen considerably in response to this WAA, which is reflected
by SREF DGZ 100mb depth probabilities reaching 50% from eastern SD
through the Twin-Cities of MN and into central WI. With the column
being quite cold, this could result in fluffy and efficient
dendrites that can accumulate rapidly. However, it should be noted
that the best fgen lies at the top of this DGZ, so may not really
manifest as an efficient "cross-hatch" signature for snow growth,
and with the column so cold large aggregates will be difficult to
achieve. Still, an above-climo SLR seems likely, and snowfall rates
of 1-2"/hr should be supported within this translating swath of
snow.
The progressive nature of the system combined with the slightly
less than ideal snow growth, will somewhat limit total snowfall,
but this will still be an impactful event, and in some areas may be
the heaviest snowfall of the season to date. Current WPC
probabilities are 50-70% for 6+ inches across central MN and into
western WI D2, with generally 3-6" expected on either side of the
core of this snowfall from SD into MI.
Farther to the southeast, the WAA will lift a warm nose more
robustly immediately downstream of a developing surface low into
the Upper Midwest and Ohio Valley, primarily after 12Z Saturday.
This
will cause a stripe of mixed precipitation to develop from
Illinois through eastern Ohio, resulting in another round of
accumulating sleet/freezing rain across these areas. WPC
probabilities for 0.01" of ice are moderate (50-70%) from eastern
IL through eastern OH, with locally more than 0.1" possible
(10-30%) in IN/OH.
Weiss
...Mid-Atlantic and Northeast...
Days 1-3...
The first in a series of winter weather events will be exiting
this evening as a coastal low moves progressively eastward from New
England. As this low departs this evening, it will continue to toss
moisture back to the west on cyclonic flow, resulting in areas of
still moderate snowfall across eastern New England and much of
Maine. However, the progressiveness of the mid-level pattern will
keep this system moving quickly away from the area, and snowfall
associated with it should end before daybreak Friday. Additional
accumulations of 2+ inches is likely across northern Maine as shown
by WPC probabilities reaching 50-70% in the higher elevations.
Behind this low, a potent cold front will race quickly southeast in
its wake, leading to pronounced CAA and briefly impressive low-
level convergence. This will have a two-pronged impact to winter
weather across the Northeast. The first will be an increased threat
for convective snow showers and snow squalls Friday morning,
especially across Upstate NY and into Northern New England.
Although the timing of the front is not ideal for impressive
squalls, there is forecast to be enough overlap of low level RH,
fgen, and instability to support scattered to widespread convective
snow showers and isolated squalls. This is supported by simulated
reflectivity in the high-res CAMs as well as HREF probabilities
showing low-end chances for 1+"/hr rates. Accumulations with any
squalls will be minor, but briefly heavy rates and lowered
visibility could cause hazardous travel.
Then behind this front, CAA causes a drop in 850mb temperatures to
support sfc-850mb delta T nearing 20C across Lake Ontario (Lake
Erie is now completely ice covered). EL's approaching 15,000 ft
over Lake Ontario will help support a single band of lake effect
snow (LES) with rates at times reaching 1-2"/hr beginning Friday
aftn and persisting into Saturday aftn, with some upstream
connection to Lake Huron aiding in the threat. This will likely
lead to some heavy snowfall accumulations, especially across the
Tug Hill Plateau, where WPC probabilities for 4+ inches of snow D1
are above 70%, with an additional 30% chance on D2, leading to LES
total snowfall as much as 12 inches in isolated areas.
A more significant system begins to take shape the latter half of
D2. Despite the generally flat and progressive flow across the
CONUS, a potent shortwave trough ejecting from the Plains will move
into the Ohio Valley by 00Z/Sun, and then continue rapidly east
across New England on Sunday. This will be accompanied by an
impressive upper jet streak progged to reach 170kts as it arcs
across the northern tier of the CONUS. Downstream and to the south
of this overlapping synoptic ascent, a wave of low pressure will
begin to develop along a warm front as it lifts northward in
response to intensifying WAA, leading to an expansion of
precipitation late D2 and moreso on D3 from the Ohio Valley
northeast through the Mid-Atlantic and into New England.
High pressure ahead of this developing low will retreat to the east
Sunday, with the resultant isentropic ascent maximizing along the
290-300K surfaces. Mixing ratios within this lift are quite
impressive, 4+g/kg, which will push a plume of PWs exceeding the
90th percentile according to the NAEFS climatology into the Mid-
Atlantic and Southern New England. As this low skirts east off the
NJ coast, it will likely intensify south of New England, and the
resultant ageostrophic flow and CAA in its wake will halt the
progression of the accompanying warm nose, keeping mixed
precipitation confined to the Mid-Atlantic states. While there is
still quite a bit of latitudinal uncertainty in the placement of
this low and accompanying WAA strength, confidence is increasing
that a significant winter storm will move quickly across the region
D3.
This will result in a swath of moderate to heavy icing, generally
across the Central Appalachians and Mid-Atlantic states where WPC
probs for more than 0.1" of ice are above 70% (10-30% for 0.25" in
the Laurel Highlands and MD Panhandle). Farther north, however,
the threat for heavy snow has substantially increased as impressive
850-600mb fgen overlaps with the ridge of elevated theta-e to
support heavy snow rates and possible banded structures.
Despite the fast moving system, WPC probabilities are high (>70%)
for 6+ inches from near the Capital District around Albany eastward
to the Boston metro area and SW ME. Locally double-digit snowfall
is possible, especially across the Worcester Hills of MA and
Monadnock Region of NH.
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