FOUS11 KWBC 120808
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Wed Feb 12 2025

Valid 12Z Wed Feb 12 2025 - 12Z Sat Feb 15 2025

...Mid-Atlantic... Day 1...

...Winter Storm Ongoing Across the Mid-Atlantic with Impacts from
Significant Icing the Primary Concern Today. Key Messages linked below...

Heavy precipitation is expected to wind down across the Mid-
Atlantic early D1 (12z Wednesday) as a weak surface low exits
eastward into the Atlantic along with the direct IVT. However,
lingering light snow and freezing drizzle is possible during the
day as the lower levels fail to dry out entirely due to persistent
mid-upper level flow riding overtop the surface CAD signature.
Then, as the central U.S. storm strengthens from a neutral turning
negatively tilted upper trough, IVT restrengthens and even becomes
extreme (maxing out the NAEFS climatological percentiles for 06z
into western NC) by tonight. This allows for an increase in both
WAA and precipitation this afternoon and evening across the
central Appalachians and Mid-Atlantic. Given the very strong WAA,
precipitation type will mostly fall as freezing rain and a bit of
sleet outside of some brief snow possible in northern MD/PA.
Freezing rain will also confine to areas right along the Blue Ridge
and Central Appalachians quickly this evening. For western NC and
SW VA this will only add to the already reported significant ice
accretion from the prior day. WPC probabilities for an additional
0.25"+ of freezing rain are moderate (40-60%) across far western
NC, SW VA along the Blue Ridge, and along the hilly/mountainous
terrain following the VA-WV border. Additionally, low probabilities
(<10%) exist across far western MD and the Laurel Highlands of PA.

...Central Plains to Great Lakes... Days 1-3...

...Winter Storm continues across the Central Plains and Midwest
before spreading into the Great Lakes today. Heavy lake-effect
snow is expected downwind of Lake Ontario beginning Thursday night...

An upper shortwave ejecting out of the central Rockies this morning
will take on a negative tilt and provide ample broad lift within
strong southwesterly flow across the central U.S. directed at the
Great Lakes. Additionally, a southwest-northeast oriented jet is
expected to place the system into the left exit region, with a
precipitation shield beginning this morning between CO and IA
before making it's way into the Great Lakes this evening. With
lake- enhancement on the northeasterly winds, there may be locally
heavier snow and higher accumulations southwest of Lake Michigan,
which may include the Chicago and Milwaukee metros. Of course, this
will be highly dependent on where any embedded bands associated
with the storm can line up with the lake, resulting in the
localized enhancement. Blowing and drifting of snow in the strong
winds is likely right along the lake shore. Additional enhancements
are possible along the Lake Huron shoreline into Wednesday night
for the same reason...a rather unusual direction for Lake Huron
enhanced snowfall. Snow ratios will differ drastically depending on
location, with extremely favorable DGZ and moist columns along the northern/northwest side of the system (GID confirmed 29:1 ratios at
midnight CST), with less favorable conditions closer to the
surface low center. This may help enhance snowfall totals across
the northern half of the precipitation shield.

On the southern side of the comma-head region of the low, enough
warm air is expected to move in around the center of the low to
result in an area of mixing/freezing rain just south of the swath of
snow from central Oklahoma early this morning across much of
southern Missouri, including the St. Louis area, then extending
northeast across central Indiana, including the Indianapolis area,
and into far northwestern Ohio, including the Toledo area. Each of
these areas, and of course those in between, have the potential to
see some light to moderate icing, up to 1/4 inch. Once again the
timing of any icing should be shorter than areas further east,
which should limit any icing accumulations and thus, the magnitude
of any impacts.

WPC probabilities for ice are moderate (40-60%) for 1/10 inch of
accumulation from northeast of Indianapolis northeast through
Toledo, OH. Probabilities are low (5-30%) for 1/10 inch of ice from
south of St. Louis northeast through Indianapolis. For additional snowfall, probabilities for 6 inches or more are low (10-30%) for much of
northern Missouri through eastern Iowa and northern IL, then
increase back to the moderate category (40-60%) along the
lakeshore in Wisconsin due to lake- enhancement from Milwaukee
south to northern Chicago, as well as for much of eastern Michigan
north of Detroit (highest probs (>60%) in eastern Lower Michigan).

Following the passage of the surface low across the Lower Great
Lakes at the beginning of D2 (12z Thursday), strong high pressure
building into the Mid-Mississippi Valley will prompt strong west-
northwesterly flow over the cool, but mostly open Great Lakes
(around 26% ice cover). After a brief period of favorable
conditions for snow showers forming off lakes Superior and Michigan
early D2, focus turns to Lake Ontario Thursday through Friday
afternoon as -15 to -20C 850mb temperatures pass over the area. WPC probabilities for at least 8" are high (70-80%) from the southeast
lakeshore through southern Oswego county, NY. Snowfall totals
locally up to a foot are possible.

...Northeast... Day 1-2...

The system coming from Michigan Thursday morning will bring
widespread precipitation from SW to NE over the Northeast.
Downstream of this system, moist advection will maximize in
response to low-level WAA on S/SE flow overrunning a retreating
high pressure that will try to wedge back to the west into New
England. This will result in an expansion of wintry precipitation
Wednesday night through Thursday with a wintry mix expected across
the Northeast with the far northern NY border east of Lake Ontario
and northern Maine getting the most snow. In fact, for northern
Maine a favorable setup for a lifting WAA band of heavy snowfall
appears likely on Thursday supporting snowfall rates up to 2" per
hour. Model cross sections depict strong 850-700mb fgen at the
bottom of a deep DGZ, supporting heavy QPF and efficient snow
ratios. Day 2 snow probs for >4" are limited to the Tug Hill,
Adirondacks, Greens, Whites, and interior Maine with 70% for >8" in
northern Maine. Day 1-2 ice probs for > 0.1" are 20-30% in the
Poconos, Catskills, and Berkshires into the Litchfield Hills.

...Sierra Nevada and much of the West... Days 1-3...

An expansive, positively-tilted trough lingers over the west
into today with northwest flow persisting over CA. A shortwave
impulse rides along the central CA coast early this morning
producing areas of light to locally moderate precip with snow
levels around 3000ft in the southern Sierra Nevada and around
4000ft in the SoCal transverse and peninsular ranges. Then, a
stronger low shifts toward the OR/CA border tonight and into D2
especially, opening into a trough as it drifts east over the Great
Basin through D3. A strengthening and increasingly northerly jet
shifts toward the West Coast Thursday night. An atmospheric river
with IVT around 500 kg/m/s shifts over the CA coast Wednesday night
through Thursday. A wave of moderate to topographically enhanced
heavy precip shifts east with the accompanying warm front tonight
through Thursday. Snow levels over the Sierra Nevada are initially
3000ft Wednesday evening, but quickly rise to 6000ft by Thursday
morning. Day 2 snow probs for >18" are high along the length of
the Sierra Nevada with 4-6ft forecast for the High Sierra through
Friday. Farther north into the Shasta/Trinity region and Oregon,
pronounced cold air will be slow to retreat, with snow levels
generally rising only from 2000ft to 4000ft through Thursday with
lower precip rates than in the core of the AR over CA. Day 2.5 snow
probs for >12" are 50-80% for the Shasta/Siskiyou/Trinity Mtns and
Day 3 snow probs for >8" are 40-70% for the OR Cascades.

The lingering cold air brings light lowland snow potential for
much of the PacNW with an appreciable threat for freezing rain
mainly Thursday over northwest OR up along the Coastal Ranges. Day
2 ice probs for >0.1" are 40-67% for mainly the Coast Ranges west
of the Willamette Valley.

Moisture quickly spreads east throughout the Intermountain West
Thursday through Friday ahead of the slow moving trough axis. Day
2-3 snow probs for >8" are above 70% for the Wasatch and Tushar
Mtns in Utah, western slopes of the CO Rockies, as well as into the
Sawtooths of ID and western WY ranges.

...Upper Midwest and Great Lakes... Day 3...

As the upper trough crosses the Intermountain West and Rockies on
Friday, it prompts a strengthening southwesterly oriented jet
streak across the Plains by late D3 and places the Upper Midwest in
the favorable left-exit region. Additionally, model guidance
suggests an inverted surface trough and potent axis of 700 mb WAA
may allow for additional lift beginning across the Upper Midwest
Friday before sliding eastward into Lower Michigan by Saturday
morning. Current WPC probabilities for at least 4 inches are
moderate (40-70%) across much of central/southern Wisconsin. There
remains some spread in the amplitude of the western trough and
influence from northern stream energy across Canada, so more
forecast refinements are likely in the coming days.

Snell

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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

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

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