• Major Winter Storm Discus

    From Dumas Walker@21:1/175 to All on Tuesday, April 02, 2024 15:19:00
    FOUS11 KWBC 021924

    Probabilistic Heavy Snow and Icing Discussion
    NWS Weather Prediction Center College Park MD
    324 PM EDT Tue Apr 2 2024

    Valid 12Z Tue Apr 02 2024 - 12Z Fri Apr 05 2024

    ...Great Lakes... Days 1-2...

    ...Major late season winter storm this week...

    The stage is being set for a multi-day major April winter storm
    that will bring about a myriad of precipitation types (heavy snow,
    heavy rain, sleet, freezing rain), which combined with strong
    winds from the Upper Midwest to New England will result in numerous
    impacts tonight and through the end of the week. The origins of
    this winter storm begins with a vigorous northern stream
    disturbance diving south and phasing with a southern stream
    disturbance tracking northeast into northern Illinois and southern
    Wisconsin. This interaction will result in the rapid deepening of
    a more consolidated 500mb low over the Upper Midwest and quick
    intensification of a surface low by 06Z tonight over Lake Michigan.
    The latest forecast calls for the storm to deepen by as much as
    20mb over the next 18 hours. As the 500mb low rapidly deepens,
    intense vertical velocities on the northwest and western flanks of
    the 850mb low, induced by intense PVA aloft and exceptional
    850-700mb FGEN gives rise to a potent deformation axis that will
    become primarily snow this evening from northeast Iowa to central
    Wisconsin. As the storm occludes overnight, the TROWAL on the
    backside of the of the storm will continue to be the focus for
    heavy snow over northern Wisconsin and the U.P. of Michigan. The
    U.P. pf Michigan, in particular, will sport the best chances for
    ripping snowfall rates of >2"/hr thanks to the lake enhanced bands
    off Lake Superior and along the more elevated terrain of the
    central U.P.. By 12Z Wednesday, NAEFS shows 500mb heights that,
    according to NAEFS, fall below the observed CFSR database
    (1979-2009) over Illinois, illustrating the highly unusual nature
    of a cyclone that intense over Illinois for early April. It is on
    the northern flank of the low Wednesday AM where snowfall rates
    will be most significant.

    Due to the upper low being cut off from the mean flow to the west and
    the upper level omega block over eastern Canada and the northwest Atlantic,
    the storm system will be slow to move east on Wednesday, prolonging
    the period of heavy snow in northern Wisconsin and the Michigan
    U.P.. Latest WPC probabilities show high probabilities (>70%) for
    snowfall totals >8" from northern Wisconsin to the Michigan U.P..
    Farther south, there are moderate-to-high chances (50-70%) from
    just north of the I-94 corridor in southern Wisconsin up to areas
    just west of Green Bay. The WSSI sports Major to even locally
    Extreme Impacts for areas neighboring Green Bay and in the central
    Michigan U.P. The localized Extreme Impacts are depicted along the
    Huron Mountains through Wednesday afternoon. The Hurons currently
    have high chances (>70%) for >24" of storm total snowfall. Snow
    Amount is the primary driver in the WSSI algorithm, but the WSSI is
    also showing in some parts of eastern Wisconsin and even near the
    tip of Michigan's Mitt, that some Moderate Impacts as a result of
    Snow Load and Blowing Snow are expected.

    ...Northeast... Days 2-3...

    While the upper low in the Great Lakes occludes Wednesday
    afternoon, farther east, an impressive IVT over the Southeast will
    see some of its associated moisture stream north into the Northeast
    Wednesday morning and run into an air-mass just sufficiently cold
    enough to support a wintry mix of sleet and freezing rain from the
    Catskills and Adirondacks to the Berkshires. This wintry mix will
    translate farther north and east through the Worcester Hills, the
    Green Mountains, and White Mountains by Wednesday afternoon. This
    air-mass supporting the onslaught of wintry precipitation is not
    expected to leave any time soon due to the upper level omega block
    mentioned in the Great Lakes section that is locking in a dome of
    cold Canadian high pressure over Quebec. In actuality, what this
    omega block will do in part is to help keep the storm track farther
    south. As the occluded front works north through the Mid-Atlantic
    Wednesday afternoon, intense PVA and robust 850mb FGEN from
    southern PA to the southern New England coast will rapidly
    intensify an area of low pressure tracking from the Delaware Valley
    Wednesday evening to along the Long Island coast by early Thursday

    The key to this forecast lies with when the surface low takes over.
    Most of the interior Northeast will be dealing with a wintry mix
    due to the >0C warm nose in the 800-750mb layer. However, once the
    850mb low forms, winds will shift more out of the E-NE, reducing
    the warm nose aloft and leading to a sudden changeover to snow.
    Latest guidance shows this happening somewhere in the 03-09Z
    Thursday timeframe, which given this coincides with snow falling
    overnight, will maximize the opportunity for rapid accumulations on
    all surfaces. By 12Z Thursday, just about everyone from Upstate New
    York and interior New England to even the coast of New Hampshire
    and Maine can expect to be all snow.Snow fall rates late Wednesday
    night and through Thursday morning are likely to be between 1-2"/hr
    with wind gusts topping 40mph in many cases, especially along the
    New England coast and in the higher elevations. The storm looks to
    occlude off the eastern Massachusetts coast with the cold conveyor
    belt (CCB) of snow to the north of the low lasting over the
    northern Appalachians and much of Maine through Thursday evening.
    Due to the upper level omega block still in place, the upper low
    over the Northwest will continue to keep periods of snow in the
    forecast in the Green and White Mountains, but now with the air-
    mass modifying and gradually diminishing upper level support, a mix
    of rain and snow showers will be possible through Friday afternoon.
    Last but not least, upslope enhancement in the central Appalachians
    of eastern West Virginia will also ensue Thursday evening and into
    the day on Friday with elevations >3,000ft most likely to see heavy
    snow through the end of the work-week.

    In terms of impacts, this will be an exceptional one for the
    Northeast given not only the heavy, wet snow that is expected, but
    the prolonged round of strong winds combined with highly saturated
    soils in the Northeast. The WSSI shows Major Impact potential
    (considerable disruptions to daily life, widespread closures and
    disruptions) in parts of the Adirondacks, the White Mountains, and
    as far east as the Kennebec Valley of central Maine. Moderate
    Impacts (hazardous driving conditions, closures and disruptions
    possible) are higher confidence in the areas expecting Major
    Impacts, but are also possible in parts of the Catskills, Green
    Mountains, Berkshires, Worcester Hills, and in parts of the central
    Maine Highlands and along the central Maine Coast. Snow Amount is
    the primary driver in the WSSI algorithm, but a combination of Snow
    Load and Blowing Snow is also included with Moderate Impacts
    possible. With the expected impacts from Blowing Snow correlated to
    strong winds and the Snow Load component present as well, the
    exceptionally saturated soils throughout the region is leading to
    increased concerns for extensive tree damage and power outage
    potential. WPC PWPF sports high chances (>70%) for snowfall totals
    12" in the Adirondacks, as well as the Green and White Mountains,
    and into portions of central Maine. There are even some moderate-
    to-high chance probabilities (50-70%) for >24" in parts of the
    White Mountains. Please see our Key Messages below for the

    ...Western U.S.... Days 2-3...

    An upper level trough originating in the Gulf of Alaska will plunge
    south and be located off the Pacific Northwest coast by Wednesday
    afternoon. Pacific moisture streaming out ahead of the upper trough
    will result in some mountains snow along the Cascade Range and over
    the Olympics during the day on Wednesday. By Wednesday evening, a
    mid-upper level frontal band will focus a heavier swath of
    precipitation from northern California to the Northern Rockies
    where snow will fall from the Trinity/Shasta and the Blue Mountains
    of eastern Oregon to the Boise/Sawtooth and Bitterroots overnight.
    As the upper trough continues to dived south off the West Coast,
    the current of Pacific moisture will work its way south along the
    spine of the Sierra Nevada where upslope enhancement will prompt
    heavier snowfall rates over the central and southern Sierra Nevada
    throughout the day. The enhanced snowfall rates are also a
    byproduct of a compact and robust 500mb low at the base of the
    upper trough moving into central California. This leads to falling
    snow levels that support heavier snowfall totals occurring as low
    as 5,000ft. By Thursday night, 500mb and 700mb heights over central
    California are forecast by NAEFS to be below the 0.5
    climatological percentile and will even allow for some locally
    heavy snowfall amounts in the Transverse Ranges through Friday.
    Snowfall rates will back off some across most of the Pacific
    Mountains and Great Basin on Friday, but modest lift and steep
    lapse rates will still keep mountain snow in the forecast from as
    far south as the Peninsular Range to as far north as the

    Latest WPC PWPF shows high chances (>70%) for snowfall
    accumulations >12" in portions of the Blue Mountains and both the central
    and southern Sierra Nevada above 6,000ft, while similar high
    chance probabilities for >8" of snow are present in the
    Trinity/Shasta, the Oregon Cascades, and into parts of the
    Boise/Sawtooth mountains. The WSSI-P sports moderate-to-high
    chances (50-70%) for Moderate Impacts in the central and southern
    Sierra Nevada, the Blue Mountains, and the central Great Basin in
    central Nevada through Friday afternoon.


    ***Key Messages for Early April Nor'easter***

    ---Western Great Lakes heavy snow Tue night

    Heavy snow will develop over Wisconsin and expand into Upper
    Michigan overnight. Heavy snow will continue over much of the
    region through Wednesday, with additional heavy snow across
    portions of Upper Michigan, before ending on Thursday. As much as
    1-2 feet of snow is expected in parts of northern Wisconsin and the
    U.P. of Michigan.

    ---Western Great Lakes heavy snow Tue night

    Heavy, wet snow and some sleet will envelope the Northeast
    Wednesday afternoon through Friday. Significant snow accumulations
    over 12 inches are likely over northern New York and central New

    ---Significant impacts from heavy snow and wind

    The combination of heavy snow rates and gusty winds will result in
    hazardous travel due to whiteout conditions and snow-covered
    roads. The wet snow and high snow load combined with strong wind
    gusts may also cause tree damage and power outages.

    ---Moderate coastal flooding in the Northeast

    Prolonged onshore flow late Wednesday and continuing through
    Thursday will result in moderate coastal flooding for portions of
    the Northeast coast. Impacts include widespread roadway flooding,
    coastal and bayside flooding, impassable roads, and some damage to
    vulnerable structures.

    --- SBBSecho 3.20-Linux
    * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)
  • From Dumas Walker@21:1/175 to All on Wednesday, April 03, 2024 08:30:00
    FOUS11 KWBC 030855

    Probabilistic Heavy Snow and Icing Discussion
    NWS Weather Prediction Center College Park MD
    455 AM EDT Wed Apr 3 2024

    Valid 12Z Wed Apr 03 2024 - 12Z Sat Apr 06 2024

    ...Great Lakes/Northeast... Days 1-3...

    ...Major late season winter storm this week...

    As expected, phasing northern and southern streams are supporting
    the development of a deep closed over the western Great Lakes
    region this morning. Its associated surface low is now centered
    near southern Lake Michigan where it is forecast to linger for the
    next several hours before drifting to the southeast later today.
    Snow will continue to spread across much of Wisconsin, as well as
    portions of southeastern Minnesota, eastern Iowa, and northern
    Illinois. Additional accumulations for much of this area are
    expected to be an inch or two, with pockets of locally heavier
    amounts. Much heavier amounts are likely to the north across
    portions of Upper Michigan where persistent easterly to
    northeasterly winds off of Lake Superior will contribute to several
    more hours of heavy, wet, lake-enhanced snow. The heaviest amounts
    are forecast to center over Marquette and Baraga counties, where
    WPC PWPF show high probabilities (70 percent or greater) for
    additional accumulations of a foot or more today. Moderate (40
    percent or greater) to high probabilities for additional
    accumulations of 6 inches or more extend further west into far
    northeastern Wisconsin. Snow is expected to continue over Upper
    Michigan and northern Wisconsin through the overnight, but diminish
    by early Thursday as the low moves east of the region.

    Meanwhile, warm advection precipitation will continue to spread
    into the Northeast this morning, with it expected to begin as or
    quickly mix with sleet across a good portion of eastern Upstate New
    York and central New England during the afternoon. As the upper
    low begins to interact with additional southern stream energy
    lifting out of the South, a triple-point low will begin to develop
    and deepen over the Mid-Atlantic this afternoon. Models show this
    feature continuing to deepen as it moves northeast toward Long
    Island and southern New England overnight. As the low approaches,
    strong vertical ascent afforded in part by left-exit region upper
    jet forcing will support increasing precipitation rates, with snow
    likely to become the predominant p-type across much of northern New
    York into interior central and northern New England. By this
    evening and continuing into the overnight, HREF guidance shows
    snowfall rates of 1-2 inches spreading from Upstate New York into
    New England. By daybreak, the heaviest snows are expected to
    center over New Hampshire and western Maine. As the coastal low
    tracks into and then lingers near the Gulf Maine, snow will
    spread east across Maine on Thursday, while continuing to impact
    the remainder of northern New England and parts of northern New
    York through Friday. WPC PWPF shows storm total amounts likely
    exceeding a foot over portions of the Adirondack, Green, and White
    mountains, as well as much of Maine. Some parts of the region may
    see over two feet, with the PWPF indicating moderate or higher
    probabilities for reaching these amounts over the White Mountains
    into western Maine.

    ...Central and southern Appalachians.... Days 1-3...

    Shortwave energy embedded within deep cyclonic flow will support
    showers across the region beginning Thursday, with thermal profiles
    supporting snow across the higher elevations. The heaviest
    accumulations are expected to fall along the Allegheny Mountains
    in West Virgina, where several inches are possible by the end of
    the period. WPC PWPF shows probabilities for accumulations of 4
    inches or more climbing above 70 percent across this region during
    the Day 2 period (ending 12Z Friday).

    ...Western U.S.... Days 1-3...

    An amplified upper trough associated with a low dropping south from
    the Gulf of Alaska will begin to dig into the northwestern U.S.
    today Wednesday. Backing flow ahead of the advancing trough will
    support deepening moisture and the increasing potential for snow
    along a low-to-mid level front extending northeastward from the
    southern Cascades. While widespread heavy accumulations are not
    expected, locally heavy amounts are possible from the southern
    Cascades to the Blue Mountains late today into early Thursday.

    By early Thursday, models show a deep upper low centered along the
    Northwest coast that will then dig further south into northern
    California and Nevada by early Friday. This will bring heavy snows
    into the Sierra Nevada and parts of central and northern Nevada.
    Meanwhile, energy lifting east of the low and a lingering frontal
    boundary will support additional heavy amounts across northeastern

    From Friday into Saturday, the upper low is forecast to lift
    northeast across Nevada into southern Idaho, bringing additional
    snows to portions of northern Nevada, eastern Oregon, and southern
    to central Idaho. While the mountains will likely see the heaviest
    amounts, this will be an anomalously deep system that will bring
    snow levels well down into many of the valleys.

    Meanwhile, snow with locally heavy totals will also extend south
    into the southern California mountains and along the Mogollon Rim,
    and east into portions of the central Rockies.

    Regarding three day totals, WPC PWPF shows high probabilities for
    accumulations of 8 inches covering much of the Sierra Nevada, the
    Oregon Cascades, and the Blue Mountains. Locally high probabilities
    these amounts also cover the central and Nevada mountains into
    southeastern Oregon and southwestern Idaho, as well as the central
    Idaho ranges.


    ***Key Messages for Early April Nor'easter***

    ---Western Great Lakes snow through today

    Snow will continue throughout the region today, with additional
    heavy snow likely across portions of Upper Michigan and northern
    Wisconsin. Before ending on Thursday, snowfall accumulations over 2
    feet are likely in parts of Upper Michigan.

    ---Northeast snow and wind through Friday

    Heavy, wet snow and some sleet will spread into the Northeast
    Wednesday today and continue through Friday. Snowfall
    accumulations of 1-2 feet are likely across portions of northern
    New York and New England.

    ---Significant impacts from heavy snow and wind

    Heavy snowfall rates and gusty winds will result in dangerous
    travel conditions, with whiteout conditions and snow-covered roads.
    The combination of wet snow, a high snow load, and strong wind
    gusts could also cause tree damage and power outages.

    ---Moderate coastal flooding in the Northeast

    Prolonged onshore flow late today and continuing through Thursday
    will result in moderate coastal flooding for portions of the
    Northeast coast. Impacts include widespread roadway flooding,
    coastal and bayside flooding, impassable roads, and some damage to
    vulnerable structures.

    --- SBBSecho 3.20-Linux
    * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)