• HVYSNOW: Major US Storms

    From Dumas Walker@21:1/175 to All on Sunday, January 07, 2024 09:13:00
    FOUS11 KWBC 070947

    Probabilistic Heavy Snow and Icing Discussion
    NWS Weather Prediction Center College Park MD
    447 AM EST Sun Jan 07 2024

    Valid 12Z Sun Jan 07 2024 - 12Z Wed Jan 10 2024

    ...New England
    Days 1 & 3...

    The strong coastal low will be positioned just south of the New
    England coast this morning, and will likely experience some rapid intensification through the afternoon as a stripe of vorticity
    embedded within a negatively tilting shortwave lifts northeast
    downstream of the Great Lakes trough to enhance ascent over the
    low. At the same time, the subtropical jet streak arcing out of
    the TN VLY will pivot poleward and intensify to 190 kts (nearly +4
    sigma according to NAEFS), placing intense upper diffluence within
    the LFQ atop the best PVA, further enhancing the surface low
    deepening. This evolution will result in a deepening low but also
    lead to an occlusion as the low begins to stack vertically, which
    will allow the cold air to collapse back to the southeast across
    New England. This will result in a continuation of heavy snow
    across Southern New England (SNE), especially within a deformation
    band which is still progged to pivot eastward across the region.
    Some lengthening of the heavy snow is possible during the first
    half of Sunday as well as interaction with the primary trough
    occurs, driving additional ascent from the west even as the low
    begins to pull away. This overlap of ascent will likely produce
    snowfall rates in excess of 1"/hr as reflected by the WPC
    prototype snow band tool, and although some drier air will begin
    to impinge on the area from the west, additional snowfall across
    SNE will likely exceed 6 inches as reflected by WPC probabilities
    for more than 6 inches reaching 40-80% from the southern Green
    Mountains of VT eastward through far southwest ME.

    Another strong system will develop over the Southern Plains on
    Tuesday and then lift northeast into the Great Lakes by Wednesday.
    This track is not favorable for cold/snow across New England,
    especially as the surface high retreats quickly to the north.
    Moisture will likely be excessive however, with NAEFS IVT
    anomalies surging to +5 to +6 sigma into SNE by the end of the
    forecast period. This will likely be heavy rain across all but the
    highest terrain, but above 2000 ft, heavy wet snow is likely,
    which will accumulate rapidly. Uncertainty still exists by D3, but
    current WPC probabilities for more than 6 inches of snow are
    30-50% in the southern Adirondacks, and above 80% in the White
    Mountains of NH/ME where more than 12 inches is likely in the
    highest terrain.

    ..Pacific Coast, Great Basin, Northern Rockies...
    Days 1-3...

    A series of shortwaves will continue an active pattern into the
    West through mid-week. The first of these shortwaves will be
    moving onshore the WA/OR coast to start the period as an
    amplifying trough digs into the Great Basin. A sheared out
    vorticity lobe extending along the coast will provide modest
    ascent, with confluent mid-level flow in its wake driving some
    enhanced moisture into the Cascades. The overlap of forcing and
    moisture should wane by the end of D1, but in the OR Cascades, and
    generally above 2000ft, WPC probabilities for more than 6 inches
    are 30-40%.

    Shortwave ridging follows in the wake of this lead shortwave
    bringing a reduction in forcing and moisture into D2, but this
    will quickly be overwhelmed by much more significant ascent and
    moisture ahead of a more intense shortwave advecting to near the
    WA/B.C. border Tuesday evening. Downstream of this feature,
    mid-level flow will become increasingly confluent in conjunction
    with a 130kt Pacific jet streak approaching zonally to the coast.
    The overlap of these will result in increasing IVT shifting
    onshore, driving an atmospheric river (AR) towards the coast with
    moderate probabilities for IVT exceeding 500 kg/ms. This will
    initially push a warm front onshore late Monday with snow levels
    rising to 4000-6000 ft. However, a subsequent cold front following
    rapidly behind the lead WAA will quickly plummet snow levels back
    to 1500-2500 ft, with heavy precipitation occurring this
    evolution. The intense and long-lasting ascent, aided by
    pronounced upslope into the N-S ranges, combined with impressive
    moisture, will result in very heavy snowfall beginning late D2 and
    expanding across much of the region during D3. WPC probabilities
    for more than 6 inches on D2 are above 70% in the WA/OR
    Cascades,and 30-50% as far east as the Northern Rockies. However,
    by D3, this becomes much more impressive with >80% probabilities
    for 6+ inches encompassing all of the Cascades, Olympics, Shasta/Trinity/Siskiyou region, and eastward into the Salmon
    Rivers/Sawtooth, Northern Rockies, and towards the Tetons. Just
    during D3 alone, many of these ranges, especially above 2000 ft,
    will receive more than 2 feet of snow. As snow levels crash,
    considerable snow will also impact most of the area passes across
    the Cascades and Northern Rockies.

    ...Four Corners States across the Plains and through the western
    Great Lakes...
    Days 1-3...

    A potent shortwave and accompanying vorticity maxima will rotate
    across the Great Basin this morning and then amplify rapidly into
    a closed low near the Four Corners, with 500mb height anomalies
    reaching -4 sigma according to NAEFS ensemble tables in eastern
    NM, with even further deepening to as low as -5 sigma progged over
    the Southern Plains by Tuesday morning. This extreme mid-level low
    will be accompanied by increasing upper diffluence as coupled jet
    streaks intensify downstream of the amplifying longwave trough
    axis, which will result in an impressive surface cyclone
    developing in the lee of the Southern Rockies Sunday night and
    then intensifying as it lifts northeast through Tuesday. This will
    likely become an intense cyclone with heavy precipitation and
    strong winds in many areas.

    During D1, as the low begins to consolidate, most of the forcing
    for ascent will be driven by deep layer forcing within the LFQ of
    a jet streak pivoting around the amplifying trough, and within a
    region of robust height falls/mid-level divergence downstream of
    the deepening closed low. Moisture on D1 will be somewhat modest
    across the Four Corners states as reflected by near-normal PW
    anomalies, but the intense forcing will wring out what is
    available, falling as moderate to heavy snow above 3000-4000 ft.
    Increasing SW flow between 700-500mb will help advect at least
    modest Pacific moisture into the region, and this will also
    upslope into the higher terrain, especially around the San Juans,
    Wasatch, and Mogollon Rim. It is in these ranges where WPC
    probabilities for more than 6 inches maximize, reaching above 80%,
    with locally higher amounts reaching 1-2 feet likely. With snow
    levels crashing rapidly behind the cold front accompanying the
    surface low, light snow accumulations are likely even into many of
    the valley floors before 12Z Monday.

    The system really begins to ramp up on Monday and Tuesday in
    response to the most intense deep layer lift resulting from the
    overlap of coupled jet streaks and impressive mid-level height falls/divergence. At the same time, the intense confluent and
    southerly flow downstream of the closed low will drive strong
    moist advection northward into the Southern and Central Plains.
    This is noted in model progs via extreme mixing ratios of 8g/kg
    surging northward on 295K moist isentropic upglide, with the
    resultant theta-e ridge axis lifting N/NW into a TROWAL around the
    surface low, and PWs forecast by NAEFS reaching +1 to +2 sigma.
    This will fully saturate the column, and as deep layer ascent
    maximizes both through synoptic forcing and the WAA, an expanding
    shield of heavy precipitation will result. While the column in the
    Southern Plains will be too warm for snow, areas into KS/MO and
    points north will likely experience a long duration of moderate to
    heavy WAA snow, with snowfall rates of 0.5-1"/hr likely as fgen
    maximizes beneath a deepening DGZ. Cross sections at this time do
    not suggest convective rates within the WAA snowfall, but a long
    duration of snow as the storm lifts northeast will still produce
    significant accumulations, and locally enhanced banding cannot be
    ruled out. On the back side of this system, the setup does appear
    to support a pivoting or laterally translating deformation band
    surging eastward behind the low from the High Plains of CO/NM,
    through the OK/TX panhandles Monday, and then lifting into lower
    Missouri Valley Tuesday morning, and eventually into the Great
    Lakes by the end of the forecast period. These bands are extremely
    sensitive to initial position errors of the models, so confidence
    in the exact placement is still uncertain, but it is possible that
    the WAA snow will transition immediately to more intense
    deformation snow across parts of MO/IA which may explain why
    current PWPF data is most aggressive in that region. Heavy snow is
    likely to spread from the Southern High Plains into the western
    Great Lakes Monday and Tuesday, and WPC probabilities for more
    than 4 inches exceed 70% on D2 from the Raton Mesa northeast
    through the Corn Belt, and on D3 extend as far northeast as the
    Door Peninsula and northern L.P. of MI. Locally up to 12 inches of
    snow is possible, as reflected by WPC probabilities of 20-40% near
    the MO/IA border. Note that ensemble trends have been shifting NW
    the past few runs, so it is likely that additional adjustments to
    the heaviest snow axes will continue over the next few model
    cycles, and a subsequent additional shift to the NW is possible.

    ...Southern/Central Appalachians...
    Days 2-3...

    An intense 500mb low lifting across the Upper Midwest will result
    in downstream divergence across the Appalachians on Tuesday.
    Downstream of this feature, extreme moist advection is likely on
    impressively pinched flow driving IVT as high as +6 sigma
    according to NAEFS, and extreme mixing ratios of 8g/kg surging
    northward on the 295K-300K isentropic surfaces. This will result
    in an expanding shield of heavy precipitation emerging from the
    Gulf Coast, and although the associated WAA will be intense,
    initially the surface wet-bulb temperatures will be below 0C.
    These sub-freezing temperatures will likely erode rapidly from
    south to north during Tuesday, but precipitation at onset could
    feature a mix of sleet and freezing rain before changeover to
    rain. Heavy rates and limited duration of freezing rain should
    limit ice accretions overall, but WPC probabilities peak around
    30-50% for 0.1" of ice accretion on D2.5 along the NC/VA Blue
    Ridge and into the Laurel Highlands.


    ***Key Messages for Jan 7-11 Major Winter Storm***

    --Heavy Snow Will Create Hazardous Travel
    The winter storm will deliver heavy snow in the Plains and Midwest
    on Monday and Tuesday. A heavy, wet snow will develop in the
    central Appalachians and interior Northeast Tuesday Night. The
    heaviest snow totals are most likely in parts of the Midwest,
    where local maxima up to 12 inches are likely.

    --Extremely Dangerous Blizzard Possible
    Blizzard conditions are most likely in the central Plains where
    wind gusts in excess of 50 MPH will lead to near zero visibility
    at times and extremely dangerous travel. Blizzard conditions are
    possible in the Midwest as well.

    --Significant River, Flash, and Coastal Flooding
    Widespread and potentially significant river and flash flooding
    are likely for sections of the central Gulf Coast and the Eastern
    U.S. early this week. Powerful onshore winds will lead to
    widespread coastal flooding along the eastern Gulf Coast and much
    of the East Coast. Significant coastal flooding is likely in some
    areas, especially in the Carolinas and Mid Atlantic.

    --Damaging Winds May Lead to Power Outages
    Widespread wind gusts in excess of 50 MPH are likely in the
    eastern Gulf Coast, central Appalachians, much of the East Coast,
    and New England. Prepare for power outages.

    ***Key Messages for Jan 8-11 Northwest Winter Storm***

    --Major Winter Storm for the Cascades
    A winter storm will peak on Tuesday and Wednesday in the Cascades
    in Washington and Oregon, with several feet of heavy snow and
    gusty winds. Snow levels will quickly fall to between 1500 and
    2500 feet behind a cold front on Tuesday, leading to considerable
    impacts for many mountain passes.

    --Heavy Snow Expected Elsewhere in the West
    Over a foot of snow is likely (70-90%) in the higher elevations of
    the Northern Rockies, with the heaviest snow falling on Tuesday
    and Wednesday. Snow levels falling to below 1000 feet later on
    Wednesday and early Thursday may lead to some snow accumulations
    in the valleys also.

    --High Surf, Gusty Winds on the Pacific Northwest Coast
    The active storm track into the Pacific Northwest is likely to
    lead to renewed periods of high surf and gusty onshore winds early
    this week on the coastlines of Washington and Oregon.

    --Low Pressure System Likely to Move into Central U.S.
    The storm system is likely to advance through the Western U.S. and
    reach the Plains by late in the week. People in the Central U.S.
    should be aware of the potential for another winter storm and stay
    tuned for updates.

    --- SBBSecho 3.14-Linux
    * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (21:1/175)