• HVYSNOW: East Coast Storm

    From Dumas Walker@21:1/175 to All on Thursday, January 04, 2024 09:36:00
    FOUS11 KWBC 040943

    Probabilistic Heavy Snow and Icing Discussion
    NWS Weather Prediction Center College Park MD
    442 AM EST Thu Jan 04 2024

    Valid 12Z Thu Jan 04 2024 - 12Z Sun Jan 07 2024

    ...Southern Rockies into the Southern High Plains...
    Days 1-3...

    A potent and closed mid-level low centered over the southern Great
    Basin to start the period will advect eastward while continuing to
    deepen, reaching -2 to -3 sigma with respect to 500mb heights over
    the Desert Southwest Thursday night. This will result in
    impressive downstream height falls into the Four Corners and
    Southern High Plains Thursday through Friday, with ascent
    maximizing beneath the LFQ of a robust subtropical jet streak
    overlapping the greatest height falls. This overlap will also
    drive surface cyclogenesis, with a wave of low pressure moving
    eastward from near the AZ/NM border through the Red River Valley
    of the South. Downstream of this low, moisture advection will
    steadily intensify on nearly unidirectional southerly flow noted
    in regional forecast soundings, with 290-295K moist isentropic
    upglide driving PW anomalies to +1 to +2 sigma as far east as the
    High Plains. Across the Southern Rockies Thursday, this will also
    result in increasing upslope enhancement to the ascent, driving
    locally higher precipitation amounts and rates, especially in
    southern and eastern facing slopes, and this is where the greatest
    WPC probabilities exist for more than 4 inches, reaching 80% in
    the San Juans, Jemez, and Sangre de Cristos, with lower
    probabilities extending into the northeast High Plains, as well as
    south into the Sacramento Mountains. Late D1 into D2, the surface
    low briefly intensifies more robustly, resulting in an area of
    more intense WAA and associated fgen downstream and to the north
    of the surface low, potentially focusing near the OK/KS border
    Friday morning where theta-e lapse rates fall to near 0C/km
    accompanying the more intense fgen. This could result in a band of
    heavier snowfall which is reflected by the WPC prototype snow band
    tool indicating at least a low potential for 1"/hr rates, aided by
    dynamic cooling to overcome marginal surface temperatures. For D2,
    WPC probabilities for 4+" peak at just 5-10%, and will be reliant
    on this band setting up robustly enough and long enough to
    accumulate significantly.

    This first system ejects later D2, but is followed almost
    immediately but yet another shortwave digging through the
    exceptionally active flow. This shortwave is progged to dig out of
    the Great Basin Friday night and then deepen as it approaches the
    Four Corners and into the Southern Rockies once again Saturday
    morning. Height falls and PVA should again drive ascent supporting
    a wave of snowfall. Available column moisture for this second
    impulse will be less, but the column will also be colder and the
    best ascent may efficiently overlap the DGZ to support periods of
    moderate snowfall, especially in the terrain. WPC probabilities
    late D2 into D3 are modest, generally 10-30%, for an additional 4+
    inches from the Sangre de Cristos through the southern Front Range.

    ...Pacific Northwest into the Great Basin and the Sierra...
    Days 1-3...

    An active pattern will bring multiple rounds of heavy snow to much
    of the West into the weekend, with snow levels lowering with time
    as well. During D1, broad zonal flow will surge modest moisture
    onshore ahead of a shortwave which is progged to lift into the
    Pacific Northwest Friday morning. This overlap of moisture and
    modest ascent will bring a quick wave of precipitation to the
    region, with snow levels generally 4000-5000 ft within the axis of
    greatest PW and heaviest precip. Above these levels, briefly
    moderate to heavy snow is likely before some subsidence develops
    behind the first shortwave, and WPC probabilities for more than 6
    inches of snow are above 30% only in speckles of the highest
    terrain of the Cascades and into the Northern Rockies.

    After brief shortwave ridging early D2, a more pronounced trough
    will dig across the Pacific Ocean and then dive southward into the
    Pacific Northwest, eventually advecting into portions of central
    CA by the end of D3 while the primary longwave trough axis pivots
    into the Great Basin. Pronounced confluent mid-level flow
    downstream of the primary vorticity streamer will advect anomalous
    moisture eastward, and the GEFS IVT probabilities are forecast to
    exceed 60% for 250 kg/ms. At the same time, a potent Pacific jet
    streak will dive southward along the Pacific coast, placing
    impressive upper diffluence atop the best PVA/height falls, and
    merging with the onshore flow to produce widespread precipitation
    across the West. Although IVT is relatively modest, the prolonged
    overlap of moisture due to onshore flow and synoptic ascent will
    result in heavy precipitation, especially in the Cascades and
    Sierra where upslope flow will contribute. Snow levels within this
    deepening trough will fall to as low as 1500 ft in the Cascades,
    and 3000 ft across the Sierra, so impactful snowfall is likely
    even into lower elevations around the foothills of these ranges,
    with snowfall also impacting the northern CA ranges, some of the
    coastal ranges, and even spilling as far east as the
    Northern/Central Rockies and higher terrain of the Great Basin. By
    D3, WPC probabilities for more than 6 inches of snow are above 80%
    in the Cascades and Sierra, with locally more than 18 inches
    possible in the highest terrain.

    ...Southern Appalachians through New England...
    Days 2-3...

    A shortwave digging across the Four Corners Friday night into
    Saturday will advect rapidly eastward across the Southern Plains
    and then get sheared into pinched westerlies across the MS VLY
    before arcing northward across the Mid-Atlantic and into the
    Northeast late Saturday night /end of D3/. This will lead to a
    modest intensity, fast moving area of low pressure which is
    progged to develop near the Gulf Coast Saturday morning and then
    lift northeast as a Miller-A type cyclone, reaching the southern
    New England coast by Sunday morning. While this system continues
    to look progressive, and has shifted a bit NW with recent runs, at
    least through the Mid-Atlantic, it will likely result in a
    significant winter event from the Southern Appalachians, the
    interior Mid-Atlantic states, and into much of New England.
    Uncertainty remains high due to the variations in speed and track
    of this low, however, significant icing for the Southern/Central
    Appalachians, and at least plowable snow, is becoming more likely
    for much of the area.

    As the shortwave and associated vorticity max lift northeast D2
    into D3, they will be accompanied by a rapidly intensifying
    subtropical jet streak which may reach as high as 170 kts, or +4
    sigma according to NAEFS, over the TN VLY late in the forecast
    period. This will drive tremendous LFQ diffluence for ascent,
    ideally overlapping the best height falls and PVA from the modest vort/shortwave. At the same time, moist advection will gradually
    intensify downstream of the surface low, and while much of the
    best theta-e advection and isentropic upglide appear to lift
    parallel to the track (no significant TROWAL), there should still
    be plentiful moisture for heavy precipitation as noted by NAEFS PW
    anomalies rising above +1 sigma, resulting in an expanding
    precipitation shield. The impressive WAA and fast-moving low will
    likely result in a p-type challenge from the Southern Appalachians
    through the Mid-Atlantic states, with additional changeover likely
    along the coast of southern New England. The greatest risk for
    freezing rain is likely from NW GA through the Piedmont and into
    far northern VA where where WPC probabilities for more than 0.1"
    reach as high as 70%, and there is some potential for more than
    0.25" in Upstate SC and into southwest VA. Some sleet and snow is
    also possible in this area, but the predominant p-type outside of
    the highest terrain is likely to be ZR as the wedge holds surface
    wet bulb temps below freezing.

    Farther north into the Mid-Atlantic, the guidance has continued to
    trend a bit farther NW as the low lifts along the coast and begins
    to deepen. This will likely produce at least a swath of moderate
    snow at onset across most areas, but onshore/SE flow within the
    WAA and from the still warm Atlantic waters will shift p-type
    quickly over to rain generally along and east of I-95, and some
    mixing with sleet/freezing rain is possible much farther NW from
    there as well as the 850mb warm nose surges. WPC probabilities for
    at least 0.01" of freezing rain reach above 10% as far west as
    eastern KY and southwest PA, and as far north as the southern
    Poconos during D2-D3. Significant snowfall exceeding 4" during
    this event will likely be confined to D3, and from the higher
    terrain of eastern WV through the interior Mid-Atlantic and
    northward through Upstate NY and southern/central New England.
    There is increasing confidence that as the low approaches southern
    New England it will stop its northward progression and track
    eastward as it deepens, leading to a band of heavy deformation
    snow from near the Poconos through southern New England, with
    heavy snow also collapsing back to the southeast during this time.
    Some uncertainty continues near the immediate coast, including the
    Boston area, due to onshore flow off the warm waters, but a period
    of intense snow rates and rapid accumulation is likely during D3.
    WPC probabilities D3 for more than 6 inches of snow have
    increased, and are now above 50% from central PA through the
    Catskills and into much of southern New England except the
    immediate coast, and locally more than 12" of snow is possible in
    some areas as reflected by the WSE mean and low, but increasing,
    PWPF. Some of this snow could be heavy and wet, and the WSSI-P
    suggests a high potential for at least moderate impacts from this
    event D3 into D4, including the Boston metro area.

    ...Northern MN into the Western U.P. of MI
    Days 2-3...

    A closed low over the Corn Belt region Friday night will drive
    downstream divergence and periods of PVA to produce ascent into
    the northern portions of the Upper Midwest and into the western
    Great Lakes. This synoptic ascent will combine with increasing WAA
    ahead of a weak surface wave which may develop near the ND/MN
    border, and then push eastward followed by a cold front Saturday
    night driven by a secondary axis of vorticity strung out across a
    northern stream. The accompanying WAA will surge PWs to +1 sigma
    according to NAEFS in a thin ribbon focused in northern MN, which
    will result in a corridor of moderate snow beginning Friday night,
    and expanding with time through Saturday. The best frontogenetic
    ascent appear to lie within the 925-700mb layer, which is well
    below the elevated and shallow DGZ, and even then the total
    forcing is modest. This should limit snowfall intensity across the
    region, although there may be a brief uptick ahead of the cold
    front as the DGZ deepens/lowers coincident with increasing ascent,
    or along the Iron Ranges of the Arrowhead where additional
    moisture from Lake Superior may upslope into the region. This will
    produce generally light accumulations of snow, with locally higher
    amounts across the Arrowhead. WPC probabilities for more than 4
    inches reach 20-40% for much of northern MN, and above 50% along
    the shore of Lake Superior into the Arrowhead.

    ***Key Messages for Jan 6-7 East Coast Winter Storm***

    --Developing coastal low pressure
    An area of low pressure is forecast to develop near the Gulf Coast
    late Friday, and then track northeast to the DelMarVa coast
    Saturday. This low will then likely deepen south of New England
    before ejecting eastward into the Atlantic Ocean by Monday

    --Significant Icing Possible
    There is increasing potential for significant accumulations of ice
    for portions of the southern and central Appalachians. Ice
    accumulations exceeding 0.1" could produce hazardous travel due to
    slippery roads Friday night and Saturday.

    --Heavy snowfall amounts becoming likely
    For the interior Mid-Atlantic, Northeast, and New England, there
    is increasing confidence that an axis of heavy snow will develop
    Saturday afternoon into Monday morning. Although the details are
    still uncertain, significant accumulations of heavy and wet snow
    may cause considerable travel impacts as well as possible impacts
    to infrastructure.

    --Coastal Flooding a concern
    As the low intensifies on Sunday, gusty onshore winds may lead to
    minor flooding along the Mid-Atlantic and southern New England

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  • From Dumas Walker@21:1/175 to All on Friday, January 05, 2024 09:37:00
    FOUS11 KWBC 050956

    Probabilistic Heavy Snow and Icing Discussion
    NWS Weather Prediction Center College Park MD
    456 AM EST Fri Jan 05 2024

    Valid 12Z Fri Jan 05 2024 - 12Z Mon Jan 08 2024

    ...Southern Appalachians through New England...
    Days 1-3...

    Major winter storm to bring combination of freezing rain and heavy
    snow from the Southern Appalachians through New England.

    A large trough across the center of the CONUS will expand to the
    east Friday night in response to a subtle vorticity impulse and
    accompanying weak shortwave trough rotating cyclonically through
    the flow and into the Mid-Atlantic. This will be followed quickly
    by a more substantial vorticity impulse streaking out of the Gulf
    Coast, and it is this secondary trough that will drive the more
    intense ascent and support cyclogenesis as a Miller-A type low
    develops in the Southeast and then moves northeast into the
    Mid-Atlantic and into New England. This shortwave will remain
    progressive at least into Sunday morning, but guidance has
    continued to become more aggressive with it tilting negatively
    near the New England coast, which could slow/stall the surface
    low, while yet a third piece of energy approaches from the Ohio
    Valley to re-invigorate ascent and lengthen the period of
    precipitation. At the same time, an intense subtropical jet streak
    will arc out of the TN VLY, with wind speeds approaching +4 sigma
    according to NAEFS, and the intense diffluent LFQ of this jet will
    overlap the best height falls sufficiently to help rapidly
    strengthen the low south of New England. While there is still some
    uncertainty into the exact track which will wreak havoc with the
    rain/snow line, there is increasing confidence that a stripe of
    heavy and wet snow with significant accumulations will occur just
    inland from the coast from WV through ME.

    Late D1 /Friday night into Saturday morning/ the expanding
    precipitation shield will begin to overrun cold Canadian air
    across the Appalachians. Moist isentropic upglide at 285K will
    feature impressive mixing ratios of 5-6 g/kg, suggesting a rapidly
    moistening column and potentially heavy precipitation. This
    overrunning will likely result in periods of snow changing to
    freezing rain in the foothills of the Appalachians where the
    surface wedge will at least briefly become reinforced by falling
    precipitation before slowly eroding to the northeast. Guidance
    tends to over-do the rate of this erosion, and with some weak
    dry-drainage on NE winds, it is possible some areas could receive
    around 0.25" of freezing rain. However, this system remains
    progressive into Saturday so precip will either wane or turn to
    rain quickly Saturday across VA/NC/SC, with the best freezing rain
    potential shifting northward along the Blue Ridge where WPC
    probabilities for more than 0.25" peak at 20-40%.

    North of the freezing rain/mixed zone, an area of heavy snow will
    develop early Saturday as the surface low tracks from near GA to
    off the NJ coast by Sunday morning. This low will be progressive
    during this time, but should be accompanied by strong WAA at onset
    which when combined with the impressive synoptic lift will result
    in some front end heavy snow most areas, before quickly changing
    over to rain along and east of I-95, with some sleet-to-rain
    farther inland. The WAA is impressive, 850mb flow is out of the
    S/SE, and the Canadian high is not in a favorable place to lock in
    cold air, so it is likely only elevated areas and regions well
    inland that will receive significant accumulations across the
    southern Mid-Atlantic states, but a burst of heavier snowfall
    rates within the WAA/fgen band could still result in hazardous
    travel for a time, and moderate accumulations as far south as the
    Laurel Highlands and Blue Ridge. However, the more significant
    snowfall is likely the latter half of D2 and into D3 as the low
    stalls south of New England and rapidly deepens in response to a
    strong baroclinic gradient offshore and better synoptic lift.
    During this time, onshore flow will maximize noted by U-wind
    anomalies approaching -3 sigma according to NAEFS, pushing
    impressive theta-e advection into New England and lifting into a
    TROWAL. While this will likely push warm air into the coast,
    locations just inland and in higher elevations could experience a
    longer duration of heavy snow as the setup seems to at least
    marginally support a pivoting band of heavy snow N/NW of the
    maturing and then occluding cyclone. During this time, likely late
    Saturday night into the first half of Sunday, the occlusion and
    isallobaric response to the strengthening low could cause the cold
    air to collapse back to the southeast, bringing heavy snow back
    towards the coast before the system winds down Sunday evening.
    Despite marginal low-level thermals, the strong ascent should
    result in heavy snow rates exceeding 1"/hr, especially in elevated
    inland areas. There remains some uncertainty as to where this
    impressive band will pivot, but WPC probabilities, in general,
    have increased and shifted a bit NW with this update, and now are
    above 50% for 6+ inches on D2 from the Poconos through the
    Catskills and into much of southern New England away from the
    immediate coast, shifting into northeast MA, southeast NH, and far
    southwest ME on D3. Locally more than 12 inches of snow is likely
    in some areas, especially in the higher terrain of the Catskills,
    Berkshires, and Worcester Hills.

    ...Southern Rockies and Four Corners...
    Days 1-3...

    A series of shortwave rotating through broad cyclonic flow
    encompassing much of the CONUS will bring waves of precipitation,
    primarily as snow, to the Four Corners and Southern Rockies each
    day. One impulse moving into the region late tonight into Saturday
    morning will bring a round of snow, generally to the higher
    terrain of UT/CO and the Sangre de Cristos, with NW flow providing
    favorable upslope into these regions. The shortwave is rather
    transient and moisture is modest, but above 3000 ft WPC
    probabilities for more than 4 inches reach 30-50% in these ranges.

    As this first shortwave ejects to the east and into the Plains
    Saturday, brief shortwave ridging will follow in its immediate
    wake, before another, stronger, impulse digs in from the Pacific
    Northwest. This next feature will amplify into a potent closed low
    centered over the eastern Great Basin Sunday evening and continue
    to deepen as it pivots into the Southern High Plains by the end of
    the forecast period. The result of this evolution will be a
    pronounced longwave trough which sets up over the Intermountain
    West, leaving both upstream and downstream jet streaks to amplify
    and support enhanced coupled divergence aloft. The overlap of this
    synoptic ascent will likely lead to surface cyclogenesis around
    the Four Corners Sunday evening, and this low is progged to then
    strengthen as it shifts to the east on Monday. Downstream of this
    wave, at least modest moisture advection will emerge from the
    south and pivot into the Four Corners, with mid-level RH
    increasing dramatically late Sunday. Snow levels ahead of the wave
    may rise to as high as 4000-5000 ft within the WAA, but will fall
    quickly to as low as 1500 ft behind the accompanying cold front.
    The duration of this impressive ascent into a moistening column
    will result in areas of heavy snow beginning late D2 but
    especially on D3. WPC probabilities for more than 4 inches during
    this second wave reach above 80% from the Wasatch Front and Uintas
    along the Wasatch, southeast into the San Juans, and then much of
    the higher terrain from the Mogollon Rim into the Sangre de
    Cristos of northern AZ and NM. Locally more than 12 inches of snow
    is possible on D3, with the highest probability of that occurring
    in the White Mountains of AZ and the southern San Juan Range.

    ..Pacific Northwest/Great Basin/Sierra...
    Days 1-3...

    An active period of weather will be accompanied by lowering snow
    levels as an expansive trough amplifies across the West. The
    period begins with one shortwave impulse racing eastward across
    the inland Pacific Northwest and diving into the Great
    Basin/Northern Rockies. This will have limited impact to the
    sensible weather/precipitation, but will result in more zonal flow
    in its wake to surge additional moisture onshore. Within westerly
    flow, a shortwave will approach from British Columbia, pinching
    the flow to become more intense, while also producing downstream
    divergence ahead of the approaching impulse. This feature will
    then intensify as it drops southeast into the Pacific Northwest
    and northern CA Saturday evening, amplifying into a closed low
    over the Great Basin by Sunday morning before slowly pulling off
    to the east by the end of the forecast period. Broad but
    impressive synoptic lift will accompany this evolution, with lift
    being provided through substantial height falls, pockets of
    impressive PVA, and robust upper diffluence as the LFQ of a 150kt
    Pacific jet streak arcs down the coast and into the Intermountain

    Although IVT will remain modest during the duration of this event,
    generally AOB 300 kg/ms, the prolonged moisture fetch within the
    large scale ascent will result in heavy precipitation across much
    of the West, especially late D1 through early D3. The heaviest
    precipitation will spread southeast with time, starting in the
    Pacific Northwest (Cascades, Olympics) Saturday morning and then
    expanding and extending across most of the region through Sunday.
    For D1, the surface cold front will drop southeast through the
    PacNW with upslope flow enhancing snowfall across the Cascades,
    primarily above 2500 ft. WPC probabilities for more than 4 inches
    are 70-90%, with 1-2 feet likely in the highest terrain. During
    D2, WPC probabilities for more than 4 inches expand considerably
    as the front and waves of low pressure move east beneath the
    amplifying trough. The highest risk areas for more than 4 inches
    on D2 are again across the Cascades, but also extending south into
    the Northern CA ranges and along the Sierra, with additional high
    probabilities above 80% reaching the higher terrain of the
    Northern Rockies, Blue Mountains, and ranges in the Great Basin.
    Although the heaviest snowfall, which could exceed 1 foot, should
    be confined to the higher terrain of these areas, snow levels will
    be collapsing to as low as 1000 ft in WA and the Northern Rockies,
    and 2000-3000 ft elsewhere indicating an increasing potential for
    impactful snow even into lower elevations and many of the area
    passes to produce hazardous travel. By D3, the most intense ascent
    shifts southeast again into the Four Corners, but renewed
    precipitation is expected in the Cascades where WPC probabilities
    for more than 4 inches reach 20-40% in advance of yet another
    shortwave moving onshore.

    ...Northern Minnesota...
    Days 1-2...

    A cold front will waver across northern MN Friday night before
    returning north as a warm front by Saturday morning in response to
    a shortwave emerging from the Central Plains and lifting northward
    into the Upper Midwest. At the same time, a weak secondary
    shortwave will dig southeast from Saskatchewan, and the
    interaction of these features will drive weak cyclogenesis which
    will shift eastward, trailed by a cold front into the Great Lakes
    by Sunday morning. Increasing moist isentropic upglide on the
    285K-290K surfaces concurrent with the warm front will result in
    an expanding shield of light to moderate snowfall, although modest
    ascent and low probabilities for even 50mb of DGZ depth from the
    SREF indicate rates should remain modest. The column will be cold,
    so efficient and fluffy SLRs are likely, and a long duration of
    this snow should result in moderate accumulations across the area.
    As the low shifts eastward Saturday into the Great Lakes, the
    trailing cold front may be accompanied by some enhanced 850-600mb
    fgen which could drive a narrow band of heavier snowfall across
    these same areas, adding light accumulations through D2. The
    heaviest snow this period will likely be across the Iron Ranges in
    the Arrowhead where some additional moisture from Lake Superior
    will contribute, and low-level flow with the warm front will
    upslope effectively to wring out additional moisture, but WPC
    probabilities for more than 4 inches D1.5 peak around 50% along
    the lake shore, with locally more than 6 inches possible.
    Elsewhere across northern MN WPC probabilities for more than 4
    inches reach 20-40%.


    ***Key Messages for Jan 6-7 East Coast Winter Storm***

    --Heavy Snow in the Northeast
    For the interior Mid-Atlantic and New England, heavy snow is
    expected from Saturday Afternoon through Sunday. Although there is
    uncertainty into where the rain-snow transition will occur, there
    is a high chance (50-90%) of exceeding 8 inches of snow from the
    Poconos eastward through much of interior southern New England.
    Locally more than 12 inches is possible at higher elevations.

    --Impacts from heavy, wet snow and wind
    The combination of heavy, wet snow and gusty winds from northeast
    Pennsylvania through much of southern and central New England will
    result in difficult travel, and may lead to some power outages and
    tree damage.

    --Minor Coastal Flooding Possible in the Northeast
    Gusty onshore winds may lead to minor flooding along the
    Mid-Atlantic and southern New England coasts, particularly for the
    Sunday morning high tide cycle.

    --Ice Impacts to Travel in Southern Appalachians
    Along and east of the Southern Appalachians, accumulations of ice
    in excess of 0.1 inches, are likely (70-90%) with locally higher
    accumulations possible. This icing, along with some areas of
    sleet, may produce hazardous travel conditions on Friday Night and

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