• HVYSNOW: Us Winter Storms

    From Dumas Walker@21:1/175 to All on Tuesday, January 09, 2024 10:26:00
    FOUS11 KWBC 090944

    Probabilistic Heavy Snow and Icing Discussion
    NWS Weather Prediction Center College Park MD
    443 AM EST Tue Jan 09 2024

    Valid 12Z Tue Jan 09 2024 - 12Z Fri Jan 12 2024

    ...Midwest through the northern Great Lakes...
    Days 1-2...

    An intense closed mid-level low with height anomalies around -4
    sigma will bowl its way from the Southern Plains early Tuesday
    into the Great Lakes by Wednesday morning and then continue
    northeast into Quebec Wednesday night. This extremely anomalous
    low will be accompanied by intensifying jet streaks both upstream
    and downstream, leaving an increasingly coupled jet structure
    across the Upper Midwest. This setup is conducive to a rapidly
    deepening surface low pressure area which will lift northeast from
    near St. Louis, MO to the L.P. of Michigan by Wednesday morning.
    Downstream of this low, intense moisture advection will continue
    to surge out of the Gulf of Mexico on 290-295K moist isentropic
    ascent, driving PWs up towards +2 sigma according to NAEFS. This
    moisture laden air is then progged to wrap cyclonically within the
    WCB into a modest TROWAL which will pivot back into the Great
    Lakes and Upper Midwest, resulting in some enhanced elevated
    instability to increase ascent and drive more impressive snowfall

    There are likely to be two primary areas of snow as this low
    translates northeast through Wednesday. The first will be along
    and just ahead of the 850mb low where the most intense warm/moist
    advection will drive enhanced 850-600mb fgen for ascent, while
    also yielding an expanding area of precipitation. The accompanying
    WAA is quite robust, and areas east of the low track will likely
    start as snow but then change over to rain as far north as
    Detroit, MI. However, this leading WAA will also result in a
    front-end "thump" of heavy snowfall with rates of 1-2"/hr at times
    as reflected by the WPC prototype snow band tool. Farther to the
    NW but still within the best theta-e ridge, a longer duration of
    these rates is likely, especially where they pivot beneath the

    Farther to the W/NW and along/west of the 700mb low track, there
    is likely to be a rejuvenated deformation axis which will pivot
    around the backside of the low and then translate northeast with
    time. This is expected from far eastern KS early, through southern
    IA, and into southern WI Tuesday night. The setup looks extremely
    favorable for a strong deformation band, and both CSI/CI
    parameters are noted in cross-sections indicating at least the
    potential for thundersnow. With SREF DGZ depth probabilities for
    100mb exceeding 50% from IA into WI coincident with the
    deformation axis, if thundersnow does occur, rates could reach
    3"/hr at times, which is now reflected by the latest WPC prototype
    snow band tool, and could rapidly enhance snowfall totals. The
    combination of these intense rates and gusty winds will result in
    considerable impacts through D1, and the WSSI-P probabilities for
    major impacts reach as high as 20-40% from southern IA through
    southern WI. WPC probabilities on D1 for more than 6 inches are
    above 70% from far northeast MO through the Door Peninsula of WI,
    with the maximum snowfall potential exceeding 12" where the
    WAA/deformation band both occur, possibly focused near Davenport,
    IA. It is important to note that the track of this low, which had
    been trending NW the last few nights, has corrected a bit back to
    the southeast tonight. This has resulted in a modest SE shift in
    the heaviest snow, and if this trend continues that potent
    deformation axis could shift back into the Chicago metro area late

    As the low pulls into Canada late D1 and into D2, the best
    synoptic ascent and deepest moisture will shift out of the region,
    but increasing N/NW flow may result in some area of heavy lake
    effect snow (LES), especially across the eastern U.P. and
    northwest L.P. of MI. Inversion depths in regional soundings are
    modest and winds are quite strong which will reduce parcel
    residence time, but multi bands with heavy snow rates are likely,
    and WPC probabilities for more than 6 inches are above 70% in the
    Huron Mountains, and 30-50% across much of the eastern U.P. and
    near the Tip of the Mitt.

    ...Southern Appalachians through the Northeast...
    Days 1-3...

    Extreme upper low with NAEFS standardized anomalies reaching -4
    sigma at 500mb will progress northeast into the Great Lakes and
    then Quebec through Wednesday, while spokes of vorticity shed from
    the primary gyre and lift northward within pronounced southerly
    flow downstream. Impressive moist advection embedded within this
    southerly flow will drive IVT as high as +8 sigma over the
    Mid-Atlantic, indicative of how anomalous the moisture is lifting
    across the region. Ascent through PVA, mid-level divergence, and
    increasing upper-level diffluence will act directly upon this
    saturated column, resulting in a shield of heavy precipitation
    lifting from the Southern Appalachians Tuesday morning, to New
    England by Wednesday morning. Initially, this precipitation will
    be either frozen or freezing, but will eventually transition to
    rain in most areas due to the intense WAA. The exception will be
    in the higher terrain of New England where significant heavy wet
    snow accumulations are likely.

    Across the Southern Appalachians, primarily for the Blue Ridge of
    VA into the Alleghenies and Laurel Highlands, some snow at onset
    will transition to freezing rain as warm air overruns the cold
    surface layer. The intensity of the WAA will eventually cause a
    transition to all rain, and heavy rainfall rates should limit ice
    accretion efficiency, there is likely to be at least light icing
    Tuesday morning across this area, but WPC probabilities for more
    than 0.1" of ice are less than 10%.

    The more significant threat is heavy snowfall from the Catskills
    and Adirondacks northeast through the Greens and into much of the
    higher terrain of NH and ME. Here, extreme moisture flux will
    impinge into the higher terrain to drive more pronounced ascent
    through upslope enhancement, which should produce intense snowfall
    rates of 1-2"/hr. This is reflected by the WPC prototype snow band
    tool as well as NBMv4.1 1-hr snowfall probabilities. What is most
    concerning about this snowfall is that SLRs are likely to be well
    below climo, so in places above 4000 ft where the p-type may stay
    all snow for the event, the heavy wet snow could result in major
    impacts as reflected by the snow load parameter within the WSSI-P.
    WPC probabilities for more than 6 inches of snow on D1 are above
    80% in the White Mountains and higher terrain of Maine, and 40-70%
    in the Adirondacks and Greens, generally above 2000 ft. The
    heaviest snow is almost certainly going to occur in the White
    Mountains, where D1 into D2 snowfall could exceed 18" in the
    highest terrain.

    Then, as the low over the Great Lakes occludes to a secondary
    center lifting across ME on Wednesday, increasing NW flow in its
    wake will result in strengthening CAA over the anomalously warm
    Great Lakes. This could result in efficient lake effect snow (LES)
    with inversion heights climbing to around 8000 ft coincident with
    lake-induced instability rising towards 250 J/kg. There is some
    uncertainty as to how much of the DGZ will be saturated, so
    extremely intense and efficient LES is not currently progged, but
    with the lake temperatures still well above normal for early
    January, this could result in heavy accumulations exceeding 6
    inches in the Tug Hill both D2 and D3 as reflected by WPC
    probabilities exceeding 50% both days.

    ...The West...
    Days 1-3...

    An extremely active pattern continues across the Pacific Northwest
    and into the Northern Rockies/Great Basin through late week as a
    series of shortwaves drop onshore from the Pacific

    The first of these will will move onshore this morning and drive a
    surface low/associated cold front into the Pacific Northwest and
    as far east as the Northern Rockies tonight. Although the synoptic
    forcing responsible for driving this feature eastward will weaken
    with time into D2, the modest IVT (less than +1 sigma according to
    NAEFS) will still push elevated mid-level RH above 90% across much
    of the Northwest, which will manifest as snow above snow levels
    which will fall gradually through the period from 2500 ft early,
    to as low as 500 ft east of the Cascades late, 1500 ft in the
    Cascades. With impressive and confluent mid-level flow advecting
    onshore and impinging orthogonally into the terrain, the heaviest
    snow is likely in the Cascades.

    Following immediately in the wake of this first shortwave, a more
    intense shortwave and accompanying vorticity maxima will drop
    along the PacNW coast and come onshore near the OR/CA border
    before amplifying into a more potent longwave trough near the Four
    Corners Thursday night. This feature will again be accompanied by
    impressively confluent mid-level flow to surge moisture inland,
    and although PW anomalies are modest, intense ascent in the
    vicinity of a translating low and strong surface cold front, and
    within the LFQ of a more potent jet streak will drive strong
    ascent to wring out available moisture. Snow levels late D1 into
    D2 may briefly rise on the warm/moist advection, but will
    generally continue to fall, and will likely reach the surface
    across much of the interior Northwest, and fall to around just
    1000 ft near the coast of WA/OR, higher into the Sierra. The
    heaviest snow with this second wave will likely shift a bit
    southeast from the greatest accumulations on D1.

    Maybe more importantly, the guidance has become more intense with
    this low pressure and cold front moving towards Portland, OR late
    tonight into Wednesday morning. Steep lapse rates beneath the cold
    pool aloft suggest some elevated instability which could support
    convective precipitation rates aided by the impressive synoptic
    deep layer lift overlapping concurrently the most intense
    850-600mb fgen surges onshore. This suggests that snowfall rates
    in excess of 1"/hr are possible, which is reflected by the latest
    WPC prototype snow band tool. During the period of greatest
    snowfall rate potential, RAP regional soundings indicate that WBZ
    heights are below the critical 750ft level, which indicates that
    snow accumulations could reach down into the lowlands around
    Portland, OR and the Columbia River Gorge/Basin. The high-res
    guidance is most intense with this potential, but latest WPC WSE
    plumes have also shown an increase in snowfall, and after
    coordination with SEW/PDT/PQR, some enhancements to snow has been
    included in the lowlands, and WPC probabilities for 2+" reach
    30-40% in the Portland, OR metro area.

    By D3 the pattern begins to shift a little as the primary trough
    amplifies over Alberta and then digs southward, driving a surface
    cold front into a more NW to SE orientation as it drops into
    WA/ID/MT by the end of the forecast period. South of this trough,
    mid-level flow becomes increasingly pinched with W/NW flow
    impinging again into the Cascades. Although the highest IVT/PWs
    will remain just offshore by the end of the period, increasing
    moisture being acted upon by intensifying fgen along the cold
    front and amplifying upper diffluence ahead of an approaching jet
    streak should result in widespread snowfall once again spreading
    across the Northwest, with snow levels low enough to support light accumulations even into the lowlands around Seattle, WA by the end
    of the forecast period.

    WPC probabilities D1, D2, and D3 all feature high potential (>80%)
    for more than 6 inches across much of the western terrain from the
    Olympics eastward through the Northern Rockies and southward along
    the Cascades, into the Sierra, across much of the Great Basin, and
    even into the Wasatch. The most significant snowfall D1 is likely
    in the WA/OR Cascades where several feet of snow is likely above
    2500 ft, and major impacts to mountain passes is expected. Also on
    D1 as moisture spills eastward, 1-2 feet, with locally higher
    amounts, is likely across parts of the Northern Rockies. During D2
    the heaviest accumulations shift to focus primarily in the OR
    Cascades and down into the Sierra, where 1-2 feet of additional
    snowfall is likely, which will again cause major impacts to the
    passes. By D3 the focus returns to the WA/OR Cascades and Northern
    Rockies where multiple feet of additional snow is possible, and it
    is also during this time when snow levels crash to support more
    widespread, albeit light, snowfall accumulations in the valleys
    and lowlands.

    Finally, the strong cold front moving across Oregon Tuesday night
    into Wednesday /late D1 into D2/ will continue to shift rapidly
    southeast across the Great Basin and into the Central Rockies by
    Thursday morning. This could spread convective snow showers and
    snow squalls across the area, with the CIPS snow squall parameter
    showing values above +1 spreading as far south as the Four Corners
    states. Evaluation of the fields which drive the snow squall
    parameter suggest coverage of snow squalls may be modest as there
    exists only pockets of overlapping sufficient 0-2km RH/925mb
    fgen/SBCAPE>100 J/kg to support intense squalls, but scattered to
    widespread convective snow showers are possible, which could cause
    significant travel impacts even beyond the areas of heavier
    synoptic snow.


    ***Key Messages for Jan 8-10 Major Winter Storm Central Plains
    through the Midwest and Eastern U.S.***
    --Dangerous Travel in Heavy Snow
    Heavy snow bands with rates of 1-2"/hr will shift from the central
    Plains to the Upper Midwest today, and then into the Great Lakes
    tonight. These snow rates will be accompanied by gusty winds
    reaching 35-45 mph, making for dangerous travel due to low
    visibility and snow covered roads. More than 12 inches of
    storm-total snowfall is likely (70-90%) in some areas.

    --Impactful Snow in the Northeast
    Heavy/wet snow will move across interior Northeast terrain tonight
    into Wednesday. This snow will cling to trees and power lines,
    which when combined with gusty winds reaching 40 mph, could result
    in sporadic power outages and scattered tree damage.

    --Significant River, Flash, and Coastal Flooding
    Widespread and potentially significant river and flash flooding is
    likely for much of the Eastern U.S. today. Powerful onshore winds
    will lead to widespread moderate to locally major coastal flooding
    through the Mid-Atlantic and southern New England tidal rivers and
    bays through Wednesday.

    --Damaging Winds May Lead to Power Outages
    Widespread wind gusts in excess of 55 MPH are likely across the
    central Appalachians, much of the Eastern Seaboard, and New
    England today into tonight. Prepare for power outages. Severe
    thunderstorms are also possible across the Southeast today.

    ***Key Messages for Jan 8-11 Northwest Winter Storm***
    --Blizzard for the Cascades & parts of the Rockies
    A series of two strong cold fronts will cross the Pacific
    Northwest through Wednesday morning, bringing several feet of
    heavy snow, and strong winds gusting to 60 mph, to the Cascades
    and portions of the Northern Rockies. Snow levels will fall to
    between 1000-2000 ft by Wednesday, leading to considerable travel
    impacts for many mountain passes.

    --Heavy Snow elsewhere in the West
    Heavy snow exceeding 12 inches is likely (>70%) for much of the
    Northern Rockies of Montana, Idaho, and eastern Oregon. Heavy
    snowfall will also spread into the Sierra and Wasatch by Thursday.

    --Lowland Snow Accumulations
    Heavy snow rates exceeding 1"/hr combined with lowering snow
    levels may allow for light to moderate accumulations even into the
    lowlands of northern OR/southern WA tonight into Wednesday.
    Significant travel disruptions are possible.

    --Storm will move through Central U.S.
    This storm system will dig across the Western U.S. through
    Thursday before restrengthening over the central U.S. Friday.
    Those in the Plains and Midwest should prepare for another
    powerful winter storm and stay tuned for updates.
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