• MODERATE RISK MOARILKYTN

    From Mike Powell@618:250/1 to All on Wed May 8 08:36:00 2024
    ACUS01 KWNS 081256
    SWODY1
    SPC AC 081255

    Day 1 Convective Outlook
    NWS Storm Prediction Center Norman OK
    0755 AM CDT Wed May 08 2024

    Valid 081300Z - 091200Z

    ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVE PORTIONS OF SOUTHERN/SOUTHEASTERN MISSOURI...EXTREME NORTHEASTERN AR...EXTREME
    SOUTHERN ILLINOIS...WESTERN/SOUTH-CENTRAL KENTUCKY...AND
    NORTHWESTERN TO MIDDLE TENNESSEE...

    ...SUMMARY...
    Severe thunderstorms appear likely from parts of the mid
    Mississippi, Ohio, and Tennessee Valleys into the southern Plains.
    All severe hazards, including tornadoes, very large to giant hail,
    and potentially significant damaging winds are possible. Some
    tornadoes may be strong.

    ...Synopsis...
    In mid/upper levels, a temporarily blocky pattern has evolved around
    a cutoff cyclone over the northern Great Plains. The associated
    500-mb low is expected to drift erratically southeastward and fill
    slowly through the remainder of the period, while a series of
    shortwaves amplify associated synoptic troughing extending
    southwestward across the Intermountain West and Great Basin.
    Meanwhile, a northern-stream perturbation over the Upper Great
    Lakes, that peeled eastward off the cyclone's influence, has a
    compact cyclone aloft centered near ANJ. This feature should
    devolve into an open shortwave trough and accelerate eastward as it
    enters the northern stream today, reaching eastern NY and western
    New England around 00Z.

    In the southern stream, broadly cyclonic flow will prevail over the
    western CONUS. A weak perturbation -- apparent in moisture-channel
    imagery over the central Gulf of California -- should move
    east-northeastward through the period, reaching northern Coahuila
    and adjoining southwest TX around 00Z, then parts of LA/northern MS
    by the end of the period. Downstream perturbations over the
    Southeast and central/southern Appalachians should be predominantly convectively derived.

    At 11Z, a surface lows were evident over southwestern ND and between
    OFK-YKN, along an occluded front. An occluded/cold front arched
    from the NE low across northeastern KS to another weak low near CNU,
    then southwestward over southwestern OK and the TX South Plains.
    Today, the cold front will overtake the northern part of a dryline
    now over northwest TX to the Big Bend region. By 00Z, the front and
    dryline together will act as a western limit of substantial
    convective/severe potential, from eastern OK to
    north-central/central TX. A Gulf warm front over northeastern OK
    and central/northeastern AR was moving northward and should cross
    the Ozarks this morning, moving into southern MO. A progressive
    outflow boundary was apparent from northern VA across southern WV,
    southeastern KY, to northern parts of middle/western TN and
    southeastern MO. The western part of this boundary is stalling, and
    soon should retreat northward through the rest of the morning across
    parts of western KY and southeastern MO, essentially linking with
    the warm front to form a baroclinic zone focusing the greatest of
    today's severe risks.

    ...Ozarks to Mid Mississippi, lower Ohio and Tennessee Valleys...
    A complicated, potentially dangerous severe-weather event is
    possible today into this evening. Multiple rounds of strong-severe thunderstorms are expected today across this region, additively
    contributing enough severe hazard for the "moderate" (level 4 of 5)
    risk area. A swath of damaging to severe gusts and at least a few
    embedded tornadoes are likely to accompany a complex of
    thunderstorms from southern MO across parts of the multi-state area
    near the Ohio/Mississippi River junction, to at least northern parts
    of the Tennessee Valley region. The main MCS should move
    east-southeastward to southeastward along and near the instability
    gradient associated with the aforementioned outflow boundary.
    Before and during that process, several supercells are expected,
    offering tornadoes (a few potentially significant/EF2+), large to
    very large hail, and localized severe gusts.

    An ongoing area of strong-severe thunderstorms across portions of MO
    should pose mainly a severe-hail threat for another couple hours,
    though the gust potential will increase as it grows upscale. See
    severe thunderstorm watch 202 and related mesoscale discussions for
    the near-term threat. As the morning progresses, the wind threat
    will ramp up further, with some tornado potential starting as well,
    as this activity and/or additional convection developing later over
    western MO interacts with 1) a northward-moving plume of
    surface-based inflow air and 2) rich low-level moisture now found
    south of the outflow boundary. Meanwhile, diurnal destabilization
    of an already richly moist airmass south of the boundary will grow
    MLCAPE commonly into the 3000-4000 J/kg range, beneath favorable
    (50-60 kt) deep shear. Low-level shear/vorticity, SRH and hodograph
    size will peak near the boundary, supporting a corridor of
    relatively maximized tornado potential with any sustained/discrete
    or semi-discrete supercells in the area, as well as a focus for MCS
    forward propagation from the northwest. MCS activity should merge
    upscale further this evening and tonight, maintaining severe-wind
    potential into parts of the Tennessee Valley region before moving
    into gradually less instability late tonight.

    ...Eastern OK to AR, Mid-South and central TX...
    Thunderstorms should develop by late afternoon near the front and
    dryline, as a combination of lift along those boundaries and strong
    surface heating combine with very rich low-level moisture to erode
    the EML cap sampled well by the 12Z FWD RAOB. Large to giant hail,
    occasional severe gusts, and a few tornadoes are possible as
    activity shifts eastward to northeastward across central/east TX,
    the Arklatex, and AR through the evening. The northern part of this
    convection may merge with or be overtaken by MCS activity moving out
    of MO and into parts of the Mid-South.

    The afternoon/preconvective environment should be characterized by
    steep low/middle level lapse rates on either side of the weakening
    cap, along with surface dewpoints commonly in the 70s F. This will
    contribute to strong buoyancy with the peak/preconvective MLCAPE
    reaching 4500-5500 J/kg, and SBCAPE values topping 6000 J/kg.
    Although near-surface flow generally will be 10 kt or less, limiting
    lowest-km hodographs/shear, effective-shear magnitudes in the
    40-55-kt range indicate supercells will be possible. These will be
    capable of very large/destructive hail exceeding 3 inches in
    diameter. Clusters or upscale mergers of convection also may offer
    deep, precip-loaded, hail-cooled downdrafts with locally severe
    gusts. Cell mergers and interactions with boundaries will factor
    into tornado potential on the storm scale, since the environmental
    low-level shear appears on the margins.

    ...Southern Appalachians/Cumberland Plateau into Carolinas...
    Thunderstorms are ongoing along and behind the outflow boundary near
    the TN/KY border area, and may pose a severe threat through the
    remainder of the morning while moving generally southeastward. See
    SPC mesoscale discussion 693 for near-term details. Further
    intensification and/or additional development is possible into the
    afternoon over and east of the mountains, moving into an environment
    of favorable moisture and diurnal destabilization. MLCAPE in the
    1500-2500 J/kg range appears attainable, amidst roughly 35-45-kt effective-shear magnitudes. A mix of supercell and multicell modes
    is expected, with small bows possible, supporting localized threats
    for damaging wind, large hail and at least marginal tornado
    potential.

    ...Northeastern CONUS...
    Widely scattered to scattered thunderstorms may develop as soon as
    mid/late morning over parts of eastern NY and New England. Activity
    should form atop a diurnally destabilizing and weakly capped
    boundary layer, along/ahead of a prefrontal surface trough and
    behind a plume of non-severe thunderstorms and precip now crossing
    parts of the region. Thunderstorms should move generally eastward,
    offering damaging to isolated severe gusts and isolated large hail.
    A combination of low-level warm advection, spotty/irregular diurnal
    surface heating, and cooling aloft related to the approaching Great
    Lakes shortwave trough, will steepen low/middle-level lapse rates substantially. With favorable moisture (surface dewpoints generally
    50s to low 60s F), a plume of 500-1000 J/kg MLCAPE should develop,
    amidst effective-shear magnitudes that reach 40-50 kt, and despite
    strongly veered/westerly surface wind components. A mix of
    multicell and modest supercell characteristics should be observed
    before activity moves over more-stable air and/or offshore, and
    weakens by early evening.

    ...Eastern SD to eastern IA and vicinity...
    Widely scattered to scattered thunderstorms are possible from early
    afternoon in western parts to evening closer to the Mississippi
    River, with isolated damaging gusts, large hail and possibly a
    tornado or two. Activity should form in an area of deep-layer lift
    (including low-level convergence/mass response) related to a
    vorticity lobe in the southeastern part of the mid/upper-level
    cyclone. Associated cooling aloft will combine with residual
    low-level moisture (dewpoints mid 40s to mid 50s F) to yield pockets
    of around 500-1000 J/kg MLCAPE. Backed low-level winds will aid
    convergence and storm-relative low-level flow, and may contribute to
    locally enlarged hodographs.

    ..Edwards.. 05/08/2024

    $$
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