• MESO: Mesoscale Discussion 1304

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Sat Jul 17 16:01:22 2021
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    ACUS11 KWNS 171601
    SWOMCD
    SPC MCD 171600=20 NYZ000-CTZ000-MAZ000-NJZ000-PAZ000-DEZ000-MDZ000-DCZ000-VAZ000-WVZ000-17183=
    0-

    Mesoscale Discussion 1304
    NWS Storm Prediction Center Norman OK
    1100 AM CDT Sat Jul 17 2021

    Areas affected...Portions of northern VA...far eastern WV...MD...DC...DE...NJ...central/eastern PA...southern NY...and
    western CT/MA

    Concerning...Severe potential...Severe Thunderstorm Watch likely=20

    Valid 171600Z - 171830Z

    Probability of Watch Issuance...80 percent

    SUMMARY...The threat for scattered damaging wind gusts should
    increase this afternoon. Severe Thunderstorm Watch issuance is
    likely early this afternoon.

    DISCUSSION...A weak, positively tilted upper trough extends across
    parts of the lower Great Lakes and OH Valley as of 16Z. A belt of
    25-40 kt of mid-level southwesterly flow was observed with the 12Z
    sounding from PIT ahead of this feature. Recent visible satellite
    imagery shows mostly clear conditions generally along/east of the
    Appalachian and Blue Ridge Mountains. Strong diurnal heating will
    likely allow temperatures to continue to climb into the 80s and 90s
    across the much of the Mid-Atlantic and into parts of southern New
    England this afternoon. A rather moist low-level airmass, with
    surface dewpoints in the upper 60s to mid 70s, exists across this
    region. Even with mid-level lapse rates remaining poor, MLCAPE is
    expected to reach 1500-3000 J/kg, stronger across northern VA into
    MD, eastern PA, and NJ. The better mid-level flow may tend to remain
    displaced slightly to the west and north of the strongest
    instability. Still, around 25-35 kt of effective bulk shear should
    prove sufficient for mainly multicell clusters.=20

    Steep low-level lapse rates owing to the robust heating and a very
    well mixed boundary layer suggest a threat for scattered damaging
    wind gusts from convective downdrafts as scattered to numerous
    storms develop over the higher terrain and then move eastward. The
    northern extent of the severe threat will likely be constrained by a
    warm front extending across southern NY into southern New England.
    Slightly stronger low-level and deep-layer shear in the vicinity of
    this warm front may support a couple of marginal supercell
    structures, and a brief tornado or two appears possible. Latest
    mesoanalysis estimates suggest weak convective inhibition remains
    across the Mid-Atlantic. But, continued heating will erode this
    inhibition over the next couple of hours, and storms are expected to
    develop during the early afternoon. Even though some uncertainty
    still exists regarding the timing of Severe Thunderstorm Watch
    issuance, a Watch will likely be needed early this afternoon as the
    severe threat increases across much of the Mid-Atlantic into parts
    of southern New England.

    ..Gleason/Dial.. 07/17/2021

    ...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!q0E-1cL9sJN-io2u7NKGpAcC_UJHCV81i_LYfZW5o7NTHewJOcUSWI-e9iZ-TQrtiGOV1mjm$=
    for graphic product...

    ATTN...WFO...BOX...OKX...ALY...PHI...BGM...AKQ...CTP...LWX...

    LAT...LON 38777495 38447637 38487774 39027872 40297851 41457792
    42097657 42337524 42417406 42627334 42647255 42037245
    41437290 40677335 39587411 38777495=20



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