Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 201537
SWOMCD
SPC MCD 201536
NYZ000-PAZ000-OHZ000-201730-

Mesoscale Discussion 1360
NWS Storm Prediction Center Norman OK
1036 AM CDT Thu Jul 20 2017

Areas affected...Portions of western NY...northwestern PA...far
northeastern OH

Concerning...Severe potential...Watch possible

Valid 201536Z - 201730Z

Probability of Watch Issuance...40 percent

SUMMARY...Convection approaching the area ahead of a cold front and
destabilization are expected to support a threat for severe wind
gusts and marginally severe hail developing in the next few hours. A
Severe Thunderstorm Watch may be needed in the next few hours.

DISCUSSION...Ongoing convection, currently sub-severe, over the
Niagara Peninsula will continue to spread east over the Niagara
Frontier region. An eastward-moving cold front now stretching from
southern Ontario across western Lake Erie should support new
thunderstorm development in the next few hours.  Although the 12Z
BUF sounding shows very little instability and numerous dry layers
with a history of subsidence, the 12Z DTX sounding sampled a region
of air with high PW and somewhat steep 800-600 mb lapse rates that
appear to have not been overturned from the widespread convective
activity farther west yesterday and this morning.  This air mass is
expected to be drawn east and northeast ahead of the front during
the day.  The ongoing convection is expected to mostly remain north
of this region of instability in the next hour or two, but the
southern edge of the convection now approaching the Buffalo area
could regenerate toward the end of this period as it emerges into
Erie, Genesee, and Wyoming Counties.

Furthermore, substantial heating now underway over the western NY
Southern Tier region into northwest PA and northeast OH, in the
midst of 68-73F surface dewpoints, should add to the instability in
the next few hours and eventually contribute to MUCAPE values of
1500-2500 J/kg.  Deep, bulk vertical wind shear of 40 kt over
northeast OH to around 60 kt, as seen in KBUF 88D VAD wind profiles,
and likely enhanced by convective outflow from the remnant
convective system over the Ohio Valley, is more than sufficient for
convective organization, including line segments and supercell
structures.  Given the enhancement in low-level lapse rates expected
as the atmosphere destabilizes, severe wind gusts will be the
primary threat, with marginally severe hail possible in the more
organized storms.  The need for a Watch remains uncertain owing to
uncertainties in coverage, but signs of convective organization
could increase the likelihood that a Watch would be needed.

..Coniglio/Cohen/Hart.. 07/20/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...BGM...BUF...CTP...PBZ...CLE...

LAT...LON   43357861 43357802 43267756 42977664 42347634 41857676
            41557781 41327859 41107937 41118041 41348106 41748127
            42368033 42867958 43357861




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