SPC AC 221228
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0728 AM CDT Tue Aug 22 2017
Valid 221300Z - 231200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS MAINLY THIS
AFTERNOON FROM EASTERN OH AND PA ACROSS NY...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
ENHANCED RISK AREA...FROM KY TO WESTERN NEW ENGLAND...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM TN TO OK
AND THE TX PANHANDLE...
Severe thunderstorms with damaging winds, and possibly a couple of
tornadoes, are expected this afternoon from the upper Ohio Valley to
Pennsylvania and New York. Thunderstorms with isolated
strong/damaging gusts will also be possible farther southwest into
parts of the southern Plains.
...NY/PA to the southern Plains through late evening...
A pronounced midlevel trough over the upper Great Lakes will
continue to amplify while approaching Quebec tonight. Associated
surface cyclogenesis preceding the midlevel trough will drive a
surface cold front southeastward across the lower Great Lakes and OH
Valley by tonight. In addition, the remnants of a subtropical speed
max, enhanced by overnight convection across IL/MO, will accelerate
east-northeastward over the OH Valley/PA/NY in advance of the
primary midlevel trough, and in phase with the diurnal heating
It appears that the primary focus for thunderstorm development today
will a pre-frontal or the remnant outflow/differential heating zone
on the leading edge of the ongoing convection from IN to MO.
Convection will likely strengthen/redevelop by midday or early
afternoon across OH and western NY, and storms will subsequently
spread eastward across PA/NY through the afternoon. Midlevel flow
and effective bulk shear will likewise strengthen into the 40-50 kt
range from OH to PA/NY this afternoon, as surface heating results in
steepening low-level lapse rates and MLCAPE near 1500 J/kg. The
primary convective evolution should a mixed mode transitioning to
line segments, with the potential for downward momentum transfer and
swaths of damaging winds with the more intense bowing segments.
Additionally, there will be a coincident increase in low-level
shear/hodograph curvature across PA/NY during the afternoon in
response to the cyclogenesis to the north. Given the relatively
moist environment, semi-discrete storms could pose a tornado threat,
along with embedded QLCS mesovortices. The large hail threat should
remain marginal as a result of modest midlevel lapse rates.
Deep-layer flow and shear will weaken with southward extent from KY
into TN, and the weak-shear environment will extend westward along
the surface cold front in the vicinity of I-40 in OK/TX this
afternoon. Loosely organized multicell clusters are expected along
the front from TN across AR into OK/TX Panhandle, where isolated
downbursts will be the main threat this afternoon/evening.
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