Mesoscale Discussion 1023
NWS Storm Prediction Center Norman OK
1134 AM CDT Sat Jun 27 2020
Areas affected...eastern Pennsylvania and northern New Jersey into
Long Island and southern Connecticut
Concerning...Severe potential...Watch possible
Valid 271634Z - 271900Z
Probability of Watch Issuance...40 percent
SUMMARY...At least isolated strong to severe storms capable of
damaging wind gusts or marginal hail are expected to form after 17Z,
and will progress east/southeast toward the coast later this
DISCUSSION...Surface analysis shows veering winds across western PA,
west of a deepening surface trough over eastern NY and PA. Nearly
parallel to the winds a loft, a convergence zone oriented WNW to ESE
is noted over central PA with associated showers, while rain
persists from northeast PA into NY.
Temperatures are warming quickly south of these showers across
southern PA, MD and VA, and west/southwest 850 mb winds of 25-35 kt
will aid destabilization northeastward toward the rain-cooled areas.
A zone of 200 m2/s2+ effective SRH also exists along the warm/cool
air interface, and this should translate east today as the warmer
air spreads into eastern PA, NJ, and NY.
Around 1000 J/kg MLCAPE is expected to develop over the next few
hours where temperatures warm into the 80s, beneath modest westerly
flow aloft. Since this area is behind the primary midlevel
disturbance, lift will be driven mainly by warm advection, with at
least isolated storms forming along or ahead of the aforementioned
convergence zone over PA. A broken line of storms is possible,
producing damaging wind gusts given favorable deep-layer mean winds.
A supercell is possible as storms interface with the larger SRH area
along the warm/cool air interface as well, and a brief/weak tornado
could not be ruled out.
With time, greater instability may spread northeastward into Long
Island and CT, supporting isolated strong or severe storms.