Mesoscale Discussion 1139
NWS Storm Prediction Center Norman OK
1045 AM CDT Thu Jul 01 2021
Areas affected...Portions of far eastern WV...northern/central
VA...DC...MD...DE...southeastern PA...and central/southern NJ
Concerning...Severe potential...Severe Thunderstorm Watch likely
Valid 011545Z - 011745Z
Probability of Watch Issuance...80 percent
SUMMARY...The threat for scattered damaging wind gusts is expected
to increase this afternoon, and one or more Severe Thunderstorm
Watches will likely be needed.
DISCUSSION...Mid-level west-southwesterly flow over the Mid-Atlantic
associated with an upper trough/low across Ontario, Quebec, and the
Great Lakes is not as strong today compared to yesterday. Still,
about 25-40 kt is present generally around 3-6 km AGL based on
recent VWP estimates from area radars. An area of precipitation with
embedded isolated lightning flashes extends from northeastern MD
into eastern PA as of 1545Z. There is concern that this
precipitation and its associated widespread cloudiness may tend to
limit destabilization and steepening of low-level lapse rates early
this afternoon, particularly across eastern PA into NJ and vicinity.
Still, there will probably be some clearing behind this initial
activity, as evidenced by recent cloud breaks across parts of
south-central PA.
Better prospects for robust boundary-layer destabilization this
afternoon will likely be realized across parts of northern/central
VA into MD and the Delmarva Peninsula where clouds are less
prevalent. Even though mid-level lapse rates will remain poor,
around 1500-2000 J/kg of MLCAPE should develop this afternoon across
this region through diurnal heating of a moist low-level airmass.
Weaker destabilization, with MLCAPE generally ranging from 500-1500
J/kg, will probably develop farther north into eastern PA and NJ
owing to the influence of the ongoing precipitation. Storms will
probably increase in coverage over the next couple of hours
along/east of the Blue Ridge Mountains and into south-central PA as
a weak/decaying MCV moves eastward from WV. 30-40 kt of effective
bulk shear will likely foster some storm organization, with multiple
clusters likely moving eastward through the afternoon. The greatest
threat for scattered damaging wind gusts should develop where
heating of the boundary layer can steepen low-level lapse rates and
encourage efficient downdraft accelerations.