Mesoscale Discussion 1139

Issued

Jul 1, 2021, 11:45 AM EDT (15:45 UTC)

Expired

Jul 1, 2021, 1:45 PM EDT

In force

2 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...severe thunderstorm watch likely

Chance of a watch

80%

The Dover AFB radar (KDOX), 56 km from the middle of the area, began a volume at 15:48 UTC, the scan nearest the moment this went out.

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• Severe Thunderstorm Watch 331Jul 1, 12:40 PM EDT

196 
ACUS11 KWNS 011545
SWOMCD
SPC MCD 011545 
NJZ000-PAZ000-MDZ000-DEZ000-VAZ000-DCZ000-WVZ000-011745-

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.

..Gleason/Thompson.. 07/01/2021

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

ATTN...WFO...PHI...AKQ...CTP...LWX...

LAT...LON   37917817 37927881 38417897 39087820 39547736 40017643
            40317562 40407487 40207395 39867401 39467418 39187459
            38957478 38727497 38497496 38157508 38007520 37927672
            37917817

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The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.