Update, 10:15 PM: Severe Thunderstorm Watch until 2 AM for South Jersey and Delaware...this does NOT include the immediate city or Southeast Pennsylvania. Looking at radar trends through 10:15 PM, it looks like a good chunk of this storm complex goes south of the city, with the city getting in on the northern fringes of it. This probably isn't anything more than a light show for those north of the PA Turnpike...those of you in the city might get a heavy storm but the brunt of activity over the next several hours looks to be Wilmington-Barnegat on south.
Update, 9:30 PM: The Storm Prediction Center has placed areas from Wilmington to Barnegat on south under a slight risk for severe weather for the duration of the night, with DC under a moderate risk for severe weather as the squall line/derecho moves through the Mid Atlantic. The best chances for the worst weather will be to Philly's south over the next several hours but even Philadelphia could see a few rumbles of thunder after 11 PM as the complex of storms brushes through the city...and hits more heavily south of town.
The ring of fire around the heat high has been pretty impressive -- this morning's thunderstorms did a number on the Delaware Valley as they passed through between 4 and 7 AM and now we have a second round of thunderstorms approaching from the west. These storms are part of a squall line that developed in Illinois this morning; one that can easily be termed a derecho for its long track, multiple wind damage reports (wind gusts to 90+ miles per hour in Fort Wayne earlier today), and ability to sustain itself as it travels along the northern fringes of heat. The squall line/derecho is pushing southeast through Southwestern Pennsylvania and West Virginia, with its sight set on DC and Northern Virginia over the next few hours.
Of interest locally is the northern fringes of this squall -- storms are firing ahead of the squall line and moving east or slightly south of east through Southern Pennsylvania. A few cells are as close as Shippensburg and along the PA Turnpike. These are still about three hours away from Philadelphia but there's a chance that a few of these could cross through the region after 11 PM. Gusty winds, hail, and lots of thunder and lightning can't be ruled out as they move in...if they hold together. There is plenty of heat and instability in the atmosphere to support them -- the question is how far north they end up tracking.
Best chances of thunder are south of the city...Wilmington, Millville, Dover, down through Baltimore and DC look like they will get in on thunderstorms over the next few hours. Philly may get in on them -- perhaps even as far north as the PA Turnpike. Not a lock in the city or not but at minimum, a decent light show will await after 10 or 11 PM tonight.
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