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By Alex Johnson

Thunderstorms stretched over the eastern half of the country Thursday and were heading straight for the Northeast — particularly Philadelphia, where the worst could arrive Friday just as thousands of delegates and spectators were hoping to head home from the Democratic National Convention.

A flash flood watch was in effect through Friday morning for Philadelphia, where delegates, journalists and protesters were packed in for Hillary Clinton's address Thursday night.

Rain was already washing out many protests even before Clinton took the podium.

The bathroom of a home in Bellevue, Ky., is damaged after floodwaters reached 6 feet Thursday.WLWT-TV via NBC News Channel

Moist, tropical air mass and an upper-level disturbance were fueling flash floods across the Midwest, including the Ohio Valley and the Appalachians, said Chris Dolce, a meteorologist for The Weather Channel.

By Friday, "the heavy rain will likely cause plenty of delays for travelers in the Northeast," he said.

In the Chicago suburb of Berwyn, a 34-year-old woman was in serious condition after she was struck by lightning Thursday afternoon, NBC Chicago reported.

More than 3 inches of rain pummeled metropolitan Cincinnati in just two hours Thursday morning. At least 22 homes were damaged by lightning-fast flash floods rising 6 feet in Bellevue, Ky., just across the Ohio line.

Residents said they watched as air conditioners, refrigerators and vehicles floated away.

"I thought it was, like, my dream," Andrew Groh of Bellevue told NBC station WLWT of Cincinnati. But "then I walked out, and my car's gone, my motorcycle's gone. And I just saw chaos. That's all it was."

Two firefighters in Anderson Township southeast of Cincinnati were "launched into the water" Thursday morning when their rescue boat flipped over as they were working to rescue five stranded motorists, Township Trustee Josh Gerth said.

The firefighters and the motorists were "shaken up but otherwise OK," said Gerth — but the boat had vanished.

Bernie Sanders supporters flee a rally during a rain storm Thursday in Philadelphia.John Minchillo / AP

As the system moves northeastward, major cities along the I-95 corridor could get frequent lightning and as much as 3 inches of rain Friday, including Washington D.C., New York and Philadelphia.

"What we've got going on is really not a concentrated, big, bad, severe weather threat," said Ari Sarsalari, another meteorologist for The Weather Channel. "But there's going to be a lot of action over a large area.

"The flooding is going to be the big story over the next couple of days," Sarsalari said. "The rain just keeps going over the same areas, and it's not going to stop until at least the end of the week."