Hundreds of people were evacuated from suburban homes west of Philadelphia after a morning downpour sent streams over their banks and into yards and streets.
There were no immediate reports of injuries.
Several schools shut down because of the flooding (search), some motorists had to be rescued from stranded cars, and thousands of homes lost power after more than 8 inches of rain fell. The National Weather Service (search) issued a flash flood warning for Chester County through the afternoon.
In Downingtown, about 30 miles west of Philadelphia, entire blocks were inundated with muddy water, and roads in and out of the borough were closed. The mayor declared an emergency.
"We have numerous reports of houses with their basements flooded, numerous reports of vehicles stuck in the water," said Patty Mains, spokeswoman for the Chester County Department of Emergency Services.
About 200 people were evacuated from an apartment complex in Avondale, she said.
Katherine Deel of Kennett Square (search) sat atop the roof of her truck for two hours Monday while the Red Clay Creek raged around her. Rescuers finally reached her by boat and took her to safety.
"I was just praying to God that I'd make it out of there," Deel said.
Flash flooding also closed several roads in northern Delaware, just southwest of Philadelphia. Firefighters and police rescued more than a dozen motorists from stalled cars, and a Coast Guard helicopter plucked at least three people from the roof of a chemical facility near Wilmington that was inundated by flooding.