The National Weather Service issued warnings for up to three inches of rain per hour, leading to flooding in parts of the region.
The tropical-like downpours triggered flash flooding, stranding some motorists and forcing them to abandon their ride.
The rain came down fast and furious, overwhelming storm drains.
In Delaware, a U.S. Postal Service truck and another vehicle became stranded in high water on Governor Printz Boulevard in Edgemoor, New Castle County.
A police officer hopped into construction equipment to get to a man trapped in his car on State Hill Road in Wyomissing, Berks County.
Video shows that another car was almost completely submerged under an overpass in Reading.
Tropical-like downpours pummel region, triggering flash flooding
Nearby, a person was also seen wading through shallower flooding.
In Montgomery County, floodwaters consumed a car near Clearview Avenue and Donna Drive.
"He was able to get out himself. He rolled down his window because when he came through, the water just comes up really quickly," recalled Kathy Gendelman, of Huntingdon Valley.
Heavy rain pummels region and sparks isolated flash flooding
Several lightning strikes were reported, including one in Aston, Delaware County, where a person using a landline phone felt an electric shock. Emergency crews responded, and the individual was reported to be unharmed.
In Towamencin Township, a home on Red Oak Way caught fire after being struck by lightning. Fire officials said the family was displaced, but no injuries were reported.
The saturated ground also led to trees crashing down.
One landed on top of a car in Caln Township, Chester County, where firefighters were able to free the trapped driver. Incredibly, he is okay.
Flash flood warnings were up for parts of New Jersey and Philadelphia, while states of emergency were declared in New Jersey and New York City, where state workers were sent home early.
Wind-driven rain lashes Reading, Pa.
Acting New Jersey Gov. Tahesha Way said Thursday that flooding was reported across several major highways.
The storms also caused a four-hour-long ground stop for flights at Philadelphia International Airport.
Airports in New York, New Jersey and Philadelphia reported significant delays and cancellations. More flight delays and cancellations are already reported for Friday, according to FlightAware's tracker.
Amtrak suspended service between Philadelphia and Wilmington, Delaware, due to high water over the tracks. Services resumed shortly after 8 p.m. on Thursday.
"Rail traffic has been restored between Philadelphia (PHL) and Wilmington (WIL), with water receding from the tracks and providing open routes. Expect residual delays. We appreciate your patience," company officials posted on X.
The heart of the storm reached Delaware County around 2 p.m., where officials reported at least five water rescues.
"It's something you can't take lightly. Floodwaters happen and floodwaters occur, especially in Delco. We need to get the first responders out there and we've stood up the teams to do that," said Edward Beebe, Delaware County's emergency manager. "They are trained to go do these types of rescues, but nonetheless, they're going into a dangerous and delicate situation as well."
Tornado warnings were issued in New Jersey, though no twisters were confirmed. Some residents reported power outages.
In New York, over 100 passengers were rescued from a stranded Long Island Rail Road train near the Bayside station, which had become stuck in rising floodwaters during rush hour, according to WABC. Passengers had to wait about 2 hours before emergency crews could get them to safety.
New York floods after torrential rain pummeled parts of the Northeast
Nearby, a retaining wall behind a home gave way in a rockslide, sending debris into the backyard.
During the height of the storm, heavy rain had closed sections of major New York City roads, including the Long Island Expressway, Cross Island Parkway, FDR Drive and the Clearview Expressway. Some services on the Long Island Rail Road and New Jersey Transit were also suspended.
New York Gov. Kathy Hochul told ABC News' New York station WABC that the worst weather was in Queens, where storms dumped 5 inches of rain.
In Maryland, a person died after being caught in rising floodwaters and then became trapped in a storm drain, police in Mt. Airy, Maryland, said. Police have not provided the age of the person who died. Mt. Airy is located about 40 miles north of Washington.
The Associated Press and ABC News contributed to this report.