Jessica Louise Dye, a New York City musician, thought the packed subway car she was riding uptown through Manhattan on Monday had ground to a halt because of the usual rush hour delays.

Then Ms. Dye, 39, heard the gasps. She looked out the window and saw water cascading down the stairs from a station entrance at 28th Street and Seventh Avenue. In front of a turnstile, a murky torrent was spewing like a geyser.

Ms. Dye, who has lived in New York for more than a decade, said she felt a little nervous about being stuck underground under those circumstances. Still, her first thought was about how long she would have to wait.

“I was like, great, of course, I should have just walked,” she said.

As torrential rains drenched large swathes of the Mid-Atlantic and dumped several inches of rain on a region stretching from central Virginia to the New York metropolitan area on Monday evening, flash floods inundated multiple New York City subway stations, submerged major roads and prompted the National Weather Service to issue flash flood warnings for all five of the city’s boroughs.

Videos posted to social media showed floodwaters gushing into subway cars, inundating streets and submerging vehicles across the region. Water pressure appeared to lift manhole covers from the road.

In New Jersey, social media videos appeared to show a person kayaking down the street and the news media filmed rescue crews helping people stuck on flooded roads.

Several minutes after the No. 1 train Ms. Dye was riding stopped at the 28th Street station, the driver opened the doors. Water rushed in, prompting some passengers to climb onto their seats. Then the doors closed again.

One passenger, Juan Luis Landaeta, later said that he had never seen flooding like that in 12 years of living in the city.

“It was a disaster,” he said.

Ms. Dye said it was worse than anything she had seen in 2012, when Hurricane Sandy swamped parts of the city in waist-deep waters.

She said some tourists on the No. 1 train, including two women in open-toed sandals who were consulting one another about how they might wade through the floodwaters, seemed concerned. “The real New Yorkers were just on their phones, not caring, not paying attention,” she added.

The conductor kept the passengers informed throughout, she said, though he too seemed exasperated. “He was like, ‘Oh, my God, it’s only Monday,’” she said. “And then he was like, ‘I don’t get paid enough for this.’”

After 10 to 15 minutes, the train departed and continued uptown, Ms. Dye said. When she got out at Times Square, a light fog had settled over the city.

“The heat had calmed down because it finally rained,” she said. “So we got a little bit of a break from the heat wave.”