Flooding killed at least nine people, swept away cars, submerged subway lines and temporarily grounded flights in New York and New Jersey as the remnants of Hurricane Ida brought torrential rains to the area.

New York City Mayor Bill de Blasio described the flooding and weather on Wednesday night as a "historic weather event," and the National Weather Service issued a flash flood emergency in New York City for the first time.

Recovery efforts were underway early on Thursday to bring back transportation systems serving millions of residents in the densely populated metropolitan area.

The governors of New York and New Jersey, who had declared emergencies in their states on Wednesday, urged residents to stay home as crews worked to clear roadways and restore service to New York City subways and commuter rail lines.

"It is not safe to drive," New Jersey Governor Phil Murphy said on Twitter. "Our crews are working to clear and open roads, and we need everyone to stay off them so crews can safely do their job."

Ida's remnants brought six to eight inches (15 to 20 cm) of rain to a swath of the Northeast from Philadelphia to Connecticut and set an hourly record of 3.15 inches for Manhattan, breaking the previous one that was set less than two weeks ago, the National Weather Service said.

The 7.13 inches of rain that fell in New York City on Wednesday was the city's fifth highest daily amount, it said.

“Right now my street looks more like a lake,” said Lucinda Mercer, 64, as she peered out her apartment window in Hoboken, New Jersey, just across the Hudson River from New York.

Mercer, who works as a crisis line fundraiser, said flood waters were lapping halfway up the hub caps of parked cars, and some residents put plastic garbage bags over their shoes to cross the street.

Subway service in New York City remained "extremely limited" while there was no service at all on commuter rail lines to the city's northern suburbs on Thursday morning, the Metropolitan Transit Authority (MTA) said. Janno Lieber, the MTA's acting chair and CEO, told local media it was going to take until later in the day to restore full service.