A flood warning remained in effect until Tuesday morning for parts of New Jersey, while the slow-moving summer storm was expected to continue showering the Mid-Atlantic region into the middle of the week, according to the National Weather Service (NWS).

"A moisture-rich Summer-time airmass over much of the eastern/central U.S. will continue to lead to areas of scattered to widespread thunderstorms capable of heavy downpours and flash flooding," the government forecaster wrote.

Torrential rain inundated parts of New York, New Jersey, Pennsylvania and Maryland on Monday night.

New Jersey Governor Phil Murphy declared a state of emergency, urging people to stay indoors.

Rescue workers in Scotch Plains, New Jersey waded into gushing floodwaters armed with red flotation devices after several drivers became stranded in the sudden surge, according to footage from CBS.

Authorities across the region, including as far south as Virginia, warned of hazardous driving conditions due to the risk of flash floods.

The NWS urged drivers encountering flooded roads to "turn around."

"Most flood deaths occur in vehicles," it said.

In New York City, video shared on social media showed muddy brown water spewing like a geyser in front of a subway station's turnstiles.

Multiple subway lines were briefly suspended or running with delays, while above ground, massive traffic jams paralyzed several of the city's main thoroughfares.

JFK, LaGuardia and Newark airports temporarily suspended departures Monday night, forcing the cancellation of dozens of flights.

In Lancaster Pennsylvania, severe flash flooding prompted a disaster declaration, while emergency responders plucked people from flooded basements and conducted 16 water rescues.

"Intense rainfall dropped over 7 inches of rain in less than five hours," the fire department in the county's Mount Joy Borough posted on Facebook.

Staten Island recorded between 4 to 6 inches (10 to 15 centimeters) of rain on Monday night, according to the New York borough's emergency notification system.

Zohran Mamdani, the Democrat running for mayor of New York, wrote on social media that the rapid flooding emphasized the need for climate-proofing the city.

"We must upgrade our infrastructure for this new climate reality," he wrote.

The NWS said the storm would be concentrated "across the southern Mid-Atlantic/Appalachians on Tuesday, before shifting northward" on Wednesday.

The latest bad weather follows historic Fourth of July flooding that devastated parts of central Texas, killing at least 131 people, including three dozen children, and leaving more than 100 people still missing.