Two people in New Jersey were killed after their vehicle was swept up in flood waters during a storm that moved across the U.S. Northeast overnight, authorities said Tuesday.

Gov. Phil Murphy noted the deaths occurred in the northern New Jersey city of Plainfield, where there were two other storm-related deaths on July 3. A third person was killed in nearby North Plainfield in that previous storm.

"We're not unique, but we're in one of these sort of high-humidity, high-temperature, high-storm-intensity patterns right now," Murphy told reporters after touring storm damage in Berkeley Heights. "Everybody needs to stay alert."

The names of the two latest victims were not immediately released Tuesday. Local officials said the vehicle in which they were riding was swept into a brook during the height of the storm.

"Emergency personnel responded quickly, but tragically, both individuals were pronounced dead at the scene," says a statement the city posted online.

New York City Subway flooded

The heavy rains also caused flash floods in New York and south-central Pennsylvania on Monday night into early Tuesday, prompting road closures and snarling some service on New York City's subway.

At one stop in Manhattan, viral videos posted online showed water flooding down into a Manhattan subway station, submerging the platform while passengers inside a train watch. Another photo appears to show people standing on a train's seats to avoid the water beginning to soak the floor.

Janno Lieber, chair and CEO of the Metropolitan Transportation Authority (MTA), told ABC 7 in New York the city's sewer system got overwhelmed by the rain and backed up into the subway tunnels and to the stations. In several cases, he said, the backup "popped a manhole," creating the dramatic "geyser" seen in some videos.

WATCH | Water pours into New York subway: 

Water gushes into a New York subway station amid floods, heavy rainfall

Heavy rainfall caused severe flooding over New York and New Jersey on Monday night. New York City’s transit authority said its subway’s service was experiencing significant delays, as videos captured floodwater gushing into a station and onto the platform.

"What happened last night is something that is, you know, a reality in our system," he told the TV station, noting the backup happens when more than 4.5 centimetres of rain falls in an hour. "We've been working with the city of New York to try to get them to increase the capacity of the system at these key locations."

Lieber said there is now full subway service, as well as full Long Island Railroad and Metro North commuter rail service after hundreds of people worked overnight to restore operations.

Flooding has proven to be a stubborn problem for New York's subway system, despite years and billions of dollars' worth of efforts to waterproof them.

Superstorm Sandy in 2012 prompted years of subway repairs and flood-fighting ideas, and some have been put into practice. In some places, transit officials have installed or are installing storm barriers at subway station entrances, seals beneath subway air vents and curbs to raise the vents and entrances above sidewalk level.

Meanwhile, summer thunderstorms and the remains of hurricanes have repeatedly flooded parts of the subway system anew. In 2021, the remnants of Hurricane Ida killed more than a dozen New York City residents, largely in basement apartments, and sent water cascading again into subways, renewing attention to resiliency proposals.

This latest storm prompted multiple water rescues in Lancaster County, Pa., where streets and basements flooded after roughly 18 centimetres of rain fell. Some roads remained closed in parts of Pennsylvania and New Jersey on Tuesday. Murphy said the pavement buckled in some locations and state and local officials were assessing the level of damage in several counties, noting the White House had reached out to his office.

A major east-to-west highway in New Jersey was closed to make emergency repairs while dozens of flights were delayed or cancelled at area airports Tuesday, including at least 173 total cancellations at Newark Liberty Airport, according to FlightAware data.

Most flash flood watches and warnings had expired in parts of New Jersey, New York and Pennsylvania as the rain moved on.

Overwhelmed sewers

New York City officials said their venerable sewer system worked as well as it could, but simply was not built to handle rain that fell at the second-highest rate ever recorded in Central Park, surpassed only by the remnants of 2021's Hurricane Ida.

"Imagine putting a two-litre bottle of water into a one-litre bottle. Some of it's going to spill," environmental protection commissioner Rohit Aggarwala said at a virtual news briefing Tuesday.

The city doesn't run the subway system — it's under the separate MTA — but Aggarwala said the two entities have been collaborating to clean sewers near 45 flood-prone subway stations. The city also has sketched out plans to upgrade sewers to handle more water, estimating it would take $30 billion US to do so in about 80 areas that need it most. The city currently spends about $1 billion US a year on stormwater management.