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WASHINGTON CROSSING, PA. -- Heavy rains pounded an already saturated Northeast on Sunday for the second time in a week, spurring another round of flash flooding, cancelled airline flights and power outages. In Pennsylvania, a sudden flash flood late Saturday afternoon claimed at least four lives.

Officials in Bucks County's Upper Makefield Township in Pennsylvania said torrential rains occurred around 5:30 p.m. Saturday in the Washington Crossing area, sweeping away several cars. At least four people died and three others, including a 9-month-old boy and a 2-year-old girl, remained missing, authorities said.

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Other parts of the East Coast were experiencing heavy rain, including Vermont. Authorities there said landslides could become a problem Sunday as the state copes with more rain following days of flooding.

"My team and I continue to monitor the situation as more rain falls in Vermont. There are flash flood warnings throughout the state today. Remain vigilant and be prepared," Vermont Gov. Phil Scott said.

Sunday's strong storms led to hundreds of flight cancellations at airports in the New York City area, according to the tracking service FlightAware. More than 350 flights were cancelled at Newark Liberty International Airport in New Jersey alone, while more than 280 flights were cancelled at Kennedy International Airport in New York. Hundreds of flights were also delayed.

The National Weather Service issued flash flood warnings and tornado watches for parts of Connecticut, western Massachusetts, Vermont and New Hampshire. A tornado warning was issued for an area along the Massachusetts-New Hampshire border.

Thousands of power outages also were reported in the region.

New York Gov. Kathy Hochul urged people to stay home and not drive Sunday until the storms passed. There were no immediate reports of injuries from the storms and flooding in the state.

"As songwriter Annie Lennox would say, here comes the rain again New Yorkers, here comes the rain. It just seems unrelenting this year," she said. "You have to avoid unnecessary travel. ... A flash flood doesn't give you warning ... and in those moments your car can go from a place of safety to a place of death."

Hochul said 5 inches (13 centimetres) of rain fell within two hours in Suffolk County on Long Island.

The governor said work crews were checking the integrity of roads and dams hit by flooding around the state, which saw US$50 million in damages caused by last week's storms. Disaster declarations will cover more than a dozen New York counties.

Manchester, New Hampshire, the largest city in northern New England, opened its emergency operations centre in response to severe weather. Manchester Mayor Joyce Craig and other officials urged residents to stay inside if possible.

Flooding forced Tweed New Haven Airport in Connecticut to close its terminal Sunday. The small airport, which offers daily commercial flights from one carrier, Avelo Airlines, said in a Twitter post that the terminal was closed until further notice. Several flights were reported delayed.

Flash flooding was reported in New Haven, Hartford, Waterbury and other Connecticut towns, leaving many roads impassible. Connecticut Gov. Ned Lamont said he was headed to Bristol, home of ESPN, to view flooding.

In north New Jersey, some roads were closed Sunday as crews worked to repair stretches of fragmented concrete that had buckled under the stress of heavy rain and flooding. Local creeks washed over passageways and a rockslide blocked passage along Route 46. Thoroughfares were a mess of water and rocks covered in brown sludge.

In Pennsylvania, meanwhile, a sudden, torrential downpour turned deadly in Upper Makefield Township.

Fire Chief Tim Brewer told reporters the area got about 6 1/2 to 7 inches of rain (about 18 centimetres) in 45 minutes.

"In my 44 years, I've never seen anything like it," he said. "When the water came up, it came up very swiftly. We do not think that anybody drove into it, that they were actively on that road when it happened."

There were about 11 cars on the road at the time, and three were swept away. There was about 4 to 5 feet of water over the road, he said. The bodies of four people had been recovered and three people -- a woman and the two children -- remained missing as of midday Sunday, Brewer said.

Brewer declined to identify the relationships of the victims but said "one family has been severely affected."

Eight people were rescued from the cars and two from the creek, he said.

All three vehicles swept away were later located, and no one was found inside. One was about 1.5 miles from where it entered the creek.

"We are treating this as a rescue but we are fairly certain we are in a recovery mode at this time," Brewer said.

About 150 people were searching the creek during the night and 100 were involved Sunday morning, walking along the creek, he said.

As far south as North Carolina, floodwaters were blamed on the death of a 49-year-old woman whose car was swept off a road in Alexander County late Saturday night. A man who was in the car with her was rescued.

Meanwhile, recovery efforts were underway in Vermont from recent days of heavy precipitation.

The Vermont Agency of Transportation said 12 state roads remained closed while 12 were partially open to one lane of traffic and 87 have been reopened that were previously closed.

The agency said 211 bridge inspections have been completed this week in damaged areas and there are four state bridges closed and four town structures currently closed.

Rail lines throughout Vermont were also damaged by the rain and flooding, the transportation agency said. The agency said it reopened 57 miles (92 kilometres) of rail lines, and 64 miles (103 kilometres) of rail line remained closed.

"Our crews have been working tirelessly all week to repair the damaged state roads and bridges, and to restore the state's transportation infrastructure for Vermonters and visitors to our state," Transportation Secretary Joe Flynn said.

Heavy precipitation was not the only extreme weather plaguing the U.S. A scorching heat wave across the U.S. Southwest has put roughly one-third of Americans under some type of heat watch or warning. That includes brutal temperatures in the hottest place on Earth -- Death Valley, which runs along part of central California's border with Nevada. Las Vegas also faced the possibility of reaching an all-time record temperature Sunday.

The U.S. Environmental Protection Agency posted air quality alerts for several states stretching from Montana to Ohio on Sunday because of smoke blowing in from Canadian wildfires. Hochul, the New York governor, said she expected air quality alerts to be issued for northern and western parts of New York state Monday because of the wildfires.

"Air Quality alerts are in place for much of the Great Lakes, Midwest, and northern High Plains," the National Weather Service said. "This is due to the lingering thick concentration of Canadian wildfire smoke over these regions."

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Associated Press writers Ron Todt in Philadelphia; David Collins in Hartford, Connecticut; Patrick Whittle in Portland, Maine; and Leah Willingham in Charleston, West Virginia; contributed to this report.