Dangerous storms slam the Northeast, leaving New York City under flood warning and killing 4 in Pennsylvania

More than 5 million people in Southern Connecticut and Southeastern New York, including most of New York City, are under a flash flood warning until 4 p.m. ET., according to the National Weather Service.

A storm moving through the Northeast is threatening more than 40 million people with excessive rainfall and has already caused flooding that led to at least four deaths in Pennsylvania.

Weather conditions in New York are unstable, New York Gov. Kathy Hochul told reporters Sunday, warning the state could see record rainfall levels. She pleaded with residents to avoid unnecessary travel.

"A flash flood doesn't give you warning. It comes literally in a flash," Hochul said. "And in those moments, your car can go from a place of safety to a place of death."

The power is out in about 9,300 homes, mainly in Dutchess and Sullivan Counties, Hochul said, noting utility workers are trying to restore power.

"If you're one of those 9,300 families without your power, it is a frightening time," she said.

Hochul said Suffolk County received five inches of rain in two hours, and said five inches of rain over the course of a day would be significant, let alone over the course of two hours.

Major airports are also experiencing significant weather-related flight delays Sunday. All flights serving the major airports around New York City and Boston faced ground stops around 1 p.m., including Boston Logan International Airport, LaGuardia Airport, Newark Liberty International Airport and John F. Kennedy International Airport, according to the Federal Aviation Administration.

More than 1,300 flights within, into, or out of the US had been canceled by Sunday afternoon, and more than 5,400 were delayed, according to flight tracking website FlightAware.com.

The National Weather Service issued a tornado watch for large portions of the Northeast, including parts of the New York City metro Sunday. Much of the recently flooded New England region could face torrential rainfall. The weather service issued a Level 3 of 4 threat for excessive rainfall for areas including New York City, Boston, Philadelphia, and Washington DC.

"There are more than 5 million under a flash flood warning, including much of Manhattan. It might be one of the largest population numbers under a single warning I've seen," said CNN meteorologist Brandon Miller.

The National Weather Service cautioned most flood deaths occur in vehicles and urged people not to attempt to drive down flooded roads.

"Excessive runoff from heavy rainfall will cause flooding of urban areas, highways, streets and underpasses as well as other drainage areas and low lying spots," the weather service said.

Four people have been found dead due to flooding in Pennsylvania, officials say, as they search for three who remain missing, including a two-year-old girl and her nine-month-old brother.

Officials in Bucks County, where the children are missing, are also searching for an adult woman, Upper Makefield Fire Chief Tim Brewer told reporters Sunday.

Intense rainfall flooded roadways, catching many on the roads by surprise Saturday and leaving some trapped, the Upper Makefield Township Police Department said in a statement.

Search and rescue efforts are ongoing, Brewer noted.

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

Several area agencies were assisting with rescue missions overnight.

"Our department is assisting Upper Makefield Township Police Department with a search and rescue operation on Taylorsville Road in the area of Washington Crossing Road (Route 532) for missing persons lost in the flood," said Newtown Township Police Department.

The state's emergency management agency is monitoring the flooding and coordinating with water rescue teams in Bucks County, Pennsylvania Gov. Josh Shapiro said on Twitter. The emergency management agency is urging residents to check road conditions before traveling.

"Never drive into standing water. Mere inches of flood water can carry you away in your vehicle," the agency said on Twitter. "Mere inches of flood water can carry you away in your vehicle."

The agency also warned some homes may lose power.

Over the last month, parts of interior New England and the Northeast have seen 200% to 300% of their average monthly rainfall, leading to last week's disastrous flooding event in parts of Vermont, New York and western Massachusetts.

"Given some parts of the Northeast contain saturated and sensitive soils from recent heavy rainfall over the past 10 days, this is a setup primed to produce flash flooding that could be significant in affected areas," the weather service said.

A 35-year-old woman died last week when she was swept away by floodwaters as she tried to evacuate her Orange County home in New York. Officials say the flooding there caused tens of millions of dollars in damage.

Hochul said Sunday she would be announcing additional funding this week, particularly to help the homeowners in Orange County who were hit hardest.

"We need to have a conversation about how we can make flood insurance more affordable and accessible to New Yorkers," Hochul said, adding extreme weather events may very well be the new normal. "Homeowners are not prepared. They don't have the money in their pockets and their bank accounts and they're expected to cover these costs themselves."

Vermont faced flooding of the likes not seen since Hurricane Irene devastated the state in 2011. The intense rainfall gushed through streets and homes, prompting hundreds of evacuations and more than 200 rescues.

President Joe Biden approved an emergency declaration for Vermont, authorizing FEMA to move in needed equipment and resources.

Even with the help, "this is going to be a years - if not a decade - long recovery for the state of Vermont," said Jennifer Morrison, the state's public safety commissioner.

Steady warming and atmospheric changes are "supercharging" regular weather events, making them longer and more intense, Michael E. Mann, a climate scientist and distinguished professor at the University of Pennsylvania, told CNN.

Climate experts say it is part of a "perfect storm" this summer, leading to deadly flooding in places like the Northeast while other parts of the world - including the Southwestern US - are scorched by record-breaking heat.

CNN's Jennifer Gray, Samantha Beech and Michelle Watson contributed to this report.