The Crayon 301 NASCAR Cup Series race at New Hampshire Motor Speedway in Loudon (pictured) was canceled Sunday and postponed until Monday due to inclement weather.

Steven Senne/AP

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Steven Senne/AP

The Crayon 301 NASCAR Cup Series race at New Hampshire Motor Speedway in Loudon (pictured) was canceled Sunday and postponed until Monday due to inclement weather.

Steven Senne/AP

At least four people are dead in Pennsylvania after heavy rainfall Saturday evening produced destructive flash flooding, taking residents and drivers by surprise.

It was part of a band of strong to severe thunderstorms sweeping across the Northeast, affecting millions of people from Maryland to Maine.

Authorities in Upper Makefield Township, located along the Delaware River about 30 miles north of Philadelphia, said Sunday morning that three people died after being “swept away” by floodwaters. They said later in the afternoon that a fourth body had been found.

Rescue crews there are also searching for three other people who remain missing: an adult female, a 2-year-old girl and her 9-month-old brother.

“We cannot fathom the grief these families are experiencing and we will do everything we can to assist them during this extremely difficult time,” the township said in a Facebook post.

Emergency responders in Upper Makefield were continuing to scour “very difficult terrain” Sunday, and officials were warning residents to avoid some debris-ridden roadways.

Heavy rains douse a Northeast already soaked by recent storms

According to the National Weather Service, more than 40 million people across the Northeast were at moderate risk of excessive rainfall on Sunday, as major storms were inundating the Philadelphia, New York City, Hartford and Boston metro areas and beyond.

Officials say some parts of the region that have seen heavy rain over the past 10 days may already have “saturated and sensitive soils,” which could increase the likelihood of flash flooding resulting from the weekend storms.

A tornado watch was in effect on Sunday afternoon for parts of Connecticut, Maine, Massachusetts, New Hampshire, New York and Rhode Island. There was at least one tornado warning in Pennsylvania as well as flash flood warnings for parts of Pennsylvania, New Jersey and Delaware.

Vermont was bracing for even more heavy rainfall as it is still recovering from damaging flash floods that occurred across the state less than a week ago.

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

Because of the severe weather, ground stops were in effect at several major airports across the Northeast at times on Sunday afternoon, including Newark Liberty International, John F. Kennedy International, LaGuardia and Boston Logan International.

More than 1,300 flights in the U.S. were canceled Sunday and more than 5,600 were delayed, according to the flight-tracking website FlightAware.