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.

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. / AP

Updated July 16, 2023 at 8:30 PM ET

At least five 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 later said two other bodies had been found, bringing the total to five fatalities.

"This is a mass casualty incident the likes of which we have not seen before," the township said in a Facebook post.

Two children are still missing: a 2-year-old girl and her 9-month-old brother.

The children were part of a family from Charleston, S.C., that was visiting friends and family in the area. The family was driving to a barbecue when they got caught in a flash flood.

The father was able to get his 4-year-old son to safety, but the mother, grandmother and two younger children were swept away by the floodwaters. Officials said Sunday that the mother died but the grandmother survived and was treated at a local hospital.

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.

Forecasters said a cluster of heavy thunderstorms was expected to fall on parts of Louisiana, as well as the southern regions of Mississippi and Alabama on Sunday night.

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 1,600 flights in the U.S. were canceled Sunday and more than 8,500 were delayed, according to the flight-tracking website FlightAware.

NPR's Juliana Kim contributed to this report.

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