WASHINGTON CROSSING, Pa. — Crews in suburban Philadelphia on Monday intensified the search for a missing 9-month-old boy and his 2-year-old sister, swept away after weekend rains swelled the banks of a creek while they were driving to a barbecue with their family.

Upper Makefield Township Fire Chief Tim Brewer said Monday the effort would be a “massive undertaking” and that 100 search crew and numerous drones would be looking for the siblings along the creek that drains into the Delaware River in Bucks County, Pennsylvania. The children are members of a Charleston, South Carolina, family that was visiting relatives and friends when they got caught in a flash flood Saturday, Brewer said earlier.

“As they tried to escape the fierce floodwaters, Dad took his 4-year-old son while the mother and the grandmother grabbed the two additional children,” he said. The father and son were “miraculously” able to get to safety. “However the grandmother, the mother, and the two children were swept away by the floodwaters,” Brewer said. The mother was among those later found dead.

The grandmother survived, Upper Makefield Police said in a social media post. But the mother of the two children died. Four other people died in the flooding, but it was unclear who they were. Victims’ names have not been released.

Colleen Courtney, attending a church conference near the search scene Monday, was among those praying for the families.

“It’s just such a tragedy and just so much grief, I’m sure, and mourning that’s going on right now. I pray to find these children,” said Courtney, of Ewing, New Jersey.

Another news conference is planned for Monday afternoon.

An already saturated Northeast began drying out Monday after drenching rain over the weekend resulted in flash flooding in parts of New Hampshire, Massachusetts, Connecticut, New York and New Jersey. New Jersey Gov. Phil Murphy declared a state of emergency Sunday and planned to tour damage early Monday in the northwest part of the state.

A confirmed tornado touched down Sunday morning in North Brookfield, Massachusetts, but no injuries or major property damage were reported. In New Hampshire, where some roads caved in several towns, heavy rain postponed Sunday’s NASCAR race at the New Hampshire Motor Speedway by a day.

Vermont reported no immediate safety threats following historic flooding nearly a week ago that dumped up to two months’ worth of rain in two days. U.S. Transportation Secretary Pete Buttigieg planned to visit the state later Monday.

The Vermont Emergency Management agency reported that swift-water rescue teams conducted an additional six rescues overnight following the storm. The agency also is monitoring areas at risk for landslides due to the rainfall.

More rain was in the forecast for Tuesday.

Sunday’s strong storms led to hundreds of flight cancellations at airports in the New York City area, and hundreds were delayed.

New York Gov. Kathy Hochul said 5 inches of rain fell within two hours in Suffolk County on Long Island. The state saw $50 million in damages from storms in the past week.

In North Carolina, floodwaters were blamed for 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.

Thousands of people in Kansas and Missouri were without power Monday from weekend storms that swept those states. Kansas’ largest electric power provider, Evergy, said it could take days to restore power to all customers. That could cause hardships for those people, as more storms — and stifling heat — are expected in Kansas and Missouri early this week, according to the National Weather Service.

The deadly flash flooding in Pennsylvania called to mind the torrential rain that led to at least 25 deaths in New Jersey when the remnants of Ida passed through the state in 2021. People abandoned cars along washed-out roadways as muddy waters overtook driving lanes and flooded low-lying houses then.

In 2018 in Lancaster County, Pennsylvania, heavy rains brought up to 10 inches of water in a short time. No one died in that flooding.

Associated Press writers Ron Todt in Philadelphia; David Collins in Hartford, Connecticut; Sarah Brumfield in Silver Spring, Maryland; Kathy McCormack in Concord, New Hampshire; Patrick Whittle in Portland, Maine; Margery Beck in Omaha, Nebraska; and Leah Willingham in Charleston, West Virginia contributed to this report.