Rescue crews are scrambling to find two missing children after their mother died in turbulent floodwater over the weekend.

Officials have tripled the number of resources Monday in the search for the 2-year-old girl and her 9-month-old brother, who were swept away after a ferocious storm pummeled southeast Pennsylvania.

"This is going to be a massive undertaking," Upper Makefield Fire Company Chief Tim Brewer said Monday.

Thanks to improving weather Monday, searchers are able to use more resources, including underwater and air assets as well as drones and canine units.

The children and their family were visiting from South Carolina and were driving to a barbecue when they got stuck in flash flooding, Brewer said Sunday.

The family tried to escape the floodwater, Brewer said: The mother and a grandmother grabbed the two now-missing children, and the father took a 4-year-old boy.

The father and the 4-year-old "miraculously" made it to safety, the fire chief said. But the mother, grandmother and younger children were swept away.

The children's grandmother survived and was treated at a hospital, police said.

The mother's body was found Saturday, Brewer said. Authorities have not publicly identified her.

She is among five people who died as storms pummeled Bucks County, Brewer said.

"The mass casualty incident, like these, which we have never seen before, (is) unbelievable and devastating to all the families involved," he said.

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 in parts of Vermont, New York and western Massachusetts.

Floods are among the deadliest weather hazards in the US, according to the National Weather Service. It only takes 2 feet of rushing water to carry away most vehicles, including pickups and SUVs, according to the weather service.

A flash flood can happen anywhere intense rain falls faster than the soil can absorb, and generally happen within a short time period after rain, making them more life-threatening, according to the National Weather Service.