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.