Torrential rain on Long Island caused flash flooding and led to several water rescues early Monday morning, as the National Weather Service declared a flash flood emergency.

More than 900,000 people in Suffolk County on Long Island were under a flash flood emergency until 4:30 a.m. on Monday, according to the National Weather Service. The agency said just after midnight that multiple rescues were ongoing in the Nesconset, Ronkonkoma, Smithtown and St. James areas.

“This is a particularly dangerous situation,” the Weather Service said. Between four and six inches of rain had fallen, it said, and another two to four inches of rain were possible.

The same storm brought heavy rainfall in southwestern Connecticut on Sunday, leading to mudslides, washed-out roads, flash flooding and rescues. It also forced all airports in the New York City area to ground flights.

Flash flood emergencies or flash flood warnings were declared on Sunday for parts of Fairfield, New Haven, Litchfield and Hartford counties in Connecticut, as overfilled rivers crested their banks.

Some parts of Connecticut received nearly 10 inches of rain between Saturday afternoon and Sunday night. Emergency services conducted dozens of rescues, including, in one case, evacuating 18 patrons and employees from a restaurant in the town of Oxford.

This is a developing story.