A flash flood watch will remain in effect for most of the New York City area through early Sunday morning after heavy rain on Saturday flooded roads, caused power failures and collapsed a wall in New Jersey.

The thunderstorms started a few hours before daybreak on Saturday and brought rainfall at rates of up to two inches per hour, said Tim Morrin, an observation program leader with the National Weather Service.

Central Park received almost three inches of rain in less than six hours, and Suffolk County on Long Island had four inches of rain in the same period.

“That kind of rainfall, that falls so fast and so heavy, cannot run off and causes flooding,” Mr. Morrin said.