Several subway lines were impacted by flooding. A photo from the 28th Street station in Manhattan showed water gushing from a drain and soaking the platform. Water was pooling on the platform at 34th Street-Penn Station, one of the city's busiest subway stops, as riders carefully maneuvered around.

More than 50 million Americans throughout the Northeast are under flood watches on Monday, with a significant risk of flash flooding in some areas. Flash flood warnings have been issued for cities including New York City, Newark, New Jersey, and Arlington, Virginia -- including the Reagan National Airport -- through Monday evening.

Forecasters warned more rain and flash flood risk is expected on July 15 after storms flooded out roads the day before.