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.

Heavy rain swept across parts of the Northeast on Monday night, inundating sections of New York and New Jersey with flash flooding that stranded vehicles in roadways and closed subway lines.