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.

Storms brought torrential rain to the Lehigh Valley on Monday, causing flash flooding. Flooded roadways were a common site around the region, but the Upper Saucon Township area appeared to see the worst of it. The area of Route 309 at Hopewell Road, near Saucon Creek, was particularly hard hit.