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 prompted flash flood warnings across the New York City metropolitan area Monday night, with photos showing severe flooding across the city and in parts of New York state and New Jersey.

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.