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.

Life-threatening flooding was possible in western Union County, New Jersey, the National Weather Service warned.

Washington, D.C., Baltimore and Philadelphia are also under flood watches Monday night.

Slow-moving storms are expected to drop a lot of rain quickly. Rainfall rates up to 3 to 4 inches per hour are possible.

Flooding was reported in parts of southern Pennsylvania, including Lancaster County, after heavy rain moved through the area earlier Monday.

This is a developing story. Please check back for updates.