Flooding caused delays on multiple New York City subway lines Monday as torrential rain pummeled the Tri-State Area.

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.

Several NYC subway stations flooded on Monday night, causing severe delays and closures after rain drenched parts of NY, NJ and CT.

The region was hit with heavy rain Monday evening, resulting in flash floods that not only impacted roads and air travel, but also the transit system.

A state of emergency was declared in New Jersey after slow-moving thunderstorms unloaded a significant amount of rain, resulting in dangerous flash flooding across the Northeast into Monday night. At least 2 were killed.

The National Weather Service has expanded a warning for potential 'life threatening' flooding in NY to include Rockland and central .