Flash floods are occurring across the U.S., from Texas to New Jersey. Here's what you should know as risks escalate.

The National Weather Service urged people to avoid travel if a flash flood warning was in place for their area.

A flash flood warning was in effect in New York City until 9:45 p.m., and New Jersey Gov. Phil Murphy declared a state of emergency. Middlesex, Morris and Somerset counties remained under a flood warning until 7:15 a.m. Tuesday.

The Center Valley area in Upper Saucon Township, where flooding was reported along Route 309, had a recorded 6 inches of rainfall, making it the highest rainfall total in the Lehigh Valley, according to the National Weather Service in Mount Holly, New Jersey. Other areas had about 1 or 2 inches of rain.

Camp Mystic's executive director began evacuating campers approximately 45 minutes after the National Weather Service issued a "life-threatening flash flooding" alert.

New Jersey is under a state of emergency after a flash flood caused at least two deaths in Plainfield and damage across Union County.

A flood watch is in effect into July 15 for more than a dozen counties as saturated ground gets drenched again.

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.