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.

Officials in one county told residents to only dial 911 for life-threatening emergencies Monday night, according to a social media post from officials.

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

Heavy rain caused flash flooding and road closures around the Lehigh Valley on Monday evening as storms made their way through. More rain is expected this week.

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

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.

Here’s what else you need to know to get up to speed and on with your day.