Heavy rains and thunderstorms are drenching much of the East Coast on Monday, and 50 million Americans, including those in New York City, are under flash flood watches.

Flash flood warnings have been issued for cities including New York City, Washington, D.C., Baltimore, Newark, New Jersey, and Arlington, Virginia -- including the Reagan National Airport -- through Monday evening.

According to the National Weather Service, life-threatening flooding is possible for parts of Union County, New Jersey. Bergen County in northeastern New Jersey and Westchester County in New York are also under a flash flood warning.

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Forecasters said the storm systems' remarkably slow speed, combined with the hot and humid air, is drastically increasing the risk of flash floods on the East Coast and Mid-Atlantic regions. The areas particularly at risk are along the Interstate 95 corridor, from New York City all the way down to Washington, D.C.

"The concern is one storm will follow another, after another, and multiple rounds over areas that can't handle that much rain, all falling in a short amount of time," Joseph Wegman, a meteorologist with the Weather Prediction Center, told The New York Times.

The most severe thunderstorms and heavy rain were expected to be in the afternoon on Monday, from Northern Virginia through southern New England. The Weather Prediction Center has issued a "moderate" (Level 3 out of 4) risk for flash flooding in that corridor through Tuesday morning.

Rainfall rates have the potential to reach two to three inches per hour in some areas, with widespread totals reaching two to four inches and isolated areas seeing as much as six inches per hour.

In New York City, the National Weather Service warned of "scattered instances of flash flooding" and issued a Level 2 out of 4 risk for flash flooding on Monday. The highest risk of flash flooding was in New Jersey, just west of the Hudson River. The worst of the storms over New York City are expected to hit between 7 p.m. and 9 p.m. EST.

The Level 2 out of 4 risk for excessive rainfall extends from northern North Carolina through parts of New England and into southeast New York State. On Tuesday, the storms are expected to shift more towards the Mid-Atlantic and South, where the risk of flash flooding is lower due to the less urbanized areas.