A large portion of the Mid-Atlantic and the Northeast was under threat of flash flooding on Monday as a sluggish storm moved east, unleashing showers and thunderstorms from the Ohio Valley to the East Coast.

Key Things to Know

• Forecasters warned of the risk of intense rainfall in several places, including New York City, Philadelphia and Washington, D.C. Localized totals over five inches “certainly seems plausible,” forecasters said Monday afternoon.

• Flash flood watches were in effect from North Carolina to New York. A watch means conditions are favorable for severe weather, and a warning is issued once an event is occurring, or about to.

The Forecast

Forecasters said the system’s slow speed and the presence of unusually hot and humid air were contributing to the high flood risk, particularly along the Interstate 95 corridor from Washington to northern New Jersey.

“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,” said Joseph Wegman, a meteorologist with the Weather Prediction Center.

The heaviest rainfall and the most widespread thunderstorms are expected to develop from Northern Virginia through southern New England in the afternoon. The Weather Prediction Center has issued a “moderate” (Level 3 out of 4) risk for flash flooding in that area through Tuesday morning.

Rainfall rates could reach two to three inches per hour in some areas, with widespread totals of two to four inches and isolated areas nearing six inches.

With the potential of heavy rain falling in a short period of time, forecasters were concerned about urban areas where flash flooding could occur rapidly as water collects on concrete, unable to soak into the ground as it does on grass.