A large swath of the mid-Atlantic and the Northeast was under threat of flash flooding Monday as a sluggish storm moved east, unleashing showers and thunderstorms from the Ohio Valley to the East Coast. Forecasters warned of the risk of intense rainfall in several places, including New York City, Philadelphia and Washington. Localized totals over 5 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.

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, D.C., to northern New Jersey.

The heaviest rainfall are expected to develop from Northern Virginia through southern New England in the afternoon. The Weather Prediction Center has issued a “moderate” risk for flash flooding in that area through Tuesday morning.

— The New York Times