All told, the area the weather service considered to be under heightened risk of dangerous weather included 74.7 million people in 19 states.
Last year, a derecho caused at least $1 billion in damage from Chicago to Washington, killing 13 people and leaving more than 4 million people without power, according to the weather service. Winds reached nearly 100 mph in some places and in addition to the 13 people who died from downed trees, an additional 34 people died from the heat wave that followed in areas without power.
Derechoes, with winds of at least 58 mph, occur about once a year in the Midwest. Rarer than tornadoes but with weaker winds, derechoes produce damage over a much wider area.
Tornadoes and a derecho can happen at the same time. Straight-line winds lack the rotation that twisters have, but they can still cause considerable damage as they blow down trees and other objects.
For Washington, Philadelphia and parts of the Mid-Atlantic the big storm risk continues and even increases a bit Thursday, according to the weather service.
The term derecho was coined in 1888, said Ken Pryor, a research meteorologist at the Center for Satellite Applications and Research at the National Oceanic and Atmospheric Administration in College Park, Md. The word is Spanish for "straight ahead" or "direct," Pryor said.
The structure of a derecho-producing storm looks distinctive in radar and satellite imagery, Pryor said. "The systems are very large and have signatures that are very extreme," he said. "You get large areas of very cold cloud tops that you typically wouldn't see with an ordinary thunderstorm complex. The storms take on a comma or a bow shape that's very distinctive."
The Storm Prediction Center: www.spc.noaa.gov
Associated Press writers Charles Wilson in Indianapolis, Barbara Rodriguez in Des Moines, Iowa, and Seth Borenstein in Washington, D.C., contributed to this report.