Derecho storms bears down on New England, Mid-Atlantic

Extreme weather is headed eastward after drenching the Midwest, as thunderstorms and high winds threatened the East coast from as far south as Georgia and as far north as Philadelphia and Baltimore Thursday.

Western Philadelphia, Washington D.C., and Maryland could also see hail and dreaded derechos -- straight-line storms that pack hurricane-force winds -- and even tornadoes.

The system was expected to head north toward New York and as far as Boston into Friday, bringing with it threats of flooding and wind damage.

Tornados, high winds and heavy rains knocked out power to at least 55,000 in Illinois and Indiana Wednesday night and threatened Cleveland with baseball-sized hailstones.

Derechos are fast-moving storms that form in a straight line, most often forming in the Northern Hemisphere in June and July. In the U.S., they generally begin in the Midwest and move eastward and span at least 250 miles long.

These types of severe storms are particularly damaging not only because they are capable of spawning tornadoes, but seem to strike very suddenly.

A similar storm in along the East Coast last summer knocked out power to more than 3.7 million people.