The storm that devastated much of Maryland on Friday, known as a "derecho", not the first of its kind to strike the state, but its impact was among the most severe and widespread.
Derechos are widespread storms in which multiple bands of strong storms packing damaging winds move hundreds of miles. According to the National Weather Service Storm Prediction Center, their name comes from the Spanish word for "direct" or "straight ahead", which is the way the storms typically move.
The heavy winds, typically upward of 60 mph, come from downbursts in storm clouds, caused by differences in the heat and density of air within the storm systems.
Most of the storms occur between April and August in the southern Plains or Mississippi and Ohio valleys, but they have been known to reach the mid-Atlantic and even New England. They happen, on average, once every two to four years in Maryland.
A derecho with the intensity of Friday's storms happens in the mid-Atlantic once every five to seven years, said Greg Carbin, warning coordination meteorologist for the storm center in Norman, Okla. But systems with, say, half of its intensity pass through every summer, Carbin said.
Here are some notable past derechos that reached Maryland, according to the storm center:
- July 4-5, 1980: The storm formed in Omaha, Neb., and moved across nine states to Virginia and Maryland, where winds reached 63 mph. In the strongest part of the storm, winds reached 81 mph in Virginia, Ohio and Illinois, and six people died.
- April 9-10, 1991: Storms formed in the morning of April 9 in Arkansas, moving to the northeast throughout the day, reaching Western Maryland after midnight April 10. Two people died in West Virginia, where winds approached 100 mph in some parts.
- Aug. 4, 2004: The storm formed in Kentucky, moving northeastward spawning high wind reports in Ohio, West Virginia, Maryland and Pennsylvania. Power lines and trees were downed from Oakland in Western Maryland to Frederick and Taneytown, Cockeysville and Bel Air.
- May 21, 2004: The derecho formed in Nebraska and South Dakota, moving to the east across the Great Lakes to Ohio, West Virginia and Western Maryland. A 72 mph wind gust was recorded in Garrett County and trees and power lines were reported downed in Oakland, Frostburg and Darnestown.