As a powerful summertime derecho moved from Illinois to the Mid-Atlantic states on June 29, expanding and bringing destruction with it, NASA and other satellites provided a look at various factors involved in the event, its progression and its aftermath.

According to NOAA's Storm Prediction Center web site, a derecho (pronounced "deh-REY-cho") is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms. Damage from a derecho is usually in one direction along a relatively straight track. By definition an event is classified a derecho if the wind damage swath extends more than 240 miles (about 400 kilometers) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length.

These storms are most common in the United States during the late spring and summer, with more than three quarters occurring between April and August. They either extend from the upper Mississippi Valley southeast into the Ohio Valley, or from the southern Plains northeast into the mid-Mississippi Valley.

NOAA's GOES-13 satellite, which watches the movement of weather systems in the eastern half of the U.S., captured the expansion and movement of the derecho from its birthplace in Illinois. The satellite data was compiled and animated by the NASA GOES Project at the NASA Goddard Space Flight Center in Greenbelt, Md.

The movie begins on June 28 at 1515 UTC (11:15 a.m. EDT) and ends on June 30, 2012 at 1601 UTC (12:01 p.m. EDT). In the animation, the derecho's clouds appear as a line in the upper Midwest on June 29 at 1432. By 1602 UTC, they appear as a rounded area south of Lake Michigan. By 2132, the area of the derecho's clouds were near Lake Erie and over Ohio expanding as the system track southeast. By 0630 UTC, the size appears to have almost doubled as the derecho moves over West Virginia, Maryland, Pennsylvania and Virginia. At 0232 UTC on June 30 (10:32 p.m. EDT), the Derecho was over the mid-Atlantic bringing a 100 mile line of severe storms and wind gusts as high as 90 mph to the region.

"It is interesting how the process is a self-sustaining process that is fed by a combination of atmospheric factors that all have to be in place at the same time," said Joe Witte, a meteorologist in Climate Change Communication at George Mason University, Va. and a consultant to NASA Headquarters, Washington. "That is why they are relatively rare: not all the elements line up that often."

Witte said that one could think of the strong winds as a combination to two main wind flows: the downburst winds from very high altitudes, and the forward speed of the storms.

A downburst occurs when cold air in the upper atmosphere is cooled more by the evaporation of some of the rain and melting of the frozen precipitation pushed up into the high levels of the towering cumulonimbus (thunderclouds). That cold air becomes much denser than the surrounding air and literally falls to the ground, accelerating like any other falling body. "The huge blob of very cold air from the upper atmosphere has a higher forward wind speed since it is high in the atmosphere, " Witte said. "This gives the 'blob' great forward momentum. Add that speed to the falling speed and the result is a very powerful forward moving surface wind."

The process of a derecho can become self-sustaining as hot and humid air is forced upward by the gust front and develops more (reinforcing) towering clouds. When one adds in a rear low level jet stream, there is nothing to stop the repeating process.

NASA's Aqua satellite flew over the derecho on June 29 and June 30, using the Atmospheric Infrared Sounder instrument (AIRS) onboard to capture infrared imagery of the event.

Excerpt from:

NASA Satellites Examine Powerful Summer Derecho