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A super-sized storm

Two images side by side, the first from radar and the second from the WRF model.

The radar image on the left, taken at 11:56 a.m. on May 8, 2009, shows the super derecho’s bow-shaped structure, with a tropical-storm–like eye in the center of the comma head visible in light blue surrounded by heavier echoes. (Image courtesy NOAA/NWS.) The 27-hour WRF model forecast of the event, shown on the right, accurately predicts this rare reflectivity configuration, although the forecast system occurs somewhat southwest and two hours earlier than the observed system. (Image courtesy Morris Weisman.)

A windstorm that swept across Kansas, Missouri, and Illinois on May 8, 2009, was so remarkably fierce that NCAR scientists coined a new term to describe it: super derecho.

A derecho (the Spanish word for “straight”) is a long-lived, straight-line windstorm that is associated with a bow echo, or line of severe thunderstorms. The word was first used in the American Meteorological Journal in 1888 by Gustavus Hinrichs, who sought to describe a derecho event in Iowa.

The May 8 event, however, was no ordinary derecho. The bow echo produced an eye-like structure similar to tropical cyclones. It gained strength as it moved across Kansas in the early morning hours, spinning off 18 tornadoes and packing wind speeds from 70 to 90 miles per hour when it hit Springfield, Missouri. It plowed a path of destruction through the state about 100 miles wide, crossing the Mississippi River with 90 to 100 mph wind gusts before dissipating at Illinois’ eastern border.

NCAR scientists forecast the super derecho 24 hours in advance with the Weather Research and Forecasting model (WRF), using a 3-kilometer horizontal grid interval—the most high-resolution, accurate real-time forecast to date of such an event. Using output from the model simulation, the team is both analyzing how the model was able to capture the event in advance with such accuracy and studying how and why the derecho became so intense by looking at its structure and evolution.

“We are currently in the process of determining how the super derecho developed and evolved, seeking to understand how and why it became so strong as compared to other derecho events,” says NCAR postdoctoral scientist Clark Evans, who is working on the research with Morris Weisman and Lance Bosart. “We also want to know how and why the eye-like structure and associated intense area of low pressure developed, especially given the exceedingly rare nature of these features.”