A derecho is defined as a linear thunderstorm complex that produces widespread damaging winds. The swath of damaging winds can be more than 100 miles wide and hundreds of miles long. Derechoes are typically in the shape of a bow, but not all "bow echoes" are derechoes.
Such a system formed near Columbia, MO this morning, and progressed ESE across the Saint Louis area (they got hit...again) and is currently aligned parallel to the Ohio River from Evansville, IN to the southern tip of Illinois.
Figure 1. Composite reflectivity from the Evansville, IN radar showing the location of the derecho at 330 pm EDT 07/21/06.
The Doppler radar depicts low level winds in excess of 50 knots (58 mph) just south of the radar site. These are the brightest red colors in the image.
Figure 2. Corresponding velocity image from the NWS radar in Evansville, IN.
The Storm Prediction Center has issued a severe thunderstorm watch for SE MO, southern IL, southern IN, and western KY valid until 800 pm CDT (900 pm EDT). They have labeled this a "particularly dangerous situation" - which is a term usually reserved for major tornado outbreaks. An isolated tornado is possible with this derecho, but the major threat is widespread damaging winds. Look out western Kentucky!
Figure 3. Severe weather reports as of 330 pm EDT. A blue dot is a high wind report (58 mph or greater), a green dot is a large hail report (3/4" - dime size or larger), and a red triangle is a tornado report. It's not hard to see the path this system took.