As of 2011, it's been 13 years since the widespread, severe thunderstorm wind event that raked portions of South-central and Southeast Wisconsin with wind gusts up to 100 to 128 mph. This event, which occurred during the pre-dawn hours of Sunday, May 31, 1998, was probably the most powerful, widespread, severe thunderstorm wind event ever to hit Southern Wisconsin. The meteorological community refers to these powerful, widespread extreme thunderstorm wind events as "derecho" which means "straight-ahead" in Spanish. Essentially, a derecho consists of many powerful downbursts.
On May 31, 1998, the strongest thunderstorm winds occurred across the southern parts of the counties of Columbia, Dodge, Washington, and Ozaukee, and the northern parts of the counties of Dane, Jefferson, Waukesha, and Milwaukee. However, powerful gusts occurred outside of these areas as well. The southern tier of counties from Lafayette to Kenosha escaped the strongest winds.
A write-up of this event can be found here (Part 1), and here (Part 2). Part 1 contains a simple summary and a link to a graphic that shows maximum wind gust values - both measured and estimated. Part 2 is from a formal paper that was written by a NWS employee, and contains more "science" information.
The Storm Data write-up, containing a county-by-county and event-by-event breakdown, can be found here.
Essentially, a derecho consists of many powerful downbursts. Below is a simple graphic showing a cross section through a squall line which can be part of a derecho. The strongest of the downdrafts become downbursts with an outflow (dark blue arrows in graphic) of damaging straight-line winds at or near the ground.