The storm that capsized a boat on Geneva Lake crossed Walworth County in under an hour. A National Weather Service survey later put its peak winds at between 90 and 100 mph.
Forecasters in Milwaukee had been following the system since shortly after 10:30 a.m. Friday, when it was over northwest Illinois and heading toward Southeast Wisconsin. By 11:16 a.m. conditions favored severe storms moving into the state.
The warnings, step by step
At 11:33 a.m. a severe thunderstorm warning went up for Rock County, just west of Walworth County, where Lake Geneva sits. The storm was traveling northeast at 50 mph with 60 mph gusts and quarter-size hail. Six minutes later the wind threat was raised to 70 mph gusts.
A tornado warning followed at about 11:45 a.m. for Rock County, set to lapse as the storm crossed into Walworth County. The thunderstorm warning stayed in force for Walworth County shortly after 12 p.m., with the storm approaching Delavan and still pushing 70 mph gusts. At 12:20 p.m. it was over Delavan Lake, minutes from Lake Geneva.
Ten people were in a boat on Geneva Lake when it arrived. The storm left widespread damage across the county and the boat capsized.
A downburst, not a tornado
The survey concluded that the damage came from a downburst. Unlike a tornado, which spins, a downburst is a column of air falling out of a thunderstorm. Air inside the storm turns cold and dense, drops toward the ground, and fans out in every direction on impact. That produces destructive winds over land or water.
Over open water, according to the report, a downburst with winds near 100 mph can build dangerous waves quickly and cut visibility to almost nothing, even for experienced boaters.
It lasted only minutes on Geneva Lake. The report’s conclusion was blunt: wind is a deadly danger in itself.
