At 215 pm on Saturday afternoon, August 17th, a dust devil-type circulation caused a pontoon boat to flip over. It had been on a hoist at the end of a pier on the east/southeast shore of Lake Monona.
What happened?!? Skies were clear, which rules out a waterspout. The only explanation we have is a dust devil. This isolated non-thunderstorm wind damage event was probably caused by a land-water boundary and aided by very unstable conditions near the ground (low level lapse rates were steep (10.6 Celsius/km)) that resulted in an updraft circulation. Normally, dust devils are seen with similar dry conditions and freshly plowed fields. In this case the circulation was not visible due to the lake and vegetation, and may have been aided by a slope in the terrain from the lake to the higher land over-looking the lake.
The boat owner was outside and saw the event occur. For a personal account of what happened that afternoon, check out their post on the NWS Milwaukee/Sullivan Facebook page: http://www.facebook.com/US.NationalWeatherService.Milwaukee.gov