Over the weekend a picturesque waterspout was seen on Lake Winnebago in Northeast Wisconsin. https://twitter.com/Bbass_34/status/1274474835326963714 A couple reports had the storm producing a tornado just before moving out over open waters, south of Oshkosh, Wisconsin. As it moved over the lake it began to organize and strengthen – as seen on a video also shared by Blake.
Reflectivity/Velocity data seen on RadarScope 6/20/20 5:45pm
RadarScope was able to capture the weak circulation via velocity data from the Green Bay radar site. The green here indicates hydrometers moving toward the radar site and the reds away. While not impressive on radar, it was just enough of a circulation to spin up a relatively large waterspout. No damage was reported, as it presumably weakened by the time it reached the other side of the lake.
EF0 Tornado Captured on RadarScope
Furthermore, on Saturday a weak tornado was observed to the west of Oshkosh. While preliminarily just rated an EF0, RadarScope did capture the tight circulation moving northeast Saturday evening. Two reports of tornadoes were relayed to the National Weather Service, along with some tree damage in the area.
Lake Winnebago is familiar with waterspouts yet are typically weak and not always associated with thunderstorms. The lake sits in a unique location just roughly 30 miles west of Lake Michigan, a much larger lake of course. As with all Great Lakes their vastness and depth mean they take longer to warm up during the spring and summer, compared to the land. Due to the temperature and pressure imbalance a lake breeze can often form if the wind direction does not have a strong westerly component.
Sometimes the lake breeze can work all the way inland to Lake Winnebago, bringing a cool east wind. Occasionally tight spin-ups over the lake are possible if a shower/thunderstorm/or even just a west wind under perfect conditions are in place.
While we do not have extensive or reliable waterspout data, we do have tornado data. The region has been a sort of mini tornado alley relative to the rest of Wisconsin. The five-county area of Waupaca-Outagamie-Winnebago-Calumet-Brown has seen an abnormal number of confirmed tornadoes. Including the likely tornado west of Oshkosh Saturday, the five-county area has seen 34 tornadoes from 2010-2020, 16 of which have been in Outagamie County. The data does not include waterspouts.
Many of the tornadoes over that time were associated with what meteorologists call a quasi-linear convective system (QLCS), which is a line of strong thunderstorms. While it is unclear if the lake breeze played a role in these tornadoes, archived radar does show it was prevalent during a couple of the tornado outbreaks over the past decade. Further research would need to be completed to make any conclusions.