On the morning of July 11th, an area of heavy rain and thunderstorms began to weaken and dissipate as it crossed Southern Wisconsin. The decaying showers and thunderstorms moved east over Lake Michigan by mid to late morning. However, in the wake of these thunderstorms, an interesting weather phenomenon developed. To some people looking at a visible satellite loop, this may have appeared to look like a hurricane as it swirled across Southeast Wisconsin. However, it is actually an example of a relatively common summer occurrence - a "Mesoscale Convective Vortex", or MCV.
An MCV is really just an area of mid-level low pressure caused by a large complex of thunderstorms. The circulations do not extend down to the surface. Large clusters of thunderstorms can release massive amounts of energy, warming the middle levels of the atmosphere, which can produce a small-scale area of low pressure. These low pressure areas in the middle levels of the atmosphere can sometimes be seen on visible satellite if the thunderstorms dissipate. An MCV will usually show up as a small swirl on the satellite imagery.
This case is just an extreme example of an MCV. They usually do not appear quite as large, or as obvious, on satellite as this particular one did. Below is an analyzed map of some of the weather features that were present this morning around 11:45 AM, overlayed on a visible satellite image from the same time.
And here is an image taken by a polar orbiting satellite. The image is provided courtesy of SSEC at the University of Wisconsin in Madison.
Sometimes MCVs can move along in the direction of the wind flow, and trigger thunderstorms in a different region later in the day. Therefore, tracking these MCVs can be an important tool that meteorologists can use when preparing a forecast.