By late evening, mayflies were swarming in La Crosse, La Crescent, Stoddard and points up and down the river. While the emergence of mayflies from their river bottom mud dwelling can occur at various times through the warm season depending on the species, this particular emergence was that of the larger black/brown Bilineata species. The radar loop below shows the reflected radar energy (reflectivity) from 835 pm to just after midnight.More via Smithsonian.
The picture above might look like a gas station in a blizzard, but those particular particles aren’t snow. What you’re seeing is a huge swarm of mayflies that erupted from the Mississippi River on Sunday night in Wisconsin. And those flies weren't just at the gas station, they were everywhere, as can be seen in these photos gathered by Minnesota Public Radio. There were so many flies that the National Weather Service actually picked up the swarm on radar: