A swarm of mayflies hatching along the Mississippi River July 20th was so large and so dense, it showed up on radar as the same intensity as a moderate downpour of rain!
This batch of mayflies was carried north and descended on towns in an around La Crosse, Wisconsin. The National Weather Service forecast office in La Crosse posted these pictures that were sent to them.
This swarm of mayflies was also reportedly responsible for a car crash. A swarm of mayflies as dense as the one reported along the Mississippi River can reduce visibility significantly and even cause the road to become slick!
Typically, mayflies hatch in this area around late June, but due to the unseasonably cold and long-lasting winter in the northern Midwest, this phenomena was delayed almost one month.