At about 8:45 p.m. on the night of July 20, over the Mississippi River, this happened:
The happening registered on the National Weather Services' radar as "light-moderate" rain. But what you're actually seeing is the periodic emergence -- better described as massive swarm -- of mayflies. The NWS is downright giddy about what must, for those nearby, have seemed like something out of a horror movie:
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. The higher the values (greens to yellows) indicate greater concentrations of flies. Note how the swarm is carried northward over time.
Yes, do note that. Also note this "grotesque natural phenomenon" is being blamed for a three-car crash that left one person hospitalized, as well as for leaving behind a "big, slimy mess." It's kind of what the flies are going for, as Wired explains:
Mayflies emerge synchronously around dusk to avoid their main above-water predators: birds and bats. Predators trying to capitalize on a sudden mayfly all-you-can-eat buffet are overwhelmed by the emergence of millions of insects. Some individuals make it through, and the species continues.
Oh, and the mass emergence is a sign that the Mississippi River is healthy. Hear that, all you who are caught in the swarms' path? You should be grateful.
The NWS has a photo gallery documenting the invasion; check out some of the best (read: ickiest) below:
[slide_show id ="13733167"]