The hatch captured on the National Weather Service's (NWS) Doppler radar looks like a rain storm, but is actually a cloud of Hexagenia bilineata mayflies stretching from Redwing Minnesota in the north to to Prairie du Chien Wisconsin.
National Weather Service radar image July 20, 2014
According to the NWS: "The radar detected the flies about 8:45 pm, emanating from the river (the source) with echo values similar to that of light-moderate rain (35-40 dBZ). With a general south-to-north wind flow above the surface, the mayflies quickly moved north once in the air. As the flies dispersed moving north-northeast, they also gained altitude with some of the echo being detected as far north as Black River Falls and as high as 2,500 feet above ground.
"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 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."