In La Crosse, Wisconsin, a week ago Sunday, radar from the National Weather Service detected what appeared to be a huge rain cloud developing along the Mississippi River, only it wasn’t a rain cloud at all. It was a cloud of mayflies.
A giant mayfly swarm emerged from the water and became airborne, producing on radar what the NWS described as echo values similar to that of light-to-moderate rain.
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.
The Minneapolis StarTribune reported that the natural phenomenon called to mind a biblical plague or an Alfred Hitchcock film. It’s definitely eerie.
|Mayflies cover a gas station making it difficult to pump gas.|
The mayflies started to appear on radar around 8:45 p.m. Sunday, July 20, and began invading La Crosse, La Crescent, Stoddard, and other locales up and down the river in massive numbers, as seen in the accompanying photos provided by the NWS.
The mayfly swarm is said to be an annual occurrence and a good sign for the health of the river, but this year’s event is reportedly much bigger than normal.
The NWS called it a massive mayfly emergence similar to the event that occurred on June 23, 2012, when snowplows were used to remove the mayflies from the roads.
The StarTribune reported that the mayflies produced slimy roadways and was blamed for a three-car accident near Red Wing, Minnesota.
More from the NWS:
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.
We’re pretty sure the local residents would have preferred it to be rain.