In this photo provided by the U.S. Fish and Wildlife Service a female "Hexagenia limbata" mayfly. The National Weather Service captured a massive swarm on radar Sunday, July 20, 2014 as thousands of the flies, commonly found in tributaries of the Upper Mississippi River, came out of the river and drifted north on the wind. (AP Photo/U.S. Fish and Wildlife Service)

Caught on radar: Mayfly mayhem

Mayflies have begun emerging from the Mississippi River in swarms that show up on radar like thunderstorms, coat roads and leave behind slim…