Mayfly Madness: This Massive Swarm of Mayflies Was So Big It Was Detected on NWS Weather Radar While Invading Towns Along the Mississippi River and Heading Towards the Twin Cities Friday, July 3, 2020 By Stillness in the Storm Leave a Comment (Strange Sounds) A massive swarm of mayflies, detected by National Weather Service weather radar starting after 9 p.m. Thursday, invaded downtown Burlington, Iowa along the Mississippi River.