It's been a while since this happened, so I don't have many details. About 3 years ago, we had a popup storm here in Louisville. It was completely unexpected and unpredicted. That's not the strange part though. I took at look at the radar (it was about 5:00 PM) and noticed that on a 2 hour loop, the storm appeared as if it was brewing right over the city landfill and drifting towards the north-east. That was strange. They were essentially just thunderstorms with a lot of lightning, no tornado threat.
But then I zoomed out of the radar, and noticed that the same thing appeared to be happening over other nearby major cities (Cincinnati, Nashville, Indianapolis, St. Louis...). I'm not sure where the landfills are in those cities, but I'm guessing that may have been a pattern. That night was unusually muggy, like 100% humidity. The date was 7/20/2011.
What could explain this phenomenon?