A lightning alert system gives venue staff a local trigger for action before the first loud clap of thunder reaches the crowd. Venue staff needs to know what is happening close enough to threaten one field, stage, or festival site. The Warning Now Starts at the VenueFort Worth’s outdoor warning sirens are designed to alert the public about broad threats. Lightning within a few miles of one venue is not listed as a routine activation trigger. An on-site sports field monitor clears city fields when lightning is detected within a 10-mile radius.