Ours go off if there is a tornado (or possible) anywhere in the county. What I find annoying is they would interrupt an "important" TV show to let us know there are strong storms on the very outlying areas of the viewing area. The other day the tornado sirens came on (in the St. Louis metropolitan area) and it took me about 20 minutes to figure out where the storm was. Nothing was on the news except "Tornado Warning" scrolling the bottom of the screen. That wasn't very helpful.