If you live in the south or the Great Plains, you know the feeling of going to bed without knowing if your house will be there in the morning. Tornado is the latest in the American Red Cross's series of disaster relief apps, and since tornadoes are by their very nature sudden and unpredictable, it's a welcome addition to the Play Store. The app ties into the National Oceanic and Atmospheric Administration's alert system, and emits an alarm when your area (or any area you designate) receives a tornado warning. There's also an all-clear signal when the danger has passed.
Like the previous disaster relief apps, there's also a handy "I'm Safe" button that will send a pre-recorded message to your friends and family, plus a constantly-updated list of local shelters. A strobe light and siren could be handy in the unlikely event that you're incapacitated and can't call for help, but can operate your phone. There's also a few educational sections - pretty much the same things you learned in elementary school, but handy if you're trying to remember which room in the house is best for taking shelter.
As someone who had to run from the wildfires in the western states last year, I've got a lot of respect for what the Red Cross does. Remember that while the southern and midwestern US tend to get the most frequent tornadoes, and during the spring and summer at that, they can happen anywhere and at any time.