This free app from the American Red Cross tracks a tornado as it approaches with step-by-step advice about what to do before the storm hits. A siren warning is built into the app and goes off when the National Oceanic and Atmospheric Administration issues a tornado warning in your area. There's also a customizable notification system to let friends and family know when the user is safe via social media, text, and e-mail.
Compatibility: iPhone, iPad, and Android
Cost: Free