Storm Shield, from E.W. Scripps Company, is a US$4.99 app that keeps track of severe weather for you. It supports up to five locations (besides your current location) so you can keep track of family, friends, or your vacation home.
If your location (U.S. only) falls inside an alert area from the National Weather Service, you'll get an alarm and an iOS message whether the app is open or not. It has a Follow Me mode, so it knows where you are and will give you alerts even if you haven't pre-set the location.
There are several types of alerts available, like fire, wind, hurricane, thunderstorms, ice, snow and more. Because the GPS tracking features can take a hit on battery life, you can set a throttle on the app so you see a scale between location accuracy and battery life. Then you can choose the intensity of the tracking. I think it's best to run the app when you think the weather will deteriorate. Running it all the time will certainly shorten your battery longevity.
The app is easy to set up, and it's simple to go change your default location. The app includes a detailed radar map for the whole US which is zoomable. You can view the map in a plain map view or in a more detailed satellite view. You can add layers to show all types of conditions like flooding, tornadoes, high winds etc.
There isn't much to argue with here. There are plenty of apps that will give you storm warnings, but this app has warnings as its main purpose, and it handles the job well. With the ability to save multiple locations, it is a pretty handy information tool to have as we approach the time of the years most violent storms in the U.S.
The app is not universal, so does not look its best on an iPad, which is a shame because a case can be made to have that same functionality on that larger screen.