If you use a weather app on your smartphone to see what the weather is expected to be like today, you may notice from time to time that the app is a complete bust and the forecast is dead wrong. If you compare forecasts from different weather apps, you may also see differences in the most mundane of weather. Why is that? Since each app maker uses different weather forecasts to feed their apps, you can expect some difference on any given day. But apps are particularly bad when weather is either highly variable (think summer thunderstorms) or quickly changing (a fast-moving cold front, for example). In these situations, the forecast from the app may lag behind the current weather or may just be wrong. And never use these apps for severe weather, because they are often only updated once or twice a day, which makes them useless for real-time weather nowcasts. There are other apps designed specifically for severe weather warnings. This article from Mic.com provides more details about how weather apps work here.