That number drops to 80% for seven-day forecasts and 50% for forecasts that look ten days or longer into the future. Five-day forecasts are accurate about 90% of the time. Whether in an app or on the news, weather forecasts are made with computer modeling programs that use past and present atmospheric data to predict future weather. (Lugging grocery bags around is certainly a much bigger task during a heat wave or thunderstorm than on a nice, cool afternoon.) Even a quick trip to the store can be impacted by the weather. A good weather app is essential for anyone who likes to plan ahead-from choosing an outfit for work to booking your upcoming vacation.