Yes, there are weather apps -- those are neat. But you have to check the weather before leaving the house. You may remember to do that most of the time... but not on that one day you have to leave in a rush, and of course, you just know that's when it's going to rain.
This is why I came up with Remember The Umbrella - a simple Android app that lets you know if it's going to rain, at just the right time. It's simple: Tell the app when you typically leave the house every morning, and right before you leave, it'll tell you if you should take your umbrella that day. The Pro version lets you set several alarms, too.
Try it out, and let me know if it saved you from getting wet!