I hate when I’m sitting at work only to look out the window and realize that it is starting to rain. Then when I go to look at the radar it is a major storm that is moving in, and had I left 30 minutes earlier I wouldn’t have had to worry about pulling out my umbrella.
Rain Alarm for Chrome and for Firefox are two extensions that will monitor the surrounding area for incoming precipitation. Based on the criteria you specify it will send a notification to your browser giving you a little more time to break free before the storm hits home. If your computer is constantly on the move it will try to update your location automatically so that the warnings it provides are accurate.