Here's a simple idea packaged in a great app: The MacDoctor has been playing with Horizon Calendar for iPhone. It ties together your existing calendars with daily weather reports helping you plan and adjust those weather critical events. Tapping to the weather icon to the right of any event takes you to a more detailed weather outlook for the day and time. As the developer says, you should never have to choose between checking the weather and your calendar!
It's £1.49 over at the app store. Let us know how you get on.