Sadly, the game doesn't allow you to be that specific. Your best bet is to either start the race with 50% rain or more, and set changeability to 10. But even then, it might still rain throughout the entire race. It's a rather awful system, which in my experience, results in it raining throughout the entire race more often than not. And I'm talking 30-40 lap races btw, so plenty of time for the weather to change.