Always carry an umbrella. If you don't have it, it will rain.
Good advice and I usually followed it. Too bad this site didn't exist when I was in college (ok, yes, I would have also needed a cell phone and no one had texting yet):
You type in your zip code and it says YES (as in yes, you need an umbrella) or NO (you do not need an umbrella). If you want, you can sign up for it to send you a text message on days when you need an umbrella.
I love it!