Ya know what, just do it! This has happened too many times to me and I'm always then saying, "why didn't I stop and grab that photograph"!
Remember a few months ago I mentioned about keeping your camera at the ready? This evening a storm blew threw and a beautiful rainbow appeared - actually 2, the second was hard to see. It was only there for 3 ~ 5 minutes.

Here is one of 4 shots I took just outside my home, it was still raining slightly. I wish I have a clear shot, a helicopter would be nice for this. The point here is - have your camera ready! If you're not comfortable with changing setting quickly, keep it on Auto, which I'm not a fan of, but it just may be the difference between getting a photograph or not.
Act Quickly and Just Get It! It's digital, if you don't like it - delete it, but, you'll never know!