If I told you I had a gripping vision of a terrible catastrophe in the U.S.A., would you believe me?

Any prediction has a 50/50 chance to be right or wrong. I wouldn't worry about it too much. Had dreams like that before, usually after eating something which didn't agree with me or after seeing a horror show. You can "warn them" all you like but most will either ignore you or think you are nuts. But if you push too hard and start a panic which is later unfounded, you could get yourself in a lot of trouble.