It never stormed in Camp Half Blood without his permission. So when it started hailing, thundering, and having the wind of a tornado, Chiron was quick to usher all the demigods to the Big House's basement. He was glad he'd had it renovated a few years before, because otherwise everyone would never have fit.
The campers were relatively calm, they just assumed this was a freak hurricane that had managed to get through the borders. But he knew better. This was the work of Gaea. This was the beginning of the end.