[Perhaps you saw a flash of yellow and purple out of the corner of your eye, but maybe it was just your imagination. That sudden gust of wind and thunderstorm on the other hand, is not your imagination, and you are likely now soaked. The Pokémon responsible doesn't care, though. He's just happy to be going for a run. He's been cooped up in his ball for too long, and it feels good to stretch his legs.
Either way, there's a Raikou going for a casual run all over the island, carrying the rain along behind it and not caring one bit about who it soaks in its exuberance. Hope you brought an umbrella!]