If I find Celeste on the island, does that automatically mean there will be meteor showers? She never mentions them when she visits and Isabelle never announces them either.
If Isabelle doesn't make the announcement. It means you have a light shower so you'll have very infrequent stars. You can find out the exact time of these star falls by finding your weather seed.
There are light showers, then there are heavy showers.
If Isabelle makes an announcement, it means that you have a heavy meteor shower. Shooting stars are very frequent and shows up almost every minute.
Meanwhile, light showers have large gaps between them, sometimes occurring only in a specific minute within a specific hour. That's why we think that we don't have showers when Celeste is visiting. Basically a "blink and you'll miss" kind of thing.
There can still be heavy showers without an announcement though. Mine occurred the same day as the Bug-Off, and that announcement took priority over the showers. Talking to villagers is the real key.
I had a heavy shower without an Isabelle announcement as well. She opted to announce that there is someone in the campsite, rather than to say that there'll be shooting stars at night lol Isabelle.