Checking when a roaming pokemon is on a certain map

So I had the idea to change the weather of certain routes based on whether or not a roaming Pok?mon is on said route. I plan on using a single event on each route that controls the weather. So if for example a roaming Kyogre is on Map X, then the weather event will activate a switch to start raining on Map X.


However, I need a switch that activates when a specific roamer is on the map (and deactivates when it is not). How would I go about scripting that?


EDIT: Actually now that I think about it, a variable would be a lot easier to manage for each roamer than separate switches for each. But the question still stands...