Go talk to the guy on the top floor in the Weather Institute and he will tell you there has drought or heavy rainfall in a certain area. To get to the Weather Institute go left from fortree city.When he tells you that go to that area or route and you will see a cave. You need good luck and ultra balls or catch them easily by a master ball but before you battle SAVE FIRST!!!! After you catch one of them go back to the Weather institute and talk to him again and go to the area or route he says to go to. But remember to SAVE before battling.