In my experience, it rains a lot on that one swampy route with the "haunted" house (14?)
And whenever I fly to Kiloude city, it usually starts with rain for a few minutes, then stops. Gotta have a rare candy though.
Also, the location for shiny chain fishing posted in the sticky (southern part of Ambrette town) always alters between rain and shine every few battles. So an option is to just chain fish for a bit until it starts raining.
Not changing this sig until Pat Benatar is in Super Smash Bros. (Started 8/31/2010) 3DS Friend Code: 1075 - 0844 - 9134 - GHOST: Pumkaboo, Lampent, Dusclops.