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Forecast is hardcoded to only work for Castform. The wielder's type changes to Fire under sun, to Water under rain, and to Ice under hail. Forecast does not change the wielder's type in sandstorm or fog. If this Pokemon is changed to a certain type, and then is hit with Gastro Acid, or receives a different ability via Role Play or Skill Swap, it reverts to its base forme until it switches, even if the weather changes. If a Pokemon with Air Lock or Cloud Nine is on the field, the wielder's forme reverts to normal. If the wielder uses Entrainment or is the target of Role Play, the move will fail.
Castform everywhere can breathe a sigh of relief as endless, gamebreaking transformations due to Forecast are gone for good with the Acid Rain glitch finally removed. Forecast is an interesting ability, but its sole possessor is outclassed by almost any other weather abuser. It's hard to keep up with base 135 Special Attack juggernauts when you only possess base 70. Overall, Forecast is another interesting ability ruined by being stuck on such a lousy Pokemon.
Found in Dream World