During weather conditions, Forecast will change the type of the Pokémon with this ability.
The type of a Pokémon with this ability will change as follows:
It will return to its natural type if Sandstorm or Shadow Sky is in effect, the weather returns to normal, or a Pokémon with Cloud Nine or Air Lock comes onto the field. The same is true when there is Fog.
This ability is made specifically for Castform, and as such, if other Pokémon have this ability (either via Skill Swap or Trace) it will have no effect. Conversely, if Castform loses Forecast (either via Skill Swap or Worry Seed) it will be stuck in whatever form it was in at the time.
Castform: Castform's head becomes surrounded in a transparent shape depending on the form that it will take and it transforms its body depending on the weather.