It is the only Pokémon with the "Forecast" ability and one of two that learns the move "Weather Ball" (the other Pokémon capable of learning Weather Ball is Roserade via Move Tutor in Pastoria City).
It is the only Pokémon with the ability "forecast" and one of two with the move "Weather Ball". Both are affected by the Weather, and weather can be changed either by Castform or an opposing Pokémon (the other Pokémon capable of learning Weather Ball is Roserade via Move Tutor in Pastoria City).