It will also revert to its normal form if a Pokémon on the field has Cloud Nine or Air Lock.
This Ability has no effect on Pokémon other than Castform.
Forecast does not change Castform's form until after Stealth Rock deals damage.
If a Pokémon transforms into Castform with Forecast, it will change forms with the weather just like Castform.
If Castform or a Pokémon transformed into Castform loses Forecast, it will be stuck in its current form. When it regains Forecast, it will change to the form matching the current weather, even if it is already in the correct form (which resets its type if it had changed due to an effect like Color Change).
If a Pokémon transforms into Castform, the Pokémon will remain in the same form as the target Castform, regardless of the weather.
If Castform loses Forecast or Forecast is suppressed with Gastro Acid, it will revert to its normal form and not be able to change form. If Castform regains Forecast, it will change form to match the current weather, unless it would be in its normal form in the current weather.