What it does
Forecast does one thing: when the weather changes, Castform's type changes with it. Harsh sunlight → Fire. Rain → Water. Hail → Ice. Any other weather or no weather → Normal. The form changes visually to match, but every base stat stays at 70 regardless.
The practical payoff is STAB. When Castform is Fire-type in sun, its Fire-type moves hit at ×1.5 — and harsh sunlight independently boosts Fire moves by another ×1.5, so Weather Ball in sun goes out as a base 100 Fire move with both multipliers stacked. Same logic applies in rain with Water-type Weather Ball. Without weather, Weather Ball is a base 50 Normal move — far weaker.
100% usage means every Castform in Champions runs this. No other Pokémon can have Forecast, so this is less a choice and more Castform's defining feature.
Who runs it
Castform (100% usage). It's the only Pokémon with Forecast, and removing it would leave Castform as a Normal-type with base 70 in everything. The ability is the entire reason to use it.
How to use it
Castform works as part of a weather team, not as a standalone pick. You need teammates with weather-setting abilities (like Drought or Drizzle) or weather moves to establish and maintain the weather while Castform capitalizes on the type shift.
Weather Ball is the key move. In sun it becomes a base 100 Fire move with sun's Fire boost on top; in rain it becomes a base 100 Water move with rain's Water boost. Use Heat Rock or Damp Rock on your weather setters to extend the weather duration and keep Castform's type window open longer.
Tips & strategy
The single biggest threat to Forecast is weather disruption. The moment your opponent switches in a Pokémon with a weather ability like Drought or Drizzle — or manually overwrites the weather with a move — Castform reverts to Normal-type and its damage output drops sharply. Having a way to restore your preferred weather mid-battle, whether through a second weather setter or a weather move, keeps Castform functional.
One detail worth knowing: in hail, Castform becomes Ice-type, which means it's immune to hail chip damage. That 1/16 HP per turn that hits your opponent's non-Ice Pokémon doesn't hit Castform at all once it shifts. On paper this doesn't change much, but in a close game where every HP matters, the immunity occasionally matters.