Why do weather forecasts use forecasts instead of forecasts?

I feel that it will be easier to explain if I get to the bottom of it.

The root of forecast is cast, which means to execute. Forecast, ahead of schedule, means forecast. The root of prediction is dict, which means words and language. Prediction means speaking in advance, so it is translated into prophecy.

I don't quite understand this explanation. Let's rely on language sense. If you can't say it, just understand.