Technically, you cannot use the near future for weather forecasting.  A forecast is a prediction; therefore, the future simple should be used.  The only grammatically correct way to use the near future for weather is when you are outside, the wind whips up, storm clouds roll in with thunder/lightening...then, you can say (in English, French, or Spanish) that it's "going to rain".  Perhaps in very colloquial language the near future is used for weather forecasts, but it is wrong, and this should be clearly noted in the lesson.