Note the topical vocabulary: "the weather forecast". Try to stick to this format for conciseness and precision (though the world is free and there are different ways to say the same thing): "The weather report says [quote it]." Both "it will rain" and "it is going to rain" are correct.

"Is correct this form" <-- this is not a correct phrase. As for reporting the weather prediction for tomorrow, either sentence can work. Actually, you would repeat whatever phasing you hear from the original weather prediction.