I've been through several books for teaching English as a foreign language, and these structures are usually presented as exact synonyms. But isn't there a difference?
Just by looking at the verb tense, I'd say that using Present Simple gives the idea that the speaker thinks it is likely to rain, whereas using will places the possibility of rain in the realm of 'unlikeliness'.
Am I on to something here or am I just splitting hairs? Honestly, I feel awkward telling my students 'yeah, they're interchangeable just like the book says' when I'm not convinced.