I think it has a slightly different meaning.
"It will be raining tomorrow" is not used to just tell info about the weather, same as "It's going to rain tomorrow" - sounds weird to me. Weather is always stated as future simple, don't know why. (I might be wrong, not a native speaker).
So it doesn't really mean "завтра буде дощ" but rather "Завтра йтиме дощ (о 5 годині)" - It will be raining tomorrow at 5 o'clock.