Translation:It rains at night.
Why is "It will be raining tonight" not accepted here? "It rains at night" (which is the right answer) makes a completely different sense, in this case I don't relate the rain to this particular night but to a somehow regular occurring action. How do Japanese distinguish between these two meanings?
In Japanese, you use でしょう when saying what the weather will be in the future, because you can't know what it will actually be and can only make a prediction.
The reason is that deshou/darou is a grammar function that is typically used to express that something will probably happen. This is why you hear deshou being used so much in weather forecasts:
Ashita wa ame ga furu deshou.
(It will probably rain tomorrow.)
In weather forecasts, they don’t say:
Ashita wa ame ga furimasu.
(It will rain tomorrow.)
Because it’s entirely possible that it won’t rain tomorrow! Instead, deshou is used as a conjecture.
If you wanted to emphasize that it would rain tonight, 今夜は雨がふります would work as "it will rain tonight." For this one there is no particle and it's using the generic 夜 which doesn't point out any night in particular. (Honestly, it will be raining tonight technically conveys something in english but it's a really weird statement unless used to respond to someone else.)