We wouldn't use Continuous here. Perhaps "Will it be raining when we go out?", (asking about an event in progress at a particular time).
But this is asking for a simple prediction about today - "Will it rain today?"
Changed now. Luckily "Will it rain" doesn't suggest any perfective option in Polish in this example.
Why was it marked wrong with dziś the end?
Maybe just this answer was just not entered into database.
Czy będzie padał/padać deszcz dziś/dzisiaj? - is quite normal sentence in Polish.
Well, it's a so-so option in my opinion, but I guess enough to accept it. Added.