Can" it will rain" be a correct translation? Normally in English we would leave out the be.
"It will rain" would be "Lloverá."
"Estará lloviendo mañana" is the present progressive tense, so it's always a form of to be and an -ing word.
Then how about:it is going to be raining tomorrow. I am finding many ways to get marked wrong!
That specific sentence would be "Va a estar lloviendo ..." I know how you feel, trust me it is a little picky, but remember, they do take feedback, so let's work to improve the app! XD
wether "tomorrow" is at the beginning or at the end of the sentence it means the same and should be accepted.
The same for me... "Tomorrow it will be raining" is not accepted, though appears to be fine
I believe "It will rain tomorrow" is the only grammatically correct English translation.