The accepted answer is quite clumsy. "We don't know what the weather will be like tomorrow" is a better option.
This is definitely the best translation.
No reason why "will be" can't be at the end of the sentence. Please amend your correction
It can also be "We don't know what the weather will be tomorrow." Duo has used the most stilted, least common construction of the sentence.
"We don't know what the weather will be tomorrow" is better English but is not accepted.
In another sentence, it is plural " leS prévisionS météo"; here is simply "LA metro".
Is it correct to understand that French always singular "LA metro", and plural " leS prévisionS météo"?