Here is no cloud in the sky. It --------- a lovely day.
1) will be
2) is going to be
I think you meant to write "There." The natural, native choice here is "is going to be." If you use "will be" instead, the sentence won't be ungrammatical, of course. Your sentence will simply be perceived as nonnative by native speakers.
I am planning to write an extended, fabulously researched post on the differences between will and be going to sometime in the near future, since questions in this area crop up regularly on the Grammar Exchange, so stay tuned.
As to "a lovely day" indicating opinion, that is not a reason not to use "be going to." The first sentence implies that there are present signs that the day will turn out to be what the speaker would describe as "lovely."