"If it's sunny tomorrow, we'll have a picnic." (Usually more than one person attends a picnic.) If you say, "it's sunny" without giving a time frame, the sentence is too vague and therefore, not useful to the receiver.

The first one sounds pretty weird, you should always go for the second. I'll have a picnic tomorrow if it's sunny. But I would formulate it differently, like : Provided it's sunny tomorrow we'll go for a picnic.