！
Let it be sunny tomorrow!
Literal
Tomorrow weather become-[wish-なあれ]!
天気になあれ is a children's incantation — the elongated なあれ is from なれ (imperative of なる), stretched for a sing-song, wish-making effect. Japanese children say this while making or hanging a てるてる坊主 (a handmade cloth doll hung to wish for good weather). It's a cultural set phrase, not a grammatically productive pattern.