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Will it be hot tomorrow?
Literal
Tomorrow [topic-は] hot become [probably-でしょうか]?
暑くなるでしょうか — 'will it become hot, I wonder?' 暑くなる uses the i-adjective adverbial form + なる ('become hot'). でしょうか adds polite speculation.
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Will it be hot tomorrow?
Will it be hot tomorrow?