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I'm sure the weather will get better tomorrow.
Literal
Tomorrow [if-becomes-になれば] surely weather [also-も] become-good probably.
明日になれば is a temporal conditional — 'if/when it becomes tomorrow.' The ~ば form on なる turns 'become tomorrow' into a 'when' clause. きっと ('surely') strengthens the prediction. も on 天候 ('weather') adds a touch of 'also/too' — implying the weather too will improve along with other things.