If it happens to rain tomorrow, the baseball game will probably be postponed.
Literal
By-chance tomorrow rain [into-に] become-[conditional-ば], baseball's game [topic-は] will-be-postponed [probably-だろう].
雨になる ('to become rain,' 'to turn into rain') is the idiomatic way to say 'it will rain' in Japanese — treating the weather as a change of state rather than a verb of action. ~ば is the formal conditional 'if.' The passive 延期される ('is postponed') + だろう predicts probable consequence.