If by any chance it rains tomorrow, the match will probably be canceled.
Literal
By-chance tomorrow rain [subject-が] fall-[conditional-ば] match [topic-は] will-be-canceled [probably-でしょう].
雨が降る ('rain falls') is the most common Japanese phrasing for 'it rains' — Japanese makes the rain itself the subject of 降る rather than using a dummy 'it.' The 万一+~ば+でしょう shape is the textbook frame for unlikely hypotheticals: 'just in case rain falls, the match will probably be canceled.' でしょう keeps the register politely soft.