If it happens to rain the day after tomorrow, I'll stay home.
Literal
Day-after-tomorrow, if rain [subject-が] fall [if-ば], I [topic-は] house [at-に] exist.
もし ('if') is an optional emphasis particle that often accompanies conditionals for clarity. もし...ば signals the hypothetical reading explicitly. もし is often dropped since the conditional form carries the 'if' meaning on its own, but it can add emphasis on the hypothetical nature.