、。
If by any chance it rains tomorrow, I'll probably stay home.
Literal
By-chance tomorrow rain [subject-が] falls-[conditional-ば], I [topic-は] house [at-に] will-be [probably-でしょう].
家にいる ('be at home') uses the existential いる (for animate beings) with に marking the static location of being. Combined with でしょう, the prediction is softened to a polite probability — fitting for a weather hedge or a soft commitment about plans. The 万一+~ば opening adds the 'just in case' flavor.