If it rains tomorrow, she probably won't come here.
Literal
Tomorrow rain [subject-が] fall-if, she [topic-は] here [to-へ] [topic-は] come-not [probably-だろう].
The double は — 彼女は for the topic and ここへは for contrastive scope — is a natural feature of spoken Japanese. ~だろう adds speculation to the negative. 降れば is the ~ば conditional of 降る.