。
I don't know whether it'll rain tomorrow.
Literal
Tomorrow rain [subject-が] fall [whether-かどうか] know-not.
わからない vs 知らない is a key lexical distinction: わからない implies inability to know (the information isn't currently available or workable), while 知らない implies lacking the knowledge in storage. For unfolding situations like weather predictions, わからない is the more natural default.