。
We don't know what will happen tomorrow.
Literal
Tomorrow what [subject-が] occur [question-か] [topic-は] know-not.
何が起こるかは uses は after the embedded question to topicalize it — 'as for what will happen...' The は adds a nuance of 'that particular thing, we don't know,' slightly softer than the universal-negation 誰にもわからない reading.