、。
If it were to snow in May, they'd be surprised.
Literal
By-chance if-もし May [in-に] snow [subject-が] fell-[conditional-ば], they [topic-は] would-be-startled [probably-だろう].
A curious double hedge: 万一 + もし combines two hypothetical markers, intensifying the remoteness of the possibility. This stacking is unusual and emphatic. びっくりする ('to be startled, to be surprised') — the verb form of the mimetic びっくり.