、。
Thunder is a sign that a storm is approaching.
Literal
Thunder [topic-は], storm [subject-が] approaching-is [attributive-の] sign is.
Note the comma after 雷は — a stylistic device to mark a strong topic break, more common in formal/written register. 徴候 ('sign / symptom / indication') is a more clinical word than きざし — used for medical symptoms, statistical indicators, scientific observations.