。
A storm is about to break out.
Literal
Storm [subject-が] arise-[volitional] about-to-do-is.
The structure [verb-volitional]+としている means 'about to do X / on the verge of X.' With an animate subject it means 'trying to do,' but with an inanimate subject like 嵐, it means an imminent event. 起ころう is the volitional form of 起こる.