。
The storm was accompanied by thunder.
Literal
Storm [to-には] thunder [subject-が] accompanied.
に+は marks the target with contrastive emphasis — 'in the storm (specifically).' 伴う ('to accompany, to come with') is a formal verb. Subject-inversion: 雷 (thunder) is the grammatical subject of 伴う. A weather description with a slightly formal tone.