、。
If there hadn't been a storm, I would have arrived sooner.
Literal
Storm [subject-が] did-not-exist-if, I [topic-は] more early arrived [explanatory-のだ]-but.
A counterfactual conditional with ~たら on the negative (なかったら = 'if X had not been'). The trailing のだが is a characteristic Japanese ending that softens the statement and leaves an implied contrast hanging — 'I would have arrived earlier... [but the storm happened].' Don't try to translate だが literally; treat it as an implied 'but.'