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If the storm hadn't come, I would have arrived sooner.
Literal
Storm [subject-が] come-[if-ば] not, more early had-arrived probably.
A counterfactual conditional using ~ば on the negative (こなければ = 'if [the storm] hadn't come'). The main clause stacks ~ていた (perfective state) + だろう (probability) to render 'would have [already] been arrived.' This is how Japanese expresses what English does with 'would have' — by composing past state + conjecture rather than a dedicated tense.