。
The storm gradually subsided.
Literal
Storm [subject-が] gradually calm-down-came.
~てくる attached to a change-of-state verb like おさまる ('to calm / subside') describes a gradual shift arriving into view — the storm came to calm over time, from the speaker's perspective. Paired with ~ていく, which points the change away from the speaker. しだいに ('gradually') is a set formal adverb.