>>2191707I don't see any way to do it entirely automatically. I don't know how well content-aware fill would work. (And I don't have it.)
I copied sections of background, mirrored, and moved them. Mirroring avoids obvious discontinuities. A little stretch and distortion to hide cloning.
For the rest, flood-fill with gray, "dirty" it up with some texture, and extend the white rain-streaks.
