The rattle of thunder was replaced by the rumble of Harley-Davidson motorcycles Friday afternoon as 20-year-old Tyler Southern came home from months of hospitalization at Walter Reed Army Medical Center following severe injuries from a bomb. But the sky seemed to brighten when he lit up a big smile as he rolled to his parents’ home on Gathering Oaks Drive at 5:30 p.m., arms high as he rode in a motorcycle sidecar.