Once you enter the storm coast and watch the Dragon and Giant fighting, the Dragon will never return to that specific spot.
I assume you know where Dragon Island is, and the boat to get there. After scaling (no pun intended) the mountain to get to the very top, you'll find a wide circular area which should house the Dragon. As I recall it has a specific name like Vardillius or something. It's lvl 19.
If it isn't there, try returning to skyhold and then re-enter the Storm Coast (you can fast-travel directly to the boat to Dragon Island).