During the Holiday event Winter's Veil he spawns, It just passed so no time soon will someone see him I hope that this helps you some Try during that time having a character camped there. Happy Hunting!
Per Blizzard's definition it is currently winter; WoW only has two seasons, summer and winter. That no one has encountered a Snowy Owl, however, indicates there are other conditions that have not yet been met.
Perhaps they only spawn in specific weather conditions during the winter. Or, as someone stated, during the Feast of Winter Veil (coming in December). We simply don't know anything about this spawn at the moment. It doesn't really matter, though, as the Achieve for Kalimdor cannot be earned for at least six more months anyway.