Tempest could either refer to initial definitions of;
1) swiftness/speed
2) thunder/lightning
It could either be a Rogue ability that incorporates magical or magic-like abilities, sort of what Cole will likely have, specifically pertaining to lightning damage.
Or could it simply be a specialization that focuses on fast strikes, momentum, and maximum DPS. For example, you can activate a momentum talent that momentarily (sorry for the redundancy) boost your damage per hit, with each hit stacking damage/attack points, however it drains stamina with each hit.
It's a good alternative to the Assassin specialization in my opinion, for Assassin at its core is more about hitting the absolute HARDEST you could, as opposed to quick dps strikes which Tempest seems to imply.
My personal 2 cents on the specialization ^ granted we have 0 info on it and can only speculate based its name and context in relation to the other specializations.