The tl;dr is that it's a (very) localized lightning storm made up of Force Lightning that the user calls down from above the target area/enemy, and it's something that needs to be actively channeled. So if he stops maintaining it, it will immediately dissipate.

Canon-wise, from the look of things it's also possible to combine a force storm with other abilities like Whirlwind (which he knows) and regular Force Lightning (which he also knows), but I don't plan on him getting quite that advanced with it on his first try. Even though he definitely knows about it, because someone tried to kill him with that exact combination, but I don't see him having that level of control over it until he's spent a whole lot more time practicing.

Some references can be found here, here, here, and here.