Jet Stream on a target outside of a Storm Cell gets a short repel. Inside a Storm Cell it's automatically converted to knockdown. The lightning procs in Storm Cell and Cat 5 do knockdown, but standard KD rules apply, i.e. if they're a lower level than you or a group that's susceptible to KB (e.g. Clockwork) that KD is gonna be KB. This isn't an issue with Storm Blast, it's just the way KD/KB works in the game.