When we introduced our pup to the household we had 4 cats!
After a while he started to chase the cats and he could also fit through the cat flap to persue them into the outdoors (hard to believe it now).
Anyway, we attached a house line to him for what turned out to be a few weeks and every time he went on the chase, I stepped on the line and stopped it.
It realy was just the persuit he loved, GSD's generally having a high prey drive, the chase is what excites them.
Once we did this for a few weeks he was fine and we could stop a chase with a command after that.
He continued to chase when he saw them in the garden but the same training applied outside did the trick too.
I would suggest that Storm is the same. The chase is good fun and the intention is not to harm or aggressive. My dog is great friends with the cats now and is surprisingly gentle with them. He does not chase any more and is corrected if he gets too boisterous.
Little Ted on his long house line..........