Hola Oscar

That's a good question! 

It is referring to the past but it could also be taken as a "future action" if we place ourselves at the moment of speaking; it is saying:

No podíamos hacer nada más hasta que terminara.

So we can see that subordinate clause "hasta que terminara" as something that was going to happen if you place yourself at the moment that the main clause is expressing, therefore at that moment you don't see the storm as something that finished but something that was going to finish at some point in the future.

I hope this clarified it

Saludos

Inma