Don't get me wrong, anything can happen, but not before this summer as TNT has announced its plans to run the final 3 episodes of the current season of The Closer beginning December 7.
Reports are that if TNT picks Southland up, it would make one 13 episode season out of the 7 episodes from last season and the six unaired episodes from this season. I can definitely see that happening. But it seems unlikely to happen before summer. If a decision on whether to produce additional episodes has to wait that long, I don't see it ever happening as those involved on the show will have moved on to other gigs.