It's really a combination of all kinds of factors, some I know of from reading from other wx forums, but can't thoroughly explain because of my limited knowledge. I can say in addition to a +NAO, we've been having a -PNA (which usually promotes western troughs) and a +AO (which usually promotes ridging for the eastern US).
Also most forecasters expected December to be warm for the eastern half of the US, the problem is December may end up warmer than what was initially anticipated. Unless there's another hidden monkey wrench in the works later down the road, we should begin to see a pattern flip sometime in January, when exactly is the million dollar question. Our Super El Nino is also expected to begin weakening as we head into the heart of winter.
All we can do now is sit back and watch and see what 2016 will bring.