We're mid-atlantic. We have aspects of Northern and Southern culture, but I'd say it's still more Northern than Southern, maybe like 60%-40%. I agree that the people who have lived in the DC area for generations are more Southern, but with the current cultural makeup, I can't say that Maryland/DC/NoVa are Southern. I'll make two lists:
Northern:
Maryland and DC didn't secede
very liberal---this is A LOT of what in my mind makes us more northern.
Educational performance
Low teen birth rate
not as religious---of the ones who are, most are Catholic, and Jews are a very prominent minority
fast pace of life
lots of transplants
lots of immigrants
diversity
we don't deep fry everything under the sun and we don't automatically load our iced tea with sugar
We don't have a southern accent
MoCo and Baltimore
Southern:
We are scared little bitches when it snows/we don't know how to drive in it
Maryland and Virginia were slave states, Virginia seceded
Our bagels do not compare to the ones in New York
We don't have the same accent that any other areas of the northeast have
We identify ourselves by counties and do not have townships
We are south of some stupid random line that allegedly divides north and south
Our weather is disgustingly hot
ETA: Of course there are a lot of "yes but..."s on these lists. There is no exact cultural line. I'll agree that 50 years ago the DC area probably identified as more Southern than Northern, but I think if you ask most residents TODAY, they'd identify as more Northern than Southern.