In a way, this seems more unprecedented than April 2011. NOT as bad overall, obviously, but certainly bad enough and maybe more anomalous? After all, you had April 1965 and 1974 as predecessor comparable events for April 27, 2011. April has probably had more tragic tornado outbreaks than any other month. But December? There doesn't seem to be anything like this on the record, except for the 1957 event I linked earlier -- 37 total, 1 F5, 3 F4s. But the fatality count there was 19. (From the wiki, a lot of those tornadoes were after dark, it seems.) Now, the governor of Kentucky, who's probably about as unimpeachable a source as you could get in the immediate aftermath of something like this, is speculating about more than 100 deaths (at least last I heard). With other fatalities in Arkansas and Tennessee. So, really, no comparison, at least using fatality count as the (ghoulish) metric for ranking outbreak severity.
There are gonna be a lot of studies on this one, for certain.
(Moved this from Mid Atlantic forum; seems more pertinent here.)