One of the calling cards that often accentuates a warm December is the number of 60 degree (or warmer) highs during the month. Average highs in December are dropping through the 40's over the course of the month so the 60 degree day is often 15 or more degrees above average, which means it's a statistically warm anomaly.
This December has already featured four 60+ highs so far, equaling the number from December 2006. It's not a record but it is more than 2009 (one) or 2010 (two). December 2006 also included two days of 70+ weather, which we haven't had.
The most 60+ highs in December was back in 1951 (eight). Despite the very warm start to December that year, the latter half of the month featured a cold snap and a five inch snowfall that resulted in temperatures only a couple of degrees above their era's normal, with the overall winter snowfall of 16 inches not too far removed from reality. 1982, which tied for second, was a rather warm winter thanks to a powerful El Nino but featured a 20 inch snowstorm in February. There are some craptastic winters in this warm list -- 2001, 2011 to name two, and 1998 wasn't the greatest either. Winter in Philadelphia is fickle, which we should all have as a basic understanding by now!
The good news for warm loathers is that Friday morning's front marks the end of mildness for a while. Temperatures won't be brutally cold by December standards but it will feel much more like December in the wake of the next front. With the prospect of a storm next week, we may briefly turn milder before going back colder after that but the last third of December will not be as mild and not as murky as much of the first two-thirds have been. Our odds of 60 degree weather from here on out are pretty low.