The warmest days in December can reach 70 degrees -- we were there twice in 2006 and have reached the big 7-0 on twelve total occasions since records were first officially kept in Philly. It's not a common occurrence but in the right pattern (Nino winters more often than not, although Ninas do pop up on the board from time to time such as December 1998) such warmth can occur. Philly has never had an 80 degree day in December although geographically we've come close to it -- more on that in a bit -- but we have eased into the mid 70's on a couple of occasions as the all-time monthly record for December is 73 degrees, set on December 4th and 7th, 1998.
officially hit 78 at Reagan Airport on December 7th), while Philly torched out at 73 degrees. Other really warm records on December 7th '98 included Baltimore (77), New York (75 at LaGuardia).
temperatures only topped out at 52 with an off/on rain. Not exactly "snowy" and arctic but certainly not shorts weather!
Thankfully (for those who like cold), no 70's are in the forecast anytime soon.