Friday, December 09, 2011

Weather History: Seventies in December

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.

In order to get a rather warm day in December you need a strong upper level ridge of high pressure along the East Coast or over the Mid Atlantic (see the big red dot over New York and Pennsylvania), with a trough in the western parts of North America (but cold is bottled mainly over Yukon and Alaska).  The consensus blend of the mid level pattern on 70 degree days in December shows a strong ridge east, trough west look -- not surprising to be honest, but you need to have a stronger-than-normal amount of ridging aloft over the Mid Atlantic...almost like having a heat dome that is typical in the summer occur in December.   Such ridges are harder to develop at this time of the year and much harder to keep in place...although some of the warmer winter months we've had historically can bring weeks of mild/warm patterns with them (think 1998-1999, 2001-2002, the first half of the winter of 2006-2007 when we hit 70 into January before the bottom fell out of the thermometer halfway through winter).   Often there may be a storm system separating warmth from chill (if there is any in the US) or a less-warm pattern...but typically most of the cold is bottled up in Canada near the center of the trough anomaly.

The warmest December day(s) were the 4th and 7th of 1998 -- with the 7th featuring 80 degree warmth into suburban Washington, DC (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).
As I stated before, such warmth is short-duration and not long-lasting.   We've only had two sets of back-to-back 70 degree days in December ever -- 1966 and 2001.  After this last blast of warmth in December '98, a cool front was slipping south through the state (you can see the 50's and cooler temperatures lurking on the northwest fringes of the map).   The next day, 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.