Monday, December 17, 2007

A Cold & Soggy December In The Offing

As discussed several times in September the warmth we saw last Fall was showing signs of being a good trend for colder than normal weather in Philadelphia for December and so far the trend has played out quite nicely around here, with temperatures so far this month close to being what was forecasted here back in October.

So far in December temperatures have averaged 3.2 degrees below normal, which is the coldest December around here. If current trends held, this would be the coldest December here since 2000. Even though my thought process about the latter half of December would suggest near normal temperatures, the odds are significantly in favor of a colder than normal December by the time all is said and done, probably around two degrees below normal.

What is interesting (and surprising) this month is that we are above normal in rainfall and unless the faucet shuts off between now and the end of December we will finish this month above normal in rainfall. As for cold and rainy Decembers, 2007 could join only 21 other instances since 1872 where December had below normal temperatures and above normal rainfall, the last instance of this occurring was 2003. We can thank a more active storm track up the East Coast for this taking place.

For the last five cold and wet Decembers (1977, 1981, 1983, 2002, 2003) the average winter was quite awesome. Granted 1977-78 and 2002-03 were tried and true El Nino winters but the other three were not by definition, with 1983-84 being the closest to this winter Nina wise although that winter was not even a weak Nina. This winter, so far, is somewhat rarefied air as 1966-1967 was the last winter that fit the bill of cold, wet, and La Nina (again, very weak Nina)...that winter turned out quite good (45" of snow) but the 1960's are a different time, different place.

With that said though, none of the last five winters that had cold and wet Decembers ended up featuring above normal temperatures for the winter. Snowfall ranges from slightly below normal (2003-04) to above normal (77-78, 02-03). Despite the fact that the snowiest winters were Nino winters in this instance and we are in a Nina but there has not been a cold and wet December leading into a warmer than normal winter since 1926-1927, covering nine Decembers in that sample and 80 years of time!
Past performance is certainly not an indicator of future trends but if the September-December connection can continue to play out as well as the trends say it does, perhaps the cold and wet December can continue to play into a colder winter overall.