Tuesday, March 19, 2013

Back To Back Chill

February and March will end up as with below average temperatures when all is said and done. This is the first time since December 2010 and January 2011 that Philadelphia has been below average in two consecutive months and it will be the third month in the last five where the city has experienced cooler than average temperatures.

In terms of February and March, it is the first time since 1996 where the duo of months have both finished below average.  Given our tendency locally to experience milder starts to spring, getting a cold March is a bit more unusual of late since this March will be the first since 2005 to finish below average on temperatures.

Going back into the 70's, using a temperature criteria where February and March both finish below our current normals, there are eight years where we've been below average in both months. How do their corresponding April and May end up?

A mixed bag -- some years mild, some years not so much.  There is no real trend to support warmth or chill either way but the average temperature of each of the eight years is below our current normal going forward and in some of the colder years like 1972 and 1978 are below their respective "normals" as well.

If we just looked at the last 25 years -- 1988 and on -- temperatures average above normal in April, influenced greatly by a very mild April 1994, and below average in May thanks to May 1996.

Factoring in 90 degree days, the average "first" 90 falls around June 11th on the dataset, but varies very wildly.  In more recent years, the first 90 falls in April and May during 1994 and 1996, with 1972 and 1982 bringing our first 90 degree day in July.  Obviously, different eras climatically between the 70's/early 80's and the 1990's but it's still an interesting statistic that some of our colder spring starts resulted in a very late first 90 while other years did not.

If anything, when the rubber band breaks...and it will snap at some point from the chilled pattern...we will probably see a significant warm shot. It may be very fleeting but if years like 2007 are any indication, it does not take much to go from cold to 80's...and back...in very short fashion.