Tuesday, May 28, 2013

Summer Outlook: Not As Hot As Past Years Overall, But...

Spring 2013 in Philadelphia has been pretty nice overall -- a bit extreme at times (going from near 90 to 50 on two occasions in short order as we did in April and last week was a bit much for a system shock) but you could mix in days like Sunday and Monday in there as payback for those temperature swings.

Summer from a meteorological perspective starts on Saturday with appears to be a pretty warm intro -- temperatures that will probably be around or above 90 for the first couple of days of June.  While it's a fair bit above average, we are getting our first 90 later than we have in any year since June 6th, 2008.  Summery weather will be in vogue this year but we're not looking at as persistent a heat as we have had in past years.

The first sign for that is the lack of drought in the Corn Belt and Ohio Valley -- these areas in the summer are source regions for our heat.  With the current drought picture confined further west -- over the High Plains and Southwest, the heat ridge will be more prone to develop over Texas as opposed to Georgia later this summer as the drought-prone regions show a greater tendency to warm excessively.   Our thinking on a national level is that the Plains and South have a hot summer overall but that our episodes of heat will be shorter in duration.
Intensity could still be an issue, however, meaning that when we tap into some of the core of the heat ridge during the summer we could make a run into the upper 90's or even above 100.  2001, thanks to dry weather over our region during the summer, provided such an opportunity as parts of the Plains baked in July and we tapped into some of that heat during early August.  However, on the whole 2001 was not a terribly hot summer from a temperature standpoint as July was rather cool compared to average.

Our analog, or comparison years, for this year feature the following criteria:  First, years where April and May temperatures were within a couple of degrees of what we experienced in 2013.  This gave us about 15 years to sample going back to 1874.  From there, I sliced out the pre-1940 data as those temperatures were not taken at the Airport.  That left us eight years.  Second, I focused on the years where we were in a relatively neutral Nino/Nina state in Spring such as this year.  That left us 1951, 1981, and 2001 as our best comparison years to work from.  None of the three were terribly warm, with 2001 the warmest of the set and the one I'm leaning into as it's closer to modern climate.  Each of the three years featured at least one or more cool months -- 1951 and 1981 featured two in June and August, with 2001 focusing in on July being cooler than average.

The last three years have featured 55, 33, and 39 days of 90 or warmer.  None of the three years in our data set approached that, with 2001's 23 the high water mark.  Our 30-year normal is 27 and we're forecasting 24 90 degree days this year, relatively close to that normal mark.  It would be the fewest number of 90 degree days since 2009 should our projections be right.

That said, while this summer won't be as brutal as part ones overall we will have intense shots of heat at a couple of occasions --with early August potentially where we see our hottest weather of the year, a bit later than the usual late July timeframe.  100 degree weather can't be ruled out later this summer but we don't foresee multiple runs of five, six, or seven days of high heat as was the case the past few summers.