Friday, July 06, 2012

Saturday's Hottest Of Heat Wave

We've been down this century club road before -- four times the last two years as a matter of fact.  While we are getting there a bit more frequently over the last 25 years than we have prior to the 1980's, some of the 100 degree days of yore have been a lot worse to deal with.  July 1993 featured three 100 degree days in a row, July 1988 brought five total 100 degree days (four in eight days), and August 30-September 3rd, 1953 brought four 100 degree days in five days.

While this heat wave is memorable and goes down as one of the top eight in terms of intensity plus duration in our city's history, there have been worse shorter stretches of oppressiveness.  This isn't to diminish what we're going through -- it's plenty bad -- but it is to point out that the 100 degree weather isn't something that this city has never experienced prior to 2010...we've had it much, much worse...and in the 50's, central air wasn't quite mass market although more homes were getting it installed.

Saturday's heat will challenge records in a few locations -- the most likely is Atlantic City, with possible challenges at Reading and Allentown if everything breaks right.  Philly will come close -- whether it hits 100, 101, or 102 is immaterial unless you like records breaking -- but many of the records were set just two years ago when we hit 100 on back-to-back days (July 6th and 7th).

For the most part, our heat wave has featured relatively modest humidity levels -- meaning that the dew point has been in the 50's or 60's nearly every day with the exception of last Friday when we were solidly in the middle and upper 60's during peak heat.  Those higher dew point levels will be common on Saturday, resulting in heat index values that will exceed 105 in many locations and probably break 110 in at least a few.  Heat index is a bit of a multiplier effect on its own -- as dew point increases and temperature increases, the "what it feels like" impact increases as well.

If we break 100 on Saturday and dew point values are higher than what's listed on the graphic above, the heat index would be even higher than this...if dew point values were in the mid 70's (75) with a 100 degree temperature it would feel like 115 outside.  It wouldn't surprise if places like Dover or in the Delmarva had heat index values get into the 110 or higher range, with 105-110 a reasonable bet in and around Philadelphia.  If we get some dry air mixing down from aloft, we might see lowering of the dew points somewhat but a higher nudging of temperatures (up towards the record of 103)...the tradeoff doesn't result in much difference in what it feels like unless the dew point dropped dramatically.

Regardless, Saturday's going to be the worst of it for 2012 locally...and thankfully it'll get much better from here on out as Sunday should be the last of the heat for a while.