Wednesday, July 17, 2013

Excessive Heat Warning Length & Heat Waves Of The Past

Today marks day four of a likely seven day heat wave in Philadelphia, one that is bringing with it a six day excessive heat warning. To my knowledge, this may be the longest excessive heat warning that's been in place in Philadelphia since perhaps 1995, when the current heat warning system was put in place.  I know the current heat warning is longer than any of the ones we have seen in the past few years, with the longest heat warning that's been in place running about four days in 2011.

While the warning may be longer than past and some would argue excessive, it is still the likely longest heat wave of the summer and arguably one that will have a bit of bite over the region in the coming days.  More on the heat warning part in a minute.  First, some perspective on past heat waves.

We developed the "heat wave index" statistic last year to measure the progressive impact of heat over the course of a heat wave, using the classic definition of 90 degree days.  The longer the heat wave, the more the impacts of heat compound on one's body according to research.  Heat waves that are longer in duration, hotter in intensity, can have a compounded and nastier impact on one's body than a multi-day string of days in the lower 90's with little in the way of humidity.

Through yesterday, the three days that made up the heat wave so far (not counting today) were the most intense heat of the year so far...scoring a 34 on our index over the first three days. It's certainly not the worst of a first three day stretch of heat but it's still noteworthy as it is above the average index score for the third day of a heat wave.  I looked back at the last three years prior to 2013 (to 2010), plus selected years back to 1993 to see how the sum of the summer scaled out on the whole.

While 2010 featured the most 90 degree days on record, we had several "shorter" heat waves -- two six day heat waves, four five day heat waves (32 of the 55).  Thus, its total score for the summer was lower than that of 2012, which had 16 fewer 90 degree days but had a 12 day heat wave.  It also scored half as many "points" as the summer of 1995, which had a 17 day (and gold standard) heat wave.  That heat wave, and the 1993 heat wave that included EIGHT straight days of 95 or warmer, racked up huge scores because of the intensity and persistence of the heat.

Switching back to the excessive heat warning for a second, one of the big reasons for its length this year was the higher humidity in place over the region on the front end of the heat wave and the likelihood of high dew points later this week.  While yesterday and today have been rather hot, the tolerable dew points in place (60's) brought the highest heat index in the city today to 97 yesterday, which technically is short of the heat warning criteria of 100.  However, because of the anticipated ramping up of heat index values later this week, the NWS thought it wise to just let the warnings ride out through the course of the week.

This heat wave, while still a pretty nasty one, is not anywhere on the order of last July's 12 day brutal stretch...but the excessive heat warning that's been dropped over us is one of the longest we've seen under the current configuration of warnings...if only for the relatively unorthodox way this heat wave is playing itself out.