Sunday, June 17, 2012

Why "Heat Wave" Is An Overrated Term

For us in Philadelphia, the "technical" definition of a heat wave has been, in my opinion, a cheap and soft definition.  Three days of 90 degree or warmer temperatures in a row is very subject, dependent on humidity but also more importantly, time of year.  We've had our share of days in July where temperatures hit 90, 91, or 92 with modest humidity and yet those qualify as heat waves while a two day heat event where temperatures reach 96 or 97 do not...and those two day episodes tend to have more impact to people than a lazy, hazy three day stretch of lower 90's in July.

Yet we fascinate over duration...not impact.  This week is one of those instances where a "heat wave" may very well not occur but the impact of heat over Wednesday and Thursday may be significant.

Temperatures on Wednesday and Thursday will likely reach the middle 90's for highs -- if we crack the 94 that was set last Sunday these will be the hottest day or days of the year so far.  Throwing in a healthy dose of southern humidity to the mix as dew point values will likely be in the upper 60's to near 70 each afternoon results in heat index values that will flirt with or top 100 at the worst heat of the day.  While it won't approach the 120 degree heat index values we dealt with last July, it's still pretty nasty and hot enough to cause significant impacts to those outside in the heat or in houses without air conditioning.

One thing that has changed is that Friday is starting to look less modeling is showing that cool front, which earlier looked to cross on Friday evening, now poised to cross the region on Thursday night or very early Friday morning.

Computer modeling is uncertain with how much moisture accompanies the front -- the Euro (above) shows a mostly dry frontal passage on Thursday night, with the GFS showing a few thunderstorms around on Thursday afternoon and evening around the region in advance of the front...and a few thunderstorms lurking around overnight as the front crosses.  The Euro and GFS both agree (as of this morning) that Philly does not hit 90 on Friday afternoon.

It might hit 90 south of the city...and by definition you'd get your heat wave...but for the city and north it's very possible that the region avoids the "technical" definition of a heat wave.  It won't, however, escape the heavy handed impact that Wednesday and Thursday's heat will bring us.