Wednesday, July 04, 2012

Excessive Heat Warnings & Watches

The region continues to soldier along in this lengthy heat wave. Thankfully, we're past the halfway point of it but unfortunately it's not been without loss of life.   With another day of 90 degree weather on tap for the region, our seventh in a row, and the prospect of higher humidity levels today, heat warnings and advisories are issues for portions of the region.  The Excessive Heat Warning is out for counties in dark red and is out through Thursday, with an Excessive Heat Watch issued for Friday and Saturday (for now, these likely will be bumped to warnings).  Heat Advisories are out for the counties in orange to our east and south, including the Shore and Delaware.


The "common sense" meme in heat waves is to stay cool, drink plenty of fluids, don't overexert yourself, blah blah blah...but given we're working ourselves towards a ten or eleven day heat wave when this is all done it bears repeating in a linked article so you don't hear it from just us but from someone a bit more qualified from a health standpoint.

What is the heat advisory or heat warning criteria?  Varies based on location.

Excessive heat warnings are typically issued first for Philadelphia and the surrounding metro because the criteria for "excessive heat" is lower in the city than it is in rural areas.  Heat warnings can be issued when the heat index reaches 98 in May or September, 100 in the summer months. Philadelphia (and the Delaware Valley counties) are a bit unusual in using their own heat warning system compared to the rest of the Northeast due to the high levels of brick housing stock within the region, as well as the high levels of heat-related deaths that occurred in the 1990's.  "Kalkstein Procedures" in the graphic below refers to this study that was developed in the 1990's that was the response to the 1993 heat wave that killed 118.

Heat advisories can be issued if heat index values are predicted to reach or exceed 100 degrees to the north and northwest of Philadelphia's metro counties, and 105 degrees to the south and southeast.  The region is on that fence between the 100 and 105 criteria.


Excessive Heat Warning criteria, outside of Philadelphia, also varies.  Areas to the north and northwest require a projected heat index of 105 to get an excessive heat warning while areas to the south require one of 110.  Again, Philadelphia is unique to most everyone in the country (another noted exception is Chicago, which uses a similar method to that of Philadelphia) in using excessive heat warnings almost exclusively even if the heat index would warrant an "advisory" if it were in Allentown or Reading.