First a few stats about the Memphis summer of 2011:
- Since May 28, there have only been 5 days that the temperature has NOT reached at least 90. In July, we reached 90 every day except the 7th (it was 86).
- So far, there have been 7 days in which the low has not dropped below 80.
- The average temperature for June-July 2011 is 84.9 degrees, which ties 1980 for the warmest June-July period on record. Yes, THAT 1980!
So, about all those watches and warnings. What do they mean and why does the NWS issue one over another? The following criteria are used by the Memphis NWS office, as well as many other offices that have not adopted their own criteria (which they are allowed to do).
Heat Advisory - Generally issued when the heat index will be 105-109 degrees for at least a couple of hours during the day and the overnight temperature will remain at or above 80 in the city (many times 70s are experienced in the suburbs). Issued when the criteria will be met within 12-36 hours.
Excessive Heat Watch - Issued when the heat index is forecast to be at or above 110 degrees within 24-72 hours.
Excessive Heat Warning - Issued when the heat index is expected to reach 110+ degrees and overnight temperatures will remain above 75 degrees in the upcoming 12-48 hours.
So, the main threshold to watch for is the 110-degree heat index. An Excessive Heat Watch may be issued 3 days in advance of a forecast heat index of 110. Then, the day before the event, if the heat index is expected to only reach 105-109, a Heat Advisory will replace the Watch. If the heat index is still expected to hit 110, the Warning will be issued instead. An Advisory is basically a "toned-down" Warning. A Watch is issued for longer lead times (similar to a Tornado Watch, which means conditions are favorable, versus a Warning, which means the event is imminent).
Hopefully this helps sort out the plethora of headlines the NWS has used during this heat wave. By the way, the Memphis office has put together a well-done summary of all of their watch and warning products. You can find it here: Headlines Issued by NWS-Memphis.
For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.