What exactly does it mean when a Heat Advisory is issued?

BATON ROUGE, La. (WAFB) - It’s the official start of summer and it’s hot. As anyone from around the area will say, “It’s not just the heat, it’s the humidity.“

Not only is that true, it might be a little understated. Just a slight increase in the amount of water vapor in the air (we get our humidity from the warm waters of the Gulf of Mexico), and the air can feel as much as 10° to 15° hotter than the thermometer reads. That’s the purpose of the Heat Index, a calculation that estimates how humidity (water vapor) reduces the body’s ability to cool itself and therefore adds to the apparent temperature of the air.

On Thursday, June 20 and Friday, June 21, the National Weather Service (NWS) issued Heat Advisories for the WAFB region in response to temperatures in the 90s coupled with higher than average humidity, even for June.

Yes, it’s hot and everyone should be ready for that. A Heat Advisory is not a temperature alarm and it’s not the signal of the onset of “thermal Armageddon.“ It’s simply the NWS’s way of highlighting that it’s hot. In many cases, that Heat Advisory also suggests that higher than average Heat Index values may persist a little longer during the day.

For our region, NWS Heat Advisory criteria are as follows: an air temperature of 103° or above and/or a Heat Index of 108° or above. It does happen, but air temperatures above 103° for the WAFB region are fairly infrequent. The same is true for a Heat Index over 108°. Therefore, Heat Advisories tend to be issued on a limited basis. However, their issuance can often come in a run of days just like we’re seeing this week or in July of 2018, when Heat Advisories were posted for five straight days.

There really is nothing magical about the temperature or Heat Index criteria for issuing a Heat Advisory and occasionally NWS will be a little loose with the interpretations of those criteria. A morning forecast for an extra hot day is often deemed as close enough to validate the issuance of the Heat Advisory for that afternoon.

The threshold criteria for a local Heat Advisory are consistent for all NWS offices serving the Gulf Coast rim. These criteria provide a compromise for NWS offices from Texas to Florida so the issuing of Heat Advisories will be consistent.