Thursday, July 21, 2022 9:29 PM

Excessive heat warning vs. heat advisory

Matthew Holliday

In many U.S. locations, temperatures often reach uncomfortable levels during the summer. But high temperatures, combined with elevated moisture in the air, can pose a significant risk to human and animal health. That’s why excessive heat warnings and heat advisories are sometimes necessary.

What criteria are used for issuing excessive heat warnings, watches, and advisories?

Your local National Weather Service office issues heat alert products, just as they do with severe thunderstorm warnings, winter weather advisories, etc. The National Weather Service Procedural Directive establishes criteria for excessive heat warnings and advisories, though local offices are strongly encouraged to establish their own criteria. When they do, they’ll usually have consulted with health officials, epidemiologists, and professors to establish those benchmarks.

The inconsistencies in heat advisory and warning criteria among the 122 offices can lead to confusion among the public. But remember, some U.S. regions are less acclimated to hotter weather. Also, certain local offices may not exclusively consider the heat index for issuing heat products. They may also consider projected high temperatures, duration of the heat event, time of year, local knowledge, etc. Also, local offices commonly lower their criteria in the spring and early summer. People need time to adjust to higher temperatures after winter and early spring.

Below are the criteria established by the National Weather Service Procedural Directive, used by 55 (shaded in blue) of the 122 offices (45%).

Weather offices that use NWS procedural directive for heat products

Excessive heat warning: Heat index values are expected to reach/exceed 105°F/110°F. Offices in the North use the 105°F threshold, and offices in the South use the 110°F threshold. Low temperatures are expected to remain at or above 75°F.

Excessive heat watch: Conditions will favor excessive heat warning levels in 24-72 hours.

Heat advisory: Heat index values are expected to reach/exceed 100°F/105°F. Offices in the North use the 100°F threshold, and offices in the South use the 105°F threshold. Low temperatures are expected to remain at or above 75°F.

The following map shows the heat index criteria used for excessive heat warnings by local office. As shown, most offices use the procedural directive criteria. However, some offices in southeastern U.S. and southern Texas require higher heat indices. Offices out west often incorporate HeatRisk data into establishing their criteria.

Excessive heat warning criteria

Nonetheless, if you’re unsure what criteria your local weather office uses, they will almost certainly include the heat index forecast in their heat alerts. When under a heat alert, always drink plenty of water, especially if you have to be outside. If you have the opportunity to stay indoors, do it!

Excessive heat warning example from Mississippi

The research paper from Hawkins et al. (2017) was especially useful when preparing this article.