Anextreme heat warning is a notice issued by theNational Weather Service of theUnited States within 12 hours of theheat index reaching one of two criteria levels. In most areas, a warning will be issued if there is a heat index of at least 105 °F (41 °C) for more than three hours per day for two consecutive days, or if the heat index is greater than 115 °F (46 °C) for any period of time.[1]
Extreme heat can pose a threat to human life and other animals, through conditions such asheat stroke,heat exhaustion,dehydration, and other heat-related illnesses.[1]
On March 10, 2025, the National Weather Service updated the Excessive Heat Warning and Watch, and renamed it to the Extreme Heat Warning and Watch, based on a public survey results in 2018.[2]
Local offices, particularly those where extreme heat is less frequent or in areas with deserts or mountainous terrain, often have their own criteria.[1]
Due to the extreme illnesses that can occur, athletes should limit their physical activity when an extreme heat warning is issued. According to the National High School Sports Related Injury Surveillance System, High School RIO, an estimated number of 51,943 excessive heat illnesses occurred in nine of the high school sports observed between 2005, 2011, and 2020.[full citation needed]
The following is an example of an extreme heat warning and aheat advisory issued by the National Weather Service office inSan Antonio, Texas on May 13, 2025 during a heat wave.
233 WWUS74 KEWX 131753NPWEWXURGENT - WEATHER MESSAGENational Weather Service Austin/San Antonio TX1253 PM CDT Tue May 13 2025TXZ171>173-183>193-202>208-217>223-228-141000-/O.NEW.KEWX.XH.W.0001.250514T1800Z-250515T0200Z//O.EXT.KEWX.HT.Y.0001.250513T1800Z-250514T1800Z/Llano-Burnet-Williamson-Val Verde-Edwards-Real-Kerr-Bandera-Gillespie-Kendall-Blanco-Hays-Travis-Bastrop-Kinney-Uvalde-Medina-Bexar-Comal-Guadalupe-Caldwell-Maverick-Zavala-Frio-Atascosa-Wilson-Karnes-Gonzales-Dimmit-Including the cities of Boerne, Bastrop, Georgetown, Uvalde,Leakey, Fredericksburg, Hondo, Pearsall, Lockhart, Del Rio,Floresville, Eagle Pass, Gonzales, Burnet, Brackettville, SanAntonio, Rocksprings, Kerrville, Blanco, Crystal City, Austin,Seguin, Bandera, New Braunfels, Carrizo Springs, Llano, SanMarcos, Karnes City, and Pleasanton1253 PM CDT Tue May 13 2025...HEAT ADVISORY NOW IN EFFECT UNTIL 1 PM CDT WEDNESDAY......EXTREME HEAT WARNING IN EFFECT FROM 1 PM TO 9 PM CDT WEDNESDAY...* WHAT...For the Heat Advisory, air temperatures near 104 degrees, except near 110 along the Rio Grande. For the Extreme Heat Warning, dangerously hot conditions with air temperatures of 105 to 108 degrees, with near 115 degrees along the Rio Grande.* WHERE...A portion of south central Texas.* WHEN...For the Heat Advisory, until 1 PM CDT Wednesday. For the Extreme Heat Warning, from 1 PM to 9 PM CDT Wednesday.* IMPACTS...Heat related illnesses increase significantly during extreme heat and high humidity events. Hot temperatures and high humidity may cause heat illnesses.PRECAUTIONARY/PREPAREDNESS ACTIONS...Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors.To reduce risk during outdoor work, the Occupational Safety and Health Administration recommends scheduling frequent rest breaks in shaded or air conditioned environments. Anyone overcome by heat should be moved to a cool and shaded location. Heat stroke is an emergency! Call 9 1 1.&&$$