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An excessive heat warning is a notice issued by the National Weather Service of the United States within 12 hours of the heat 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 for more than three hours per day for two consecutive days, or if the heat index is greater than 115 °F for any period of time.
Note that local offices, particularly those where excessive heat is less frequent or in areas with deserts or mountainous terrain, often have their own criteria. High values of the heat index are caused by temperatures being significantly above normal and high humidity, and such high levels can pose a threat to human life through conditions such as heat stroke, heat exhaustion, and other heat-related illnesses.