Excessive Heat Warning

An Excessive Heat Warning is the most severe heat alert issued by the U.S. National Weather Service (NWS), activated when extreme heat poses a serious threat to health and infrastructure.

Excessive Heat Warning

Overview

An Excessive Heat Warning is one of the weather alerts issued by the U.S. National Weather Service (NWS). It is issued when the heat index is expected to become extremely high on the current day or the following day, posing a serious threat to human health and social infrastructure. This warning is one level higher than a regular Heat Advisory and represents the most severe level of heat alert, requiring immediate preparedness and action upon issuance. It primarily occurs during the summer, and its frequency and intensity are increasing due to climate change.

Key Details

Issuance Criteria

The criteria for an Excessive Heat Warning vary by region and season, but it is generally issued under the following conditions:

Heat Index is forecast to be 105°F (approximately 40.6°C) or higher (in some regions, 110°F/43.3°C or higher)
Overnight minimum temperature remains at 75°F (approximately 23.9°C) or higher, providing no opportunity for recovery
High-temperature event is expected to persist for two or more days
Humidity is very high, making evaporative cooling nearly impossible

Health Risks

Extreme heat can cause the following acute and chronic health problems in the human body:

Heat Stroke: The body's temperature regulation center becomes paralyzed, causing body temperature to rise above 40°C, with risks of loss of consciousness and death
Heat Exhaustion: Dehydration and electrolyte imbalance due to excessive sweating, leading to dizziness and vomiting
Heat Cramps: Muscle cramps, especially during exercise
Heat Rash: Skin rash caused by blocked sweat glands
Worsening of underlying diseases: Rapid deterioration of conditions in individuals with chronic diseases such as cardiovascular disease, respiratory disease, and diabetes

Preparedness and Response Tips

Recommended actions when an Excessive Heat Warning is issued include:

1. Stay indoors: Remain in an air-conditioned indoor space as much as possible

2. Hydrate: Drink water regularly even if not thirsty (avoid alcohol, caffeine, and sugary drinks)

3. Avoid outdoor activities: Refrain from outdoor activities between 10 a.m. and 4 p.m.

4. Protect vulnerable groups: Frequently check on the condition of the elderly, infants, young children, individuals with chronic illnesses, and pets

5. Never leave in vehicles: Absolutely do not leave children or pets in parked cars (car interior temperature can rise by 20°F in just 10 minutes)

6. Use cooling facilities: Check and utilize information on public cooling centers

7. Be cautious with electricity use: Prepare emergency batteries and flashlights in case of power outages due to surging cooling demand

Social and Economic Impacts

Power grid strain: Increased risk of blackouts due to surging cooling demand
Transportation disruption: Deformation of railway tracks, melting of road asphalt, restrictions on aircraft takeoffs and landings
Agricultural damage: Livestock deaths, crop withering, reduced yields
Healthcare costs: Increased emergency room visits, surge in hospitalizations, rise in mortality rates
Reduced labor productivity: Work stoppages and risk of heat stroke for outdoor workers (construction, agriculture, delivery)

Latest Trends

As of 2024-2025, the issuance patterns of Excessive Heat Warnings show the following changes:

Increased frequency: The average annual number of issuances across the United States has more than tripled compared to the 1980s
Geographic expansion: Issuances have increased not only in the traditionally hot southwestern region but also in the Northeast, Midwest, and Pacific Northwest
Link to climate change: NOAA (National Oceanic and Atmospheric Administration) announced that 2024 would be one of the hottest years on record, due to the combined effects of El Niño and climate change
Worsening urban heat island effect: Overnight minimum temperatures in major cities have reached record highs, highlighting issues of cooling access, especially in low-income areas
Introduction of new warning system: Starting in 2024, the NWS piloted HeatRisk, a color-coded, health-focused heat warning system (five levels: green-yellow-orange-red-magenta)
International response: The World Meteorological Organization (WMO) recommends that all countries establish heat warning systems by 2025; South Korea has also subdivided its 'Heat Wave Special Advisory' into 'Heat Wave Warning' and 'Heat Wave Advisory' for operation