Heat Alerts Explained: Extreme Heat Warning, Watch and Advisory
The National Weather Service (NWS) issues an Extreme Heat Warning when extremely dangerous heat conditions are expected or occurring, an Extreme Heat Watch when conditions are favorable for an extreme heat event, and a Heat Advisory for dangerous heat that is not expected to reach warning criteria. The watch and warning were called Excessive Heat Watch and Excessive Heat Warning until March 4, 2025.
There is no single national temperature for any of these alerts. Each local NWS office sets its own criteria, and some now base them on the NWS HeatRisk scale.
What is the difference between an extreme heat warning, watch and advisory?
The warning is the highest level, the watch signals a possible extreme heat event, and the advisory covers heat below warning level. The NWS heat watch and warning page defines them as follows, and the third column shows the criteria used by one office, NWS Baltimore/Washington, for areas east of the Blue Ridge.
| Alert | NWS definition | Example: NWS Baltimore/Washington |
|---|---|---|
| Extreme Heat Warning | Extremely dangerous heat conditions are expected or occurring | Heat index expected to reach 110°F or higher within 12 to 24 hours |
| Extreme Heat Watch | Conditions are favorable for an extreme heat event | Heat index may reach 110°F or higher within 24 to 48 hours |
| Heat Advisory | Dangerous heat not expected to reach warning criteria | Heat index expected to reach 105 to 109°F within 12 to 24 hours |
The same office uses 105°F for warnings and 100 to 104°F for advisories west of the Blue Ridge. It also says it may issue heat alerts at lower values early in the season or during a multi-day heat wave.
When did Excessive Heat become Extreme Heat?
The rename took effect on or about March 4, 2025. NWS Service Change Notice 24-88, issued on September 5, 2024, changed the Excessive Heat Watch to the Extreme Heat Watch and the Excessive Heat Warning to the Extreme Heat Warning. The Heat Advisory kept its name.
In its announcement of the heat alert changes, the NWS said the rename is part of its Hazard Simplification project and follows similar changes to cold alerts, which became Extreme Cold Watches and Warnings in October 2024. The NWS also noted that 2024 was the hottest year on record for the contiguous United States. Older pages and apps may still show the Excessive Heat names.
How do local NWS offices set heat criteria?
The NWS says local offices often work with local partners to decide when an alert should be issued, because people in different regions are used to different heat. Its example: residents of Florida are more accustomed to temperatures above 90°F than residents of Alaska. Official criteria from four offices show the range:
- NWS Twin Cities (Minnesota): in the 7-county Minneapolis–St. Paul metro, a Heat Advisory starts at a heat index of 95°F for at least 3 hours. An Extreme Heat Warning starts at a heat index of 100°F, or after at least 4 consecutive days of advisory conditions.
- NWS Phoenix: alerts are based on HeatRisk rather than a fixed temperature. When Major or Extreme HeatRisk is forecast, the office issues an Extreme Heat Watch or Warning, and it says there is not one single, constant temperature used to decide.
- NWS Anchorage: since on or about June 9, 2026, the office issues Heat Advisories when high temperatures are forecast to reach 70, 75 or 80°F, depending on the zone, for two days or more. These replaced the Special Weather Statements it used for heat before.
- NWS Baltimore/Washington: heat index thresholds of 105 to 110°F, shown in the table above.
What is NWS HeatRisk?
NWS HeatRisk is a color-numeric index from 0 to 4 that uses weather, climate and Centers for Disease Control and Prevention (CDC) heat-health data to identify potentially dangerous heat. It gives a daily value for each 24-hour period of the next seven days. According to the NWS, HeatRisk considers how unusual the heat is for the time of year, how long it lasts, including daytime and nighttime temperatures, and whether those temperatures pose an elevated risk of heat-related impacts based on CDC data.
| Level | Color | Who is affected, per NWS |
|---|---|---|
| 0 Little to none | Green | Little to no risk from expected heat |
| 1 Minor | Yellow | Mainly people extremely sensitive to heat, especially outdoors without effective cooling or enough hydration |
| 2 Moderate | Orange | Most people sensitive to heat; impacts possible in some health systems and heat-sensitive industries |
| 3 Major | Red | Anyone without effective cooling or enough hydration; impacts likely in some health systems, industries and infrastructure |
| 4 Extreme | Magenta | Rare or long-duration extreme heat with little to no overnight relief; impacts likely in most health systems, industries and infrastructure |
HeatRisk is not an alert. The NWS says it supplements the official heat watches, warnings and advisories, and as of November 2026 it still labels HeatRisk experimental. The NWS map is on the Weather Prediction Center HeatRisk page, and the CDC offers a HeatRisk Dashboard where you enter your ZIP code.
What do the NWS and CDC advise during a heat alert?
The CDC heat and health page tells people to protect themselves and others by staying cool, staying hydrated and knowing the symptoms of heat-related illness. Its specific points:
- Use fans only if indoor temperatures are below 90°F. The CDC says that above 90°F, a fan can increase body temperature.
- Outdoors, stay in the shade, take breaks and schedule activity for the coolest parts of the day.
- Check on family, friends and neighbors, especially those who live alone or have chronic medical conditions.
The CDC lists groups at higher risk, including people 65 and older, infants and young children, pregnant women, people with heart disease or asthma, and people who work or exercise outside in the heat.
The NWS heat safety guidance adds:
- Drink plenty of water and non-alcoholic, decaffeinated fluids, even if you don’t feel thirsty.
- Use air conditioners or spend time in air-conditioned places such as malls and libraries.
- Never leave a child unattended in a vehicle, even with the windows down, even for a minute.
- Check on older, sick or frail people who may need help responding to the heat.
For the symptoms of heat-related illness, see the CDC’s guidance, and follow instructions from your local officials and NWS office.
Will a heat warning reach my phone?
Usually not by Wireless Emergency Alert. The NWS list of warnings it sends as WEA does not include Extreme Heat Warnings or Heat Advisories. Check weather.gov by ZIP code, NOAA Weather Radio, or our live list of NWS alerts by state, for example active NWS alerts for Arizona. Our pages show official NWS data in one place; they are not alerts. Our guide to how official emergency alerts reach you covers local alert sign-ups.
Bottom line
An Extreme Heat Warning means extremely dangerous heat is expected or occurring, an Extreme Heat Watch means it is possible, and a Heat Advisory means dangerous heat below warning level. The Excessive Heat names ended on March 4, 2025. Criteria depend on your local NWS office and range from highs of 70°F in parts of Alaska to a heat index of 110°F near Washington, D.C. HeatRisk adds context but is not an alert. Cold alerts were renamed the same way, as explained in our guide to winter weather alerts, and the general rules are in watch vs warning vs advisory.
FAQ
Is an Extreme Heat Warning the same as an Excessive Heat Warning?
Yes. The NWS renamed the Excessive Heat Warning to Extreme Heat Warning on or about March 4, 2025, and the Excessive Heat Watch to Extreme Heat Watch. The Heat Advisory kept its name.
What heat index triggers a heat advisory?
It depends on the local NWS office. NWS Baltimore/Washington uses a heat index of 105 to 109°F east of the Blue Ridge, NWS Twin Cities uses 95°F in its metro area, and NWS Phoenix bases alerts on HeatRisk instead of a fixed number.
Is HeatRisk an official heat alert?
No. The NWS says HeatRisk supplements its official heat watches, warnings and advisories, and it still calls the product experimental. Some offices, such as NWS Phoenix, use HeatRisk levels to decide when to issue official alerts.
Does a fan help during a heat wave?
The CDC says to use fans only if indoor temperatures are below 90°F. Above 90°F, the CDC says a fan can increase body temperature.