What Is a Heat Advisory vs a Heat Warning? How the NWS Classifies Extreme Heat
What the National Weather Service is actually saying
The National Weather Service (NWS) issues three heat products, ranked by urgency. A heat advisory means dangerous heat is likely and people should take precautions. An excessive heat watch means dangerous heat is possible in the next one to three days — the forecast is forecasting confidence is high but still short of certainty. An excessive heat warning means dangerous heat is occurring or imminent and people should act now to protect themselves and others.
The difference between an advisory and a warning is not just adjectives. It maps to different expected impacts, different levels of certainty in the NWS forecast, and different recommended public responses. A warning is the most dangerous tier and historically correlates with spikes in heat-related illness and death. Watches are issued earlier, often a couple of days ahead, so that cities, employers, and the public have time to prepare cooling centers and shift outdoor work schedules.
None of these products are triggered by the raw air temperature alone. They are triggered by a derived number called the heat index, which combines temperature and relative humidity to estimate how hot the air feels to a human body. The same 98°F air temperature feels meaningfully different at 20 percent humidity (dry, evaporation cools you) versus 70 percent humidity (sweat barely evaporates, the body struggles).
How the heat index works
The heat index is a model of human thermal comfort, not a physical instrument reading. It was built by calibrating against how human subjects reported heat stress at different temperature and humidity combinations, and it accounts for the fact that the body cools itself mainly by sweating. When humidity is high, evaporation slows, the body's cooling system fails to keep up, and the "feels like" temperature climbs well above the thermometer reading.
A few reference points:
| Air temperature | Relative humidity | Approximate heat index | Risk |
|---|---|---|---|
| 90°F | 40 percent | 91°F | Caution |
| 95°F | 60 percent | 110°F | Extreme caution |
| 98°F | 70 percent | 124°F | Danger |
| 100°F | 75 percent | 135°F | Extreme danger |
The danger column maps loosely to the NWS thresholds: heat index values around or above 100°F for sustained daytime periods, with overnight lows that do not drop enough to give the body a break, are the trigger zone for advisories. When the heat index climbs toward 105°F–110°F or higher and stays there, the local NWS office typically upgrades to an excessive heat warning.
Why the thresholds shift by city
There is no single national number at which a warning is issued, and that is by design. Local NWS forecast offices set heat criteria based on regional climatology, not a fixed national standard. A HeatRisk level or advisory threshold in Phoenix, which routinely reaches 110°F in summer, will be higher than the threshold in Seattle or Portland, where homes are less likely to have air conditioning and residents are less acclimatized to extreme heat.
The reasoning is epidemiological: heat-related illness and mortality rise sharply when temperatures exceed what a given population is used to. The same absolute temperature that is a normal summer week in one city can be a public health emergency in another. Local thresholds combine the heat index with the frequency of hot days in that region's historical record, expected duration, and how cool the nights get. Sustained heat with warm nights is treated more seriously than a one-day spike followed by cool overnight recovery.
The NWS also uses a newer tool called HeatRisk, a color-coded, five-level system that overlays climatology with heat index and duration. It distinguishes "minor" risk (yellow) from "extreme" risk (magenta), and it explicitly highlights populations most at risk: outdoor workers, the elderly, small children, pregnant people, and those without reliable cooling at home. HeatRisk is gradually supplementing, and in some cases replacing, the older advisory / watch / warning language so that the public gets a clearer sense of how unusual the heat is for where they live.
How to read a heat alert
When an alert is issued, the practical guidance varies by tier but the core advice is constant:
- Heat advisory — limit outdoor activity, drink water, check on neighbors, never leave children or pets in a car.
- Excessive heat watch — start planning: identify a cooling destination, reschedule strenuous work, prepare for possible power strain.
- Excessive heat warning — act now: stay indoors in air conditioning, use cooling centers if needed, watch for symptoms of heat exhaustion and heatstroke, which is a medical emergency.
The categories are not a forecast of how hot it will "feel nice." They are a forecast of how dangerous the heat is likely to be for human bodies — and the danger rating shifts exactly because the bodies it rates vary from place to place.