Guide

Heat advisory, warning or watch?

What is the difference between a heat advisory and an excessive heat warning?

A watch means dangerous heat is possible in the next few days. An advisory means it is expected and is reaching a level that will affect vulnerable people. A warning means it is expected and is dangerous to anyone without cooling. The thresholds are set locally, so the same heat index triggers different products in different places.

The three levels

The National Weather Service heat products, in order of certainty and severity.
Product	Means	Timing
Excessive heat watch	Conditions are favourable for dangerous heat, but it is not certain	Usually 24–72 hours ahead
Heat advisory	Dangerous heat is expected, at a level that will affect vulnerable people	Within 12–24 hours or ongoing
Excessive heat warning	Dangerous heat is expected, at a level that threatens anyone without cooling	Within 12–24 hours or ongoing

The pattern matches every other NWS hazard: watch means possible, advisory and warning mean expected, and the step from advisory to warning is severity rather than confidence.

Why the trigger is local

The single most common complaint about heat alerts is that they seem inconsistent. They are — deliberately.

Thresholds are set by each local forecast office, and the reasoning is that harm depends on adaptation and infrastructure rather than on the number alone. A 95°F afternoon in a maritime city with low air-conditioning prevalence and an unacclimatised population sends more people to hospital than a 105°F afternoon in a desert city where every building is cooled and everyone has spent June getting used to it.

An alert system calibrated purely on temperature would warn Phoenix constantly and Seattle almost never, which is precisely backwards in terms of who is at risk.

What a static page cannot tell you

Whether there is an alert where you are right now. That is a live, location-specific question, and the answer lives with the issuing agency: the NWS for the United States, and each country's own service elsewhere.

This site explains what the products mean. It does not carry them, and neither does the app — an official alert is a specific published event issued by an agency, and inferring one from a threshold is not the same thing.

The newer tool worth knowing about

The NWS also publishes HeatRisk, a five-level index that folds in how unusual the heat is for that place and time of year, how long it lasts, and overnight temperatures. It is designed to catch the cases that a fixed heat-index trigger misses — an early-season heatwave in a cool climate being the classic example.

If you find your local advisory criteria confusing, HeatRisk is often the better answer to "should I be worried", because it is normalised to your own climate rather than to a national number.

What to actually do

The advice does not vary much by product, which is worth saying plainly:

Move activity out of the afternoon. The hottest part of the day explains where that window actually is.
Check on the people who will not call. Older neighbours and relatives living alone, especially without air conditioning. Heat risk in older adults covers why their threshold is lower.
Treat warm nights as part of the event. Recovery happens overnight or not at all.
Know the difference between exhaustion and stroke, because one is an emergency: heat exhaustion vs heat stroke.

Common questions

Who issues heat advisories and warnings?

In the United States, the local National Weather Service forecast office for your area. That is why criteria vary: each office sets thresholds appropriate to its own climate and population, in consultation with local emergency management.

What heat index triggers an advisory?

There is no single national number. A common pattern is an advisory when the heat index is expected to reach roughly 100 to 105°F for a couple of hours, and a warning at roughly 105 to 110°F or above, but offices in hotter climates set higher thresholds and offices in cooler ones set lower. Check the criteria for your own forecast office.

Why does a cooler city get an alert when a hotter one does not?

Because risk depends on what people are adapted and equipped for. 95°F in Seattle, where air conditioning is less common and nobody is acclimatised, causes more harm than 105°F in Phoenix. Thresholds that reflected only temperature would systematically under-warn the places most at risk.

Does an alert account for overnight temperatures?

Increasingly yes, and it matters more than the afternoon peak. A night that does not fall below about 75–80°F gives the body no recovery period, and multi-day events with warm nights are the ones associated with excess deaths. Several offices include overnight minima in their criteria.

Are heat alerts issued in other countries the same way?

No. The UK runs heat-health alerts jointly between the Met Office and the health agency; Japan issues a national heat alert based on forecast WBGT reaching 33; many countries have nothing comparable. If you are outside the US, the terms on this page will not match what your forecaster publishes.

Sources

NOAA / National Weather Service — Heat safety and watch, warning and advisory definitions
NWS — HeatRisk
Japan Ministry of the Environment — Heat Illness Prevention Information — For the Japanese comparison

Published 30 July 2026