PlayInfoBox

Home /Education /Weather Advisory and Warning Criteria, by Hazard Type

Weather Advisory and Warning Criteria, by Hazard Type

The news says a heat advisory or a high wind warning is in effect. Here is roughly what has to happen for each one to be issued, which numbers are genuinely standard, and which ones change from place to place.

Heat advisory

Generally issued when the heat index, a combined measure of temperature and humidity rather than the thermometer reading alone, is expected to reach a level that can cause heat illness for people who are exposed for a long time. Thresholds are set locally. As one example from a US National Weather Service office, a heat advisory is used for a heat index of roughly 100 to 104 degrees Fahrenheit lasting at least a couple of hours, but other offices and other countries use different numbers. Many agencies also count consecutive hot days and hot nights, because the body recovers less when overnight temperatures stay high.

Extreme heat warning

A step above a heat advisory, issued when the expected heat is markedly more dangerous. In the United States this alert was called an excessive heat warning until the National Weather Service renamed it extreme heat warning in 2025 as part of a plan to simplify alert wording; the criteria did not change. In the same office example, the warning level starts at a heat index of about 105 degrees Fahrenheit or more. Japan runs a separate heat-stroke alert system based on the WBGT index, with an alert issued when WBGT is forecast to reach 33 somewhere in a prefecture, and a higher special alert when it is forecast to reach 35 at every observation point in a prefecture.

Cold weather advisory

Generally issued when low temperatures or wind chill values are expected to drop sharply compared to recent days, or to fall well below what is normal for the season. The exact trigger varies significantly with the region and time of year, because a temperature that is routine in one place is hazardous in another that has fewer heating systems, less cold-weather clothing and less road treatment.

Extreme cold warning

A step above a cold weather advisory, issued when an even larger drop or an even lower minimum temperature or wind chill is expected, with a real risk of frostbite and hypothermia within a short time outdoors. Pipes, livestock, unsheltered people and older adults living in poorly heated homes are the usual concerns.

Heavy rain advisory

Generally triggered by a forecast of a large amount of rain in a short window, commonly assessed as accumulation over a few hours or over roughly half a day. The goal is to flag a raised risk of urban flooding, flooded underpasses and damage to infrastructure before it happens. Agencies often set stricter numbers for areas with steep slopes or old drainage.

Heavy rain warning

A step above a heavy rain advisory, issued when even more rain is expected in a similar window, raising the risk of flash flooding, river flooding or landslides. In the United States, a flash flood warning is issued when flooding is occurring or imminent, which is closer to an emergency than a forecast. Japan adds a third, rarer tier called a special warning for events expected to be far beyond the ordinary warning level.

Heavy snow advisory

Generally issued when snow accumulation over roughly 12 to 24 hours is forecast to exceed a threshold that will make travel difficult. Mountain and low-lying areas often have different thresholds, and a given amount of snow is far more disruptive in a city that has few plows than in one that sees snow every week.

Heavy snow warning

A step above a heavy snow advisory, issued when greater accumulation, often along with wind, is expected, which can bring road closures, power outages from heavy snow on lines and roof damage from snow load. In the United States, the terminology is currently centered on winter storm warnings, which cover several types of winter weather.

High wind advisory

Generally issued when sustained winds or gusts are forecast to exceed a set threshold. For many US National Weather Service offices, a wind advisory is used for sustained winds of 31 to 39 mph for at least an hour, or non-thunderstorm gusts of 46 to 57 mph, and the high wind warning starts at sustained winds of 40 mph or gusts of 58 mph or more. Some offices use slightly different numbers, so check your local office. Coastal and open-water areas typically have separate marine criteria.

Tropical cyclone advisory / warning

Issued when damage from a tropical cyclone's strong winds, heavy rain and storm surge is considered likely, with the alert level rising as the expected wind speed, rainfall and track or intensity worsen. In the United States, the National Hurricane Center issues a tropical storm watch when tropical-storm-force winds (sustained 39 to 73 mph) are possible within 48 hours, and a tropical storm warning when they are expected within 36 hours. Other basins, such as the western Pacific, use their own numbered signal systems.

Advisory vs. warning: the general pattern

Across nearly every hazard type, a warning represents a more severe expected impact than an advisory for the same phenomenon. It is a higher tier of the same alert rather than a different kind of alert, and the recommended response escalates accordingly: with an advisory you stay alert and adjust plans, with a warning you take protective action.

Why extreme heat and cold get their own tiers

Heat and cold are graded by cumulative physiological stress rather than a single instant reading, which is why forecasters look at how long the temperature stays high or low rather than a single peak. A brief, one-day spike is treated very differently from the same temperature sustained for several days, because the health risk compounds with duration and with hot nights that give the body no relief. Heat-related illness ranges from cramps and exhaustion to heat stroke, which is a medical emergency, and people who work outdoors, older adults, infants and people without air conditioning are the most exposed.

Why the same hazard triggers different criteria in different places

A meteorological agency generally sets different numeric thresholds for the same hazard according to local terrain and typical climate. Mountains, coasts and inland plains each experience heavy snow or strong wind differently, and a rainfall total that is unremarkable in a normally wet region might be genuinely dangerous in a dry one, where soil and drainage are not adapted to it. For that reason, national averages are only a rough guide, and the criteria of your own local forecast office are what actually decide when an alert is issued where you live. Even inside one country, neighboring offices can have slightly different numbers.

How to respond to each level

For an advisory, check the forecast, adjust outdoor plans, drink water, check on neighbors and carry what you need for the conditions. For a warning, change your behavior: avoid driving into flooded roads, stay out of the worst heat, secure loose objects before strong winds and, if you are told to evacuate, do so early. Alerts reach you through phone alerts, radio and local weather services, so make sure notifications are turned on. This page offers general information and is not an official or professional safety guideline; follow your national and local authorities.

Frequently Asked Questions

Can more than one advisory or warning be in effect at the same time?

Yes. A tropical cyclone, for example, commonly brings both strong winds and heavy rain at once, which can trigger a wind alert and a heavy rain alert simultaneously. A winter storm can also combine snow, ice and wind alerts.

How does this differ from the 'watch, warning, advisory' system some countries use?

Some countries, like the US, layer a three-tier system on top of hazard-specific criteria, where a watch means conditions are favorable for a hazard and a warning means it is happening or imminent. Others apply a two-tier advisory and warning structure directly to each hazard type without a separate watch stage. The aim of alerting the public before and during a dangerous event is the same either way.

Why was a warning issued even though it did not feel that bad at my house?

Alerts cover a forecast zone, not a single address, and they are based on forecast conditions at the most affected places within it. Local terrain and building quality also change the experience.

Do the criteria ever change?

Yes. Agencies revise names and thresholds from time to time, as when the US National Weather Service renamed its excessive heat alerts to extreme heat in 2025. Always check the current criteria on your national weather service's website.

Should I wait for the warning before preparing?

No. Advisories and watches exist to give you time. Preparing early costs little, while waiting until a warning is issued can leave you too little time, especially for fast-developing hazards such as flash floods.