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 actually be issued.

Heat advisory

Generally issued when the daily maximum heat index, a combined measure of temperature and humidity, not just the thermometer reading, is expected to stay unusually high for multiple consecutive days. The specific threshold temperature varies by country and even by region within a country.

Extreme heat warning

A step above a heat advisory, issued when the expected heat index is markedly higher still and expected to persist for multiple days, signaling a materially greater risk of heat-related illness.

Cold weather advisory

Generally issued when overnight or morning low temperatures are expected to drop sharply compared to recent days, or to fall well below the seasonal average. The exact drop or threshold temperature used varies significantly by region and time of year.

Extreme cold warning

A step above a cold weather advisory, issued when an even larger temperature drop or an even lower minimum temperature is expected.

Heavy rain advisory

Generally triggered by a forecast of a large amount of rain in a short window, commonly assessed using rainfall accumulated over a few hours or over roughly half a day, that raises the risk of flooding or damage to infrastructure.

Heavy rain warning

A step above a heavy rain advisory, issued when an even larger amount of rain is expected to fall in a similarly short window, raising the risk of flash flooding, flooding, or landslides significantly further.

Heavy snow advisory

Generally issued when snow accumulation over roughly a 24-hour period is forecast to exceed a set threshold; mountainous areas and low-lying areas are frequently assigned different thresholds.

Heavy snow warning

A step above a heavy snow advisory, issued when even greater accumulation is expected, raising the risk of transportation gridlock or structural damage from snow load.

High wind advisory

Generally issued when sustained or gust wind speeds are forecast to exceed a set threshold; thresholds for land areas and for coastal or open-water areas are usually set differently.

Tropical cyclone advisory / warning

Issued when damage from a tropical cyclone's strong winds and heavy rain is considered likely, with the alert level escalating from advisory to warning as the storm's expected wind speed, rainfall, and track or intensity increase.

Advisory vs. warning: the general pattern

Across nearly every hazard type, 'warning' represents a more severe expected impact than 'advisory' for the exact same phenomenon, it is a higher tier of the same alert, not a different kind of alert, and response measures are generally expected to escalate accordingly.

Why extreme heat and cold get their own two-tier system

Heat and cold are graded by cumulative physiological stress rather than a single instant reading, which is why forecasters look at how high or low the temperature is expected to stay over multiple consecutive days rather than a single peak or trough. A brief, one-day spike is treated very differently from the same temperature sustained for several days in a row, because the health risk compounds with duration.

Why the same hazard can trigger different criteria in different places

A meteorological agency generally sets different numeric thresholds for the same hazard depending on local terrain and typical climate, mountainous terrain, coastal areas, and inland plains each experience heavy snow or strong wind differently, and a rainfall total that is unremarkable in a typically wet region might be genuinely dangerous in a typically dry one. That is also why national averages are only a rough guide, and the specific criteria for a given local area are what actually determine when an alert is issued there.

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 high wind advisory and a heavy rain advisory simultaneously, or an escalating combination of both.

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

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