Climate and Weather
What is the difference between a National Weather Service statement, advisory, watch, and warning?
The National Weather Service will issue a statement as a “first alert” of the possibility of severe weather. An advisory is issued when weather conditions are not life threatening, but individuals need to be alert to weather conditions. A weather watch is issued when conditions are more favorable than usual for dangerous weather conditions, e.g., tornadoes and violent thunderstorms. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes). A warning is issued when a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning).
Are there specific criteria to prompt the National Weather Service to issue an advisory, watch, or warning?
Certain advisories, watches, or warnings are issued when specific conditions are met while others will vary by location. Examples of when some common National Weather Service advisories, watches, and warnings issued are:
- Hurricane watch: Issued when a hurricane poses a possible threat to specific coastal areas, generally within 36 hours.
- Hurricane warning: Issued when sustained winds of 74 miles per hour (119 kilometers per hour) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning may remain in effect even when winds are below hurricane force when dangerously high water and/or exceptionally high waves continue to persist.
- Severe thunderstorm warning: Issued when either a severe thunderstorm is indicated by radar or a spotter reports a thunderstorm producing hail 0.75 inch (2 centimeters) in diameter and/or winds equal to or exceeding 58 miles per hour (93 kilometers per hour). Lightning frequency is not a criterion for issuing a severe thunderstorm warning. It is advisable to seek safe shelter immediately when a severe thunderstorm warning is issued.
- Tornado watch: Issued when conditions are favorable for the development of tornadoes in the area. They normally are issued well in advance of the actual occurrence of severe weather for a duration of four to eight hours.
- Tornado warning: Issued when a tornado is indicated by radar or sighted by spotters. People in the affected area should seek safe shelter immediately. They are usually issued for around 30 minutes.
- Wind advisory: Issued when there are sustained winds of 25 to 39 miles per hour (40 to 63 kilometers per hour) and/or gusts to 57 miles per hour (92 kilometers per hour).
- Winter weather advisory: Issued when a low pressure system produces a combination of winter weather (snow, freezing rain, sleet) that presents a hazard, but does not meet warning criteria.
- Winter storm watch: Issued when there is the potential for heavy snow or significant ice accumulations usually at least 24 to 36 hours in advance. The criteria for this watch can vary from place to place.
- Winter storm warning: Issued when a winter storm is producing or is forecast to produce heavy snow or significant ice accumulations. The criteria for this warning can vary from place to place.