This article describes severe weather terminology used by the National Weather Service (NWS) in the United States, a government agency operating within the Department of Commerce as an arm of the National Oceanic and Atmospheric Administration (NOAA).
The NWS provides weather forecasts, hazardous weather alerts, and other weather-related products for the general public and special interests through a collection of national and regional guidance centers (including the Storm Prediction Center, the National Hurricane Center and the Aviation Weather Center), and 122 local Weather Forecast Offices (WFO). Each Weather Forecast Office is assigned a designated geographic area of responsibility—also known as a county warning area—that are split into numerous forecast zones (encompassing part or all of one county or equivalent thereof) for issuing forecasts and hazardous weather products.
The article primarily defines precise meanings and associated criteria for nearly all weather warnings, watches, advisories, statements, and other products not associated with hazardous weather issued by the NWS and its sub-organizations (some of which may be specific to certain cities or regions). Related weather scales and general weather terms used by the agency are also addressed.
Definitions of severe weather alerts
The NWS divides severe weather alerts into several types of hazardous/hydrologic events: Severe local storms – Short-fused, small-scale hazardous weather or hydrologic events produced by thunderstorms (including large hail, damaging winds, tornadoes, and flash floods). Winter storms – Weather hazards associated with freezing or frozen precipitation (freezing rain, sleet, and/or snow), or combined effects of winter precipitation and strong winds. Fire weather – Weather conditions that contribute to an increased risk and help cause the spread of wildfires. Flooding – Hazardous hydrological events resulting in temporary inundation of land areas not normally covered by water, often caused by excessive rainfall. Coastal/lakeshore hazards – Hydrological hazards that may affect property, marine or leisure activities in areas near ocean and lake waters including high surf and coastal or lakeshore flooding, as well as rip currents. Marine hazards – Hazardous events that may affect marine travel, fishing and shipping interests along large bodies of water, including hazardous seas and freezing spray. Tropical cyclone hazards – Hazardous tropical cyclone events that may affect property in inland areas or marine activities in coastal waters, resulting in wind damage, storm surge, tornadoes and flooding rain. Non-precipitation hazards – Weather hazards not directly associated with any of the above including extreme heat or cold, dense fog, high winds, and river or lakeshore flooding.
Legend — Designated color codes used to identify watches, warnings, advisories or other products in NWS hazard maps and other internal products are indicated alongside the event code; — Off-white background indicates product does not have an assigned color code SVR — Product event code in bold text; all event codes are indicated following the title of each listed product; (NPW) — Product event code in parenthetical regular text; used for products that have both standard and Specific Area Message Encoding (SAME) event codes; SAME product codes assigned to each term for NOAA Weather Radio (NWR) broadcasts are included in products that do not have a specified code are identified where applicable, as defined by NOAA, as: Non-Precipitation Warnings/Watches/Advisories (NPW) Coastal Flood Warnings/Watches/Advisories (CFW) Marine Weather Statement (MWS) Mesoscale Discussion (MCD) Severe Weather Warnings/Watches/Advisories (WSW)
Severe local storms Tornado watch (yellow box in some select NWS documentation, red box in most other media) – Conditions are favorable for the development of severe thunderstorms and tornadoes in and close to the watch area. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for five to eight hours. Particularly dangerous situation tornado watch – Conditions are favorable for the development of severe thunderstorms capable of producing destructive tornadoes in and close to the watch area. These watches are occasionally issued, and usually mean that a major tornado outbreak is possible, where the potential for multiple strong to violent (EF4 and EF5) tornadoes exists. Usually only reserved for forecast "high-end" severe weather events, this type of watch is usually valid for a longer period of time and issued for a larger area by the Storm Prediction Center in Norman, Oklahoma, than a typical tornado watch. Tornado warning – Strong rotation in a thunderstorm has been indicated by Doppler weather radar or a tornado has been sighted by Skywarn spotters or other persons (such as local law enforcement). These warnings are currently issued on a polygonal basis, are usually issued for a duration of 30 minutes (but can run for as long as one hour), and can be issued without either a tornado watch or a severe thunderstorm watch being already in effect. Tornado warnings may be issued in combination with a Special Marine Warning if the storm affects nearshore or coastal waters. Particularly dangerous situation tornado warning – A large tornado has been confirmed to be producing damage and is moving into and through the warned area. The PDS wording can be incorporated into the text of a tornado warning, either upon its initial issuance or in a "severe weather statement" providing updated information on the storm, when a considerable tornado debris signature is detected on radar or a large tornado is visually observed. (The PDS wording is also incorporated into the boilerplate precautionary/preparedness action statement for tornado emergencies.) Tornado emergency – Sent as a "severe weather statement" or as the initial tornado warning, this is a high-end tornado warning issued when a violent tornado is expected to impact a densely populated area. Such warnings have been issued for, among other significant tornado events, the F5-rated tornado that destroyed much of Bridge Creek, Moore and portions of southeastern Oklahoma City, Oklahoma on May 3, 1999 (during which the product was conceived by the NWS Forecast Office in Norman); the EF5 tornado that destroyed much of Greensburg, Kansas on May 4, 2007; the EF4 tornado that hit Tuscaloosa, Alabama on April 27, 2011 (one of 16 tornado emergencies issued during the associated regional outbreak, which holds the all-time record for the most tornado emergencies issued in a 24-hour period); the EF5 tornado that affected Moore and adjacent southern portions of the Oklahoma City area on May 20, 2013; the May 31, 2013 tornado system that went over the Oklahoma City area through portions of Canadian County near and south of the densely populated city of El Reno; the EF4 tornado that affected western portions of the Kansas City area on May 28, 2019; the EF4 tornado that affected several areas in Mississippi on April 12, 2020; and two long-track EF4 tornadoes that affected the Mid-South region (one of which traveled a path through portions of western Kentucky, including the city of Mayfield) on December 10–11, 2021. This enhanced form of a tornado warning is issued mainly by Weather Forecast Offices within the National Weather Service's Central and Southern Region Headquarters; a tornado emergency is the highest level of a three-tiered Impact Based Warning system for tornadoes used by all WFOs within the Central Region Headquarters, and eight others within the Eastern, Southern and Western Regions. Severe thunderstorm watch (pink box or blue box in NWS documents, typically yellow box in other media) – Conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm contains large damaging hail of diameter or larger, and/or damaging winds greater than 58 mph (95 km/h or 50 knots) or greater. Isolated tornadoes are also possible (depending on whether atmospheric conditions are sufficient to produce tornadic activity) but not expected to be the dominant severe weather event. These watches are issued for large areas by the Storm Prediction Center in Norman, Oklahoma, and are usually valid for five to eight hours. Particularly dangerous situation severe thunderstorm watch – Conditions are favorable for the development of severe thunderstorms in and close to the watch area. Isolated tornadoes are possible but not expected to be the dominant severe weather event, hence these watches are very rarely issued. An expected severe wind event (derecho) is the mostly likely reason for a PDS severe thunderstorm watch to be issued, with widespread winds greater than 90 mph (150 km/h or 80 knots) possible. These watches are usually valid for a longer period of time and are issued for a larger area by the Storm Prediction Center in Norman, Oklahoma than a typical severe thunderstorm watch. This type of watch is usually only reserved for forecast "high-end" severe weather events. If, however, tornadoes are expected to be a major weather threat in addition to the preceding criteria, then a standard tornado watch (generally not a PDS watch) would be issued instead. Severe thunderstorm warning – A severe thunderstorm is indicated by Doppler weather radar or sighted by Skywarn spotters or other persons, such as local law enforcement. A severe thunderstorm contains large damaging hail of in diameter or larger, and/or damaging winds of or greater. These warnings are currently issued on a polygonal basis, are usually issued for a duration of 30 minutes to one hour, and can be issued without a severe thunderstorm watch or a tornado watch being already in effect. Because severe thunderstorms can produce tornadoes with little or no advance warning, a severe thunderstorm warning may be upgraded to a tornado warning if strong rotation is indicated or a tornado is sighted. Lightning frequency is not a criterion for issuing a severe thunderstorm warning. Severe thunderstorm warnings may be issued in combination with a special marine warning if the storm affects nearshore or coastal waters. Particularly dangerous situation severe thunderstorm warning – Issued within the initial severe thunderstorm warning or as a complete re-issuance of the previously issued warning, a PDS severe thunderstorm warning indicates a significant severe thunderstorm has been detected by Doppler weather radar and/or observed by Skywarn spotters or other civil defense personnel, and is moving into and through the warned area with the capability of causing significant property damage, and severe injury or fatality to people or animals caught in the storm's path. These may be issued for an ongoing severe wind event (either by a derecho, or from an intense wind core or downburst within a squall line or supercell) containing winds greater than 80 mph (124 km/h or 70 knots), and/or extreme hail of diameter or larger over a wide area. Specific to this product, the boilerplate PDS wording ("this is a particularly dangerous situation") placed between the storm summary and hazard information in the warning text was replaced in July 2021 with the notation "this is a destructive storm for[...]," highlighting specific locations under greatest threat from extreme winds or hail accompanying the warned storm.