tornado warning

A tornado warning (SAME code: TOR) is a public warning that is issued by weather forecasting agencies to an area in the path of a tornado, or a severe thunderstorm capable of producing one, and advises individuals in that area to take cover. Modern weather surveillance technology such as Doppler weather radar can detect rotation in a thunderstorm, allowing for early warning before a tornado develops. They are also commonly issued based on reported visual sighting of a tornado, funnel cloud, or wall cloud, typically from weather spotters or the public, but also law enforcement or local emergency management. When radar is unavailable or insufficient, such ground truth is crucial. In particular, a tornado can develop in a gap of radar coverage, of which there are several known in the United States. A warning should not be confused with a tornado watch, issued in the United States by the Storm Prediction Center (SPC) and in other countries by applicable regional forecasting agencies or national severe weather guidance centers, which only indicates that conditions are favorable for the formation of tornadoes. Although a tornado warning is generally a higher alert level than a tornado watch, in the U.S., it can be surpassed by a higher-level alert—structured as wording that can be added to the official warning product—to warn the public of intense tornadoes affecting a densely populated area. A tornado watch is not required for a warning to be issued; tornado warnings are occasionally issued when a tornado watch is not active (i.e. when a severe thunderstorm watch is active, or when no watches are in effect), if a severe thunderstorm develops and has a confirmed tornado or strong rotation.

History in the United States

Definition

A tornado warning is issued when any of the following conditions has occurred: a tornado is reported on the ground, or a funnel cloud has been reported, or strong low-level rotation is indicated by weather radar, or a waterspout is headed for landfall. A tornado warning means there is immediate danger for the warned area and immediate surrounding locations from the severe thunderstorm producing (or likely to produce) a tornado, if not from the relatively narrow tornado itself. Persons in the path of a warned storm are urged to immediately seek shelter in a basement, cellar, safe room or a sturdy above-ground room in the center section of a home or building (such as a bathroom or closet). In addition to the likelihood of a tornado, generally (but not always), a tornado warning also indicates that the parent severe thunderstorm has the likelihood of producing straight-line winds and/or large hail exceeding regional severe criterion as well as intense lightning, heavy rainfall and/or, through associated rain accumulations, flash flooding. (The classifiable criteria for a severe thunderstorm vary by country; e.g., in the United States, a thunderstorm is considered severe if it produces winds exceeding and/or hailstones or larger in diameter.) A tornado warning therefore implies that it is also a severe thunderstorm warning. Conversely, a severe thunderstorm warning, either in its entirety or sectionally, can be replaced by a tornado warning if a storm exhibits credible characteristics of tornadogenesis from radar and ground observations. Tornado warnings are issued by weather forecasting agencies based on mesocyclone and debris signatures identifiable on Doppler weather radar, and/or ground truth from storm spotters of signs of tornadogenesis (including wall clouds, funnel clouds and low-level mesocyclonic rotation) and of observed tornadoes. Skywarn, a U.S.-based international spotter program that provides training to citizens on how to spot severe weather phenomena, is offered by forecasting agencies in several countries including the National Weather Service (NWS) in the United States. Used in tandem with Doppler weather radar information, eyewitness reports can be very helpful for warning the public of an impending tornado, especially when used for ground truthing. Other spotter groups such as the Amateur Radio Emergency Service, news media, local law enforcement agencies, emergency management organizations, cooperative observers; members of the general public also relay information to forecasting agencies for ground truthing. When a warning is issued, people in the storm's path are advised to use local broadcast media, weather radio, weather app alert notifications and/or SMS notifications to receive warnings and updated storm information. In some tornado-prone regions, tornado sirens, if present, are usually activated within the specified warning region or an entire municipality—varying in coverage based on if a local siren network maintains a sector-organized relay structure—to inform people that a tornado has been sighted or may be forming nearby. Because sirens are generally difficult to hear indoors, residents are advised against completely depending on them as a method of receiving warnings. Local police or fire departments may dispatch crews not assigned to an existing emergency call to travel within a designated area to warn residents to take tornado safety precautions if sirens are disabled due to technical problems or are not present, while automated phone calls may be made to residents for the same purpose in some areas should such disruptions occur. Advances in technology, both in identifying conditions and in distributing warnings effectively, have been credited with reducing the death toll from tornadoes. The average warning times have increased substantially from -10 to -15 minutes in 1974 to about 15 minutes (in some cases, the lead time can extend to more than an hour's warning of impending tornadoes). In the United States, the tornado death rate has declined from 1.8 deaths per million people per year in 1925 to only 0.11 per million in 2000. Much of this change is credited to improvements in the tornado warning system, via the various advances in the detection of severe local storms, along with an increase in reports visually confirming severe weather activity via storm spotters, public officials and citizens.

Regional basis

United States

In the United States, tornado warnings are issued by local Weather Forecast Offices of the National Weather Service, which, in conjunction with the Storm Prediction Center, maintains a multi-tier public warning system to disseminate probabilistic outlooks and alerts for tornadoes: Convective Outlook (categorical and probabilistic forecasts issued at least twice per day to describe threats of general or severe convective storms; severe thunderstorm outlooks for the Day 1 and Day 2 periods include charts and maps assessing tornado probabilities) Public Severe Weather Outlook (issued when a significant or widespread severe thunderstorm/tornado outbreak is expected or, particularly from November to March, when strong tornadoes are forecast to occur after nightfall) Tornado Watch PDS Tornado Watch (upgraded wording indicating the likelihood of a significant tornado outbreak and/or a credible threat of strong to violent tornadoes within the watch area) Tornado Warning PDS Tornado Warning Tornado Emergency Local NWS forecast offices utilize WarnGen software integrated into the Advance Weather Interactive Processing System (AWIPS) to generate the warning statement, which is disseminated through various communication routes accessed by the media and various agencies, on the internet, to NOAA satellites, and over NOAA Weather Radio; additionally, the National Weather Service has the option of requesting activation of the Emergency Alert System (EAS) to interrupt television and radio broadcasts to get the bulletin out quickly. Warnings for tornadoes and severe thunderstorms are outlined in polygonal shapes for map-based weather hazard products distributed to the main agency, individual forecast office websites and the Storm Prediction Center (including open-source APIs available for free use to public weather websites and mobile apps), based on the storm's projected path as determined by Doppler radar at the time of the warning's issuance. In NWS text products, warnings are usually illustrated by individual counties, parishes or other county-equivalent jurisdictions (sections or the entirety thereof, and in list format if it covers more than one jurisdiction), particularly dependent on the jurisdiction's total land area. Prior to October 2007, warnings were issued by the National Weather Service on a per-county basis. SPC and NWS products as well as severe weather alert displays used by some television stations, and desktop and mobile radar software typically highlight tornado warnings with a red or purple polygon or filled county/parish/equivalent jurisdiction outline. (Since the late 2010s, Baron Services broadcast radar software and some public commercial radar software, such as GRLevelX, have used custom polygon coloring to denote tornado warnings based on impact level: usually, purple to indicate warnings for a confirmed tornado and inlined black-on-purple to indicate tornado emergencies.) The NWS has the option of adding intensified wording to tornado warning products and update statements issued as a Severe Weather Statement (SVS)—"particularly dangerous situation" (PDS) or "tornado emergency"—when a severe threat to human life and considerable or catastrophic property damage from a visually observed or radar-detected large tornado is imminent or ongoing. Tornado emergencies and PDS tornado warnings—which, when warranted, are usually issued when a large tornado is expected to impact a populated area—typically include action statements indicating that the tornado is extremely dangerous and life-threatening, and capable of significant if not total property destruction and severe injury or death from the intense winds and projectile debris; tornado warnings that have the "tornado emergency" wording typically incorporate a combination of the emergency and PDS phrasing above the text's basis (or "hazard") statement. In March 2012, as part of its implementation of a multi-tier Impact Based Warning (IBW) system to notify the public and emergency management officials of the severity of specific severe weather phenomena, the NWS Weather Forecast Offices in Wichita and Topeka, Kansas, and Springfield, St. Louis and Kansas City/Pleasant Hill, Missouri, began incorporating categorical tornado and damage threat indicators for visually confirmed and radar-indicated tornadoes that appear at the bottom of the text products for tornado warnings and associated Severe Weather Statements providing updated storm information. The categorical criteria—which are applicable to all NWS Weather Forecast Offices, primarily those operating within the agency's Central and Southern Region divisions—were introduced to further communicate to the public and prevent complacency of the threat of tornadoes. The NWS expanded the threat and damage indicators to 33 additional Central Region WFOs in March 2013; eight additional offices operating within the Eastern, Southern and Western Region divisions began using the IBW indicators in March 2014. The entire agency began using the format in 2016; IBW formatting was fully implemented for other individual warning bulletins in July 2021, when all NWS offices incorporated damage threat indicators into severe thunderstorm warnings. The threat indicators consist of four coded taglines, ascending by observational level and damage threat: Radar Indicated – Doppler weather radar indicates the thunderstorm is exhibiting mesocyclonic circulation supportive of tornado formation; generally requires no visual confirmation of a tornado at the time the initial warning or a subsequent severe weather statement is issued. Hazard statements will classify the tornadic rotation as "radar indicated" or "radar-indicated rotation" in the bulletin text when identifying the source of the hazard. Observed – Tornado is visually confirmed by storm spotters, law enforcement or other emergency personnel, or detected on radar in concert with an observed intense velocity couplet and/or debris signature. Hazard statements within the bulletin text will indicate a "damaging tornado." Considerable (originally "Significant" during the 2012 Kansas and Missouri tests) – Typically reserved for PDS tornado warnings, it indicates credible evidence exists (through visual or radar confirmation) that a tornado capable of producing considerable damage is imminent or ongoing. As with warnings containing the "observed" tag, bulletins with this indicator will classify a "damaging tornado" in the hazard statement. Catastrophic – Generally reserved for warnings containing Tornado Emergency wording, it indicates reliable sources have confirmed a violent tornado posing a severe threat to human life and catastrophic property damage is occurring. In most instances, bulletins with this indicator will classify a "deadly tornado" in the hazard statement. On August 15, 2020, for the first time in history, the National Weather Service issued a tornado warning for pyrocumulonimbus capable of producing a tornado in southeastern Lassen County, California, which was being affected by the Loyalton Fire. In April 2023, U.S. Sen. Roger Wicker (R–MS) introduced the Tornado Observation Research Notification and Deployment to Operations (TORNADO) Act. The bill would establish a Hazard Risk Communication Office to propose improvements in the National Oceanic and Atmospheric Administration (NOAA)’s methods for predicting severe weather events and communicating weather alerts to the public—requiring NOAA to coordinate with government and emergency management to optimize collection and sharing of storm survey data, implement high-resolution probabilistic tornado forecast guidance systems, and make recommendations for revisions to the Enhanced Fujita Scale.

Canada

In Canada, Environment and Climate Change Canada issues tornado warnings through regional Meteorological Service offices based in Vancouver, Edmonton, Winnipeg, Toronto, Montreal and Dartmouth for specified municipalities and census subdivisions. Although issuance criteria are similar to the U.S. National Weather Service, Meteorological Service-issued tornado warnings can include areas not in the immediate approximate path of the tornadic thunderstorm but are in an environment conducive to tornado development from adjacent thunderstorms during the warning timeframe (similar to but covering a smaller total area than tornado watches issued by the U.S. Storm Prediction Center). Warnings are disseminated to the public through broadcast and online media outlets, and Weatheradio Canada; depending on storm severity and regional office discretion, the warning may require activation of depending on storm severity and regional office discretion, the warning may require activation of the National Public Alerting System (Alert Ready) ( ) and feeding provincial alerting systems (such as Alberta Emergency Alert and SaskAlert) to distribute the alert to local broadcast media and cellular phones. Separately, Emergency Management Ontario—upon implementing the system in 2008—issues red alerts for sections of the province under an Environment Canada-issued tornado warning, and can sometimes override the tornado warning if local government or media outlets participate in the program. Since implementation of the colour-coded warning system in late 2025, the Weather impact guides state the following according to Environment Canada's MSC for adverse impacts expected from violently rotating, damaging winds associated with thunderstorms:

Orange

Travel routes dangerous to navigate Widespread travel disruption Widespread utility outages Significant, widespread tree damage Essential services delays; significant delays for emergency responders Significant damage to buildings and property including boats, sheds, RVs, small planes Increased risk of serious injury from flying or falling debris or collapsing structures/trees Some critical infrastructure damage They are uncommon

Red

Very dangerous and/or impassable travel routes Prolonged, widespread travel disruption including vehicles blown around or destroyed Prolonged, widespread utility/extreme power outages Transmission/utility tower collapse Stands of trees (forests, orchards, parks) flattened/destroyed Prolonged essential services disruption Homes/buildings destroyed High risk of serious injury from flying/falling debris or collapsing structures/trees Major agriculture/livestock losses Critical infrastructure compromised/destroyed; supply chain impacted They are rare

Australia

In Australia, tornado warnings are issued by regional offices of the Bureau of Meteorology (BOM) based in Melbourne Docklands, Adelaide, Darwin, Perth and Brisbane. BOM-issued tornado warnings are outlined as either a broad-based warning, covering expected impacts within a weather reporting area, or as a detailed warning, when a thunderstorm is within weather-watch radar range and includes a map depicting any existing thunderstorms and the forecast direction of movement for up to 60 minutes. Warnings are disseminated to the public through terrestrial and online media outlets, and through activation of Emergency Alert Australia to distribute the alert to local broadcast media (led by the Standard Emergency Warning Signal tone), SMS messaging and automated landline phone calls. Tornado Alert sirens and Fire Tornado Alert sirens part of the Emergency Alert Australia plan, HHEM His Highness Emergency Management plan, Emergency Management Australia plan, HH government plan and the Australian Government plan.

Related warnings and alerts

Tornado alert

For many years until the early 1980s, an intermediate type of tornado advisory known as a tornado alert was defined by the National Weather Service and issued by the agency's local forecast offices, indicating that tornado formation was imminent. In theory, tornado alerts covered situations such as visible rotation in clouds and certain other phenomena which are portents of funnel cloud formation. The National Weather Service's use of this advisory began to decline after 1974, although it was still listed on public information materials issued by various media outlets, local NWS offices and other entities for another decade or so. The criteria which called for tornado alerts in the past now generally result in a tornado warning with clarifying verbiage specifying that the warning was issued because rotation was detected in one way or another, that a wall cloud has formed or a tornado has been spotted or detected. The preferred response to both the tornado alerts and warnings is to take shelter immediately, so distinguishing them could be seen as splitting hairs, especially since storm prediction methods have improved. The tornado alert was finally eliminated outright because it was made largely obsolete by the advent of Doppler weather radar, which can detect rotational funnel cloud formations earlier than is typically possible by trained spotters and members of the public. With fewer false-positives, radar also helped reduce public confusion over storm types, strengths and precise locations. The last tornado alert to be officially issued was discussed in earnest following the 1974 Super Outbreak.

Tornado emergency

The National Weather Service has the option of issuing a tornado emergency, a severe weather statement with unofficial, enhanced wording that is disseminated when reliable sources confirm that a violent tornado is ongoing, that poses a catastrophic risk for damage and threat to human life. This category of weather statement is the highest and most urgent level relating to tornadoes, albeit an unofficial alert product. The first tornado emergency was declared on May 3, 1999, when an F5 tornado struck southern portions of the Oklahoma City metropolitan area, causing major damage exceeding $1 billion. In some cases, such as an F3 tornado that struck the Indianapolis, Indiana metropolitan area on September 20, 2002, a tornado emergency has been declared within the initial issuance of the tornado warning. Not all confirmed tornadoes will be considered a "tornado emergency", and such statements are commonly declared when it is believed that the tornado is at a severity in which it would cause a significant threat to life and property.

References

Text derived from https://en.wikipedia.org/w/index.php?oldid=1368343881 under CC BY-SA 4.0; modifications have been made.

Names in other languages