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Weather warning indicating imminent danger of tornadoes

A tornado warning (SAME code: TOR) is a public warning that is issued by weather forecasting agencies to an area in the direct path of a tornado, or a 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.

The first official tornado forecast—and tornado warning—was made by United States Air Force Capt. (later Col.) Robert C. Miller and Major Ernest Fawbush, on March 25, 1948. The first such forecast came after the events that occurred five days earlier on March 20, 1948; Miller – a California native who became stationed at Tinker Air Force Base three weeks earlier – was assigned to work the late shift as a forecaster for the base's Air Weather Service office that evening, analyzing U. S. Weather Bureau surface maps and upper air charts that failed to note atmospheric instability and moisture content present over Oklahoma that would be suitable for producing thunderstorm activity, erroneously forecasting dry conditions for that night. Thunderstorms soon developed southwest of Oklahoma City, and at 9:30 p. m., forecasters from Will Rogers Airport sent a warning to Tinker that the storm encroaching the city was producing wind gusts of and a "Tornado South on Ground Moving NE!" Base personnel received an alert written by the staff sergeant on duty with Miller, minutes before the twister struck Tinker several minutes later around 10:00 p. m., damaging several military aircraft (with total damage estimated at $10 million) that could not be secured in time before it crossed the base grounds. Following an inquiry the next day before a tribunal of five generals who traveled to Tinker from Washington, D. C., who ruled that the March 20 tornado was an "act of God not forecastable given the present state of the art", base commander Gen. Fred Borum tasked Miller and Fawbush to follow up on the board's suggestion to consider methods of forecasting tornadic thunderstorms. Over the next three days, Miller and Fawbush studied reports and charts from previous tornado events to determine the atmospheric conditions favorable for the development of tornadic activity, in an effort to predict such events with some degree of accuracy. At the time, there had not been studies on how tornadoes formed; however, military radars were being adapted for forecasting use, allowing forecasters to see the outlines of storms but not their internal attributes such as rotation. Miller and Fawbush's findings on atmospheric phenomenon present in past outbreaks would aid in their initial forecast, as the day's surface and upper air analysis charts determined the same conditions present on March 20 were present on the 25th, concluded that central Oklahoma would have the highest risk for tornadoes during the late afternoon and evening. Miller and Fawbush soon would distribute their tornado forecasts to the American Red Cross and Oklahoma Highway Patrol, after giving William Maughan, chief meteorologist at the U. S. Weather Bureau's Oklahoma City office (who provided them with additional archived weather data to help fine tune their forecasts), permission to relay their forecasts to those agencies. The relative accuracy of the forecasts restarted a debate over their reliability and whether military or civilian agencies should have jurisdiction over the issuance of weather warnings. The side effect of this policy was that the lack of warning resulted in a steady increase in the number of tornado related fatalities through the 1950s, with some events prior to 1948 (such as the deadliest tornado in U. S. history, the Tri State Tornado in March 1925, and the Glazier–Higgins–Woodward tornadoes in April 1947) having death tolls well over 100. In 1938, the Weather Bureau rescinded its ban on the usage of the word "tornado" in weather products disseminated to emergency management personnel. The Bureau would develop a network of volunteer storm spotters in the early 1940s during World War II, to provide warning of tornadoes to workers in munitions plants and strategic factories. The American Meteorological Society agreed to have Miller and Fawbush present their methodology for forecasting tornadoes during the organization's 1950 meeting in St. Louis; after garnering press coverage for their successful prediction of past tornadoes, AMS representatives decided to open the presentation to the public.

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.

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.

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.

On August 15, 2020, for the first time in history, the National Weather Service issued a tornado warning for pyrocumulonimbus capable of causing a fire tornado for southeastern Lassen County, California, which was being affected by the Loyalton Fire.

The National Weather Service has the option of issuing a tornado emergency, a severe weather statement with unofficial, enhanced wording that is disseminated when a large, extremely violent tornado is about to impact a densely populated area. 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.