Tornado sirens and emergency alert broadcast are two types of tornado warning systems. National weather forecasting services issues tornado watches via special television and radio broadcasts when conditions favorable for tornadoes occur. Siren activation may accompany other emergency-alert broadcasts, such as tornado warnings. Tornado warnings indicate the presence of a tornado confirmed visually or by radar.
Tornado warnings alert the public that severe thunderstorms and tornadic activity is either imminent or occurring. Warnings may occur when the public, storm chasers or law enforcement personal spot and report funnel clouds or tornadoes. Radar indication of tornado formation may also trigger a warning. Activation of tornado sirens typically occurs in areas where such systems are present. The sirens exist to alert the population of potential danger, although such systems are often difficult to hear when indoors.
The National Weather Service may also issue a warning known as a tornado emergency. This alert is an unofficial way to indicate a large extremely violent tornado with the potential to impact a densely populated area. Tornado emergencies are the most urgent form of warning issued. Alerts broadcasting tornado warnings may also contain added wording mentioning their life-threatening nature or their capacity to cause extensive property damage.