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 (TOA) – 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 violent (EF-4 and EF-5) 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 (TOR) – 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 (TOR) – A large tornado has been confirmed to be producing damage and is moving into and through the warned area. It is usually issued as the initial tornado warning or as a complete re-issuance of the previous tornado warning. These may also include wording for a tornado emergency. Tornado emergency (SVS) – Sent as a "severe weather statement" or a complete re-issuance of the tornado warning, this is an unofficial, high-end tornado warning issued when a violent tornado is expected to impact a heavily 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; the EF5 tornado that affected Moore and adjacent southern portions of the Oklahoma City area on May 20, 2013; the