An anticyclonic tornado is a tornado which rotates in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. The term is a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes. Many anticyclonic tornadoes are smaller and weaker than cyclonic tornadoes, forming from a different process, as either companion/satellite tornadoes or nonmesocyclonic tornadoes.
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Most strong tornadoes form in the inflow and updraft area bordering the updraft-downdraft interface (which is also near the mesoscale "triple point") zone of supercell thunderstorms. The thunderstorm itself is rotating, with a rotating updraft known as a mesocyclone, and then a smaller area of rotation at lower altitude the tornadocyclone (or low-level mesocyclone) which produces or enables the smaller rotation that is a tornado. All of these may be quasi-vertically aligned continuing from the ground to the mid-upper levels of the storm. All of these cyclones and scaling all the way up to large extratropical (low-pressure systems) and tropical cyclones rotate cyclonically. Rotation in these synoptic scale systems stems partly from the Coriolis effect, but thunderstorms and tornadoes are too small to be significantly affected. The common property here is an area of lower pressure, thus surrounding air flows into the area of less dense air forming cyclonic rotation. The rotation of the thunderstorm itself is induced mostly by vertical wind shear, specifically clockwise turning as altitude increases (called a veered vertical profile, although backed profiles can occur with anticyclonic supercells]].
Various processes can produce an anticyclonic tornado. Most often they are satellite tornadoes of larger tornadoes which are directly associated with the tornadocyclone and mesocyclone. Occasionally anticyclonic tornadoes occur as an anticyclonic companion (mesoanticyclone) to a mesocyclone within a single storm. Anticyclonic tornadoes can occur as the primary tornado with a mesocyclone and under a rotating wall cloud. Also, anticyclonic supercells (with mesoanticyclone), which usually are storms that split and move to the left of the parent storm motion, though very rarely spawning tornadoes, spawn anticyclonic tornadoes. There is an increased incidence of anticyclonic tornadoes associated with tropical cyclones, and mesovortices within bow echoes may spawn anticyclonic tornadoes.
Known "anticyclonic tornado" events
|Date||Location||Notes and References|
|8 June 1951||Corn, Oklahoma||First known tornado filmed in the US, a companion or cyclic tornado to another significant tornado|
|6 June 1975||Freedom, Oklahoma|
|13 June 1976||Central Iowa|
|6 April 1980||Grand Island, Nebraska|
|4 April 1981||West Bend, Wisconsin||1981 West Bend F4 anticyclonic tornado|
|4 May 1998||San Francisco Bay Area, California|
|19 April 2002||Lubbock, Texas|
|6 September 2004||Chek-Lap-Kok International Airport, Hong Kong, China|
|24 April 2006||El Reno, Oklahoma|
|20 June 2006||Rushville, Nebraska|
|10 May 2010||South-central Oklahoma||Two tornadoes associated with anticyclonic supercell|
|31 May 2013||El Reno, Oklahoma||EF2 southeast of the 2013 EF3 El Reno tornado|
|4 June 2015||Elbert County, Colorado|
|31 March 2016|
|5 April 2017||Shelbyville, Tennessee|
|5 January 2019||Seymour, Texas||Two possible and confirmation coming|
|15 June 2019||Estelline, South Dakota||Lasted approximately 45 seconds and damaged about 7 trees|
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