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. This is extremely rare and has only been documented 5 total times. 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. The first anticyclonic tornado associated with a mesoanticyclone was spotted on
WSR-88D weather radar in
Sunnyvale, California on May 4, 1998. The tornado was an F2 on the
Fujita Scale. ==Known "anticyclonic tornado" events==