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Apost-tropical cyclone is a formertropical cyclone that no longer possesses enough tropical qualities to be considered a tropical cyclone.[1] The word may refer to a former tropical cyclone undergoing extratropical transition or a tropical cyclone degenerating into a remnantlow. A tropical cyclone degenerating into atrough or wave, or having its low level circulation dissipate overland, lacks a cyclonic circulation and is referred as remnants instead of a post-tropical cyclone. However, post-tropical cyclones or remnants can continue producing high winds and heavy rains.[2]
Two classes of post-tropical cyclones exist
Not all systems fall into the above two classes. According to the guideline, a system without frontal characteristics but with maximum winds above 34 knots may not be designated as a remnant low. It should be merely described as post-tropical.[7] A few examples falling into this grey area are listed below.
However, there has been an occasion that theUnited StatesNational Hurricane Center went against that definition and designatedCalvin (2011) as a 35-knot remnant low.[21]
Also, if a tropical cyclone degenerates into atropical wave ortrough, then it does not qualify as a post-tropical cyclone. It would be referred as the "remnants of (tropical cyclone name)".
Météo-France classifies systems in the South-West Indian Ocean undergoing anextratropical transition or losing tropical characteristics as "post-tropical depressions", since the2012–13 cyclone season. They would be re-classified as extratropical depressions after completing the process.[22]
A post-tropical cyclone is formed when the typical characteristics of atropical cyclone are replaced with those ofextratropical cyclones, otherwise known asextratropical transition.[23] After the initial formation, a post-tropical cyclone has the potential to gain strength and intensity by forming an extratropical storm.[23] If a post-tropical cyclone does become an extratropical storm, it will eventually decay through the process ofocclusion.[24]
The re-intensification of a post-tropical cyclone can cause dangerous conditions in North Atlantic shipping routes with high seas and winds comparable to those ofhurricanes.[23]
The terminology was initiated by Meteorologist Peter Bowyer of theCanadian Hurricane Centre in 1998 duringTropical Storm Bonnie.[25] In 2008, the National Hurricane Center used this term forTropical Storm Laura to address the limitation of the two classes (extratropical/remnant low) mentioned above.[26] The term was later adopted by theNational Weather Service on May 15, 2010.[7]
TheBureau of Meteorology inAustralia refers to a former tropical cyclone as an "ex-tropical cyclone".[27] An example isex-tropical cyclone Oswald.
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