Post-tropical cyclone

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A post-tropical cyclone is a former tropical 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 remnant low. A tropical cyclone degenerating into a trough 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]

Classification/Other post-tropical cyclones

Classes

Two classes of post-tropical cyclones exist

• tropical storm force.^[1] Hurricane Paulette (2020) provides a recent example of an extratropical cyclone,^[3] in which it no longer has a warm core in higher latitudes when it was over Nova Scotia and all points north in its trajectory.^[4]
• Remnant low, which is non-frontal, has maximum sustained winds of less than 34 knots, and mainly consists of stable
  stratocumulus with little to no convective activity. These shallow systems may meander for some time before opening into a trough of low pressure, or by being absorbed into an extratropical cyclone.^[1][5][6]

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.

• Eugene (2011)^[8]
• Michael (2012)^[9]
• Nadine (2012)^[10]
• Humberto (2013)^[11]
• Joaquin (2015)^[12]
• Matthew (2016)^[13]
• Jerry (2019)^[14]
• Paulette (2020)^[15]
• Guambe (2021)
• Linda (2021)^[16]
• Batsirai
  (2022)
• Emnati (2022)
• Blas (2022)^[17]
• Frank (2022)^[18]
• Howard (2022)^[19]
• Kay (2022)^[20]

However, there has been an occasion that the United States National Hurricane Center went against that definition and designated Calvin (2011) as a 35-knot remnant low.^[21]

Also, if a tropical cyclone degenerates into a tropical wave or trough, then it does not qualify as a post-tropical cyclone. It would be referred as the "remnants of (tropical cyclone name)".

The terminology was initiated by Meteorologist Peter Bowyer of the Canadian Hurricane Centre in 1998 during Tropical Storm Bonnie.^[25] In 2008, the National Hurricane Center used this term for Tropical Storm Laura to address the limitation of the two classes (extratropical/remnant low) mentioned above.^[26] The term was later adopted by the National Weather Service on May 15, 2010.^[7]

Synonym

The Bureau of Meteorology in Australia refers to a former tropical cyclone as an "ex-tropical cyclone".^[27] An example is ex-tropical cyclone Oswald.

See also

• Glossary of tropical cyclone terms

References