Post-tropical cyclone

A post-tropical cyclone is a former tropical cyclone. Two classes of post-tropical cyclones are:

• Extratropical cyclone, which is frontal, sometimes still retains winds of hurricane or tropical storm force.
• Remnant low, which is non-frontal, has maximum sustained winds of less than 34 knots.

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. A few examples falling into this gray area are listed below.

• Celia (2010)
• Eugene (2011)
• Michael (2012)
• Nadine (2012)
• Humberto (2013)
• Joaquin (2015)

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

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)".

In the South-West Indian Ocean, Météo-France classifies a tropical system as a “post-tropical depression” when it has begun an extratropical transition. Once it finishes the process, it will be classified as an extratropical depression.

Origin

The terminology was initiated by Canadian Hurricane Centre in 1998 during Tropical Storm Bonnie. 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. The term was later adopted by the National Weather Service on May 15, 2010.

Synonym

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