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Subject: A21) What is a "post-tropical cyclone", "remnent low", and "potential tropical cyclone" ?

Contributed by Dan Brown (NHC)

Beginning in 2012, the National Hurricane Center introduced some new terminology to clarify distinctions between differnt phenomena that it deals with.

A "Post-Tropical Cyclone" is a former tropical cylone, that no longer possesses sufficient tropical characteristics to be considered a tropical cyclone. Post-tropical cyclones can continue carrying heavy rains and high winds. Note that former tropical cyclones that have become fully extratropical, subtropical, or remnant lows, are three classes of post-tropical cyclones.

A "Remnant Low" is a type of post-tropical cyclone that no longer posseses the convective organization required to be a tropical cyclone, and has maximum sustained winds of less than 34 knots (17.5 m/s). The term is most commonly applied to the nearly deep-convection free swirls of stratocumulus clouds that form in the eastern Pacfic from tropical cyclones that have moved over cooler waters and lost their thunderstorms.

A "Potential Tropical Cyclone" is a distrubance that has not yet developed a closed circulation, but has the potential to do so and threatens to bring tropical storm or hurricane-force winds to land areas within 48 hours.

Revised June 1, 2018

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