Subtropical cyclone An unnamed subtropical storm in 2000 A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone . They can form in a wide band of latitude , from the equator to 50° . Types Upper-level low The most common type of subtropical storm is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles/160 kilometres or more from the centre . In comparison to tropical cyclones , such systems have a relatively broad zone of maximum winds that is located farther from the centre , and typically have a less symmetric wind field and distribution of convection . Mesoscale low A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing zone of horizontal wind shear , also known as a dying frontal zone , with radius of maximum sustained winds generally less 30 miles/50 kilometres . The entire circulation may initially have a diameter of less than 100 miles/160 kilometres . These generally short-lived systems may be either cold core or warm core , and briefly in 1972 this type of subtropical cyclone was referred to a neutercane . As of 2006 , the warm core variety were moved under the umbrella of the tropical cyclone definiton , and removed from the subtropical cyclone definition . Transition from extratropical By gaining tropical characteristics , an extratropical low may transit into a subtropical depression/storm . A subtropical depression/storm may further gain tropical characteristics to become a pure tropical depression/storm , which may eventually develop into a hurricane , and there is at least one case of a tropical storm transforming into a subtropical storm . Generally , a tropical storm or tropical depression is not called subtropical while it is becoming extratropical , after hitting either land or colder waters . This transition normally requires significant instability through the atmosphere , with temperature differences between the underlying ocean and the mid-levels of the troposphere requiring over 40C of contrast in this roughly 20 , 000-foot/6000-metre layer of the lower atmosphere . Characteristics These storms can have maximum winds extending further from the centre than in a purely tropical cyclone . The maximum recorded wind speed for a subtropical storm is 33 m/s ( 119 km/h , 65 knots , or 74 mph ) , also the minimum for a hurricane . In the Atlantic Basin , the United States NOAA classifies subtropical cyclones similar to their tropical cousins , based on maximum sustained surface winds . Those with winds below 18 m/s , 65 km/h , 35 kts , or 39 MPH are called subtropical depressions , while those at or above this velocity are referred to as subtropical storms . Subtropical cyclones are also somewhat more likely than tropical cyclones to form outside of a region 's designated hurricane season . Subtropical Storm Ana ( which became Tropical Storm Ana ) in mid-April of the 2003 hurricane season is such a case . Naming Throughout the 1950 's and 1960 's , the term semi-tropical was used for what would become known as subtropical cyclones . In 1972 , the National Hurricane Center ( NHC ) finally designated these storms as subtropical cyclones in real-time , and updated the hurricane database from 1968-1971 to include subtropical cyclones . Subtropical cyclones were initially named from the phoenic alphabet list in the early to mid 1970's . In the intervening years of 1975 - 2001 , subtropical storms were either named from the traditional list and considered tropical or used a separate numbering system . Between 1992 and 2001 , two different numbers were given to subtropical depressions or subtropical storms , one for public use , the other one for NRL and NHC reference . For example , Karen in 2001 was initially known as Subtropical Storm One as well as AL1301 ( or 13L for short ) . In 2002 , the NHC began giving numbers to subtropical depressions and names to subtropical storms from the same sequence as tropical ones . From 2002 onwards , Subtropical Depression 13L will be known as Subtropical Depression Thirteen instead . Subtropical Storm Nicole , from the 2004 Atlantic hurricane season was the first subtropical storm that did not become tropical since the policy change . A subtropical storm from the 2005 Atlantic hurricane season also did not become tropical , but was not named since it was not recognized until post-season analysis . See also Tropical cyclones Portal Cyclones and Anticyclones of the world Extratropical - Meso-scale - Pol ar - Polar low - Subtropical - Tropical References ^ Interdepartmental Hurricane Conference Retrieved 2006-10-10 . ^ David Mark Roth ( 2002-02-15 ) . A Fifty year History of Subtropical Cyclones . Hydrometeorological Prediction Center . Retrieved on 2006-10-04 . External links NOAA FAQ on tropical cyclones . USA Today article on subtropical storms . Categories : Tropical cyclone meteorology | Midlatitude weather | Types of cyclone In other languages : Simple English A Fifty year History of Subtropical Cyclones David Mark Roth Hydrometeorological Prediction Center 2002-02-15 http : //www.hpc.ncep.noaa.gov/research/roth/Subpreprint.pdf 2006-10-04 