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A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical cyclones or extratropical cyclones. By 1972, the National Hurricane Center officially recognized this cyclone category. Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002. They have broad wind patterns with maximum sustained winds located farther from the center than typical tropical cyclones, and have no weather fronts linked into their center. Since they form from initially extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required for their formation are lower than the tropical cyclone threshold by three degrees Celsius, or five degrees Fahrenheit, lying around 23 degrees Celsius. This also means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season.

Contents 1 History of term 2 Characteristics 2.1 Transition from extratropical 3 Formation 4 Types 4.1 Upper-level low 4.2 Mesoscale low 5 Kona low 6 See also 7 References 8 External links

Throughout the 1950s and 1960s, the term semi-tropical was used for what would become known as subtropical cyclones. The term subtropical cyclone merely referred to any cyclone located in the subtropical belt near and just north of the horse latitudes. Intense debate ensued in the late 1960s, after a number of subtropical cyclones formed in the Atlantic Basin. In 1972, the National Hurricane Center (NHC) finally designated these storms as subtropical cyclones in real-time, and updated the hurricane database to include subtropical cyclones from 1968–1971. The term "neutercane" was coined by Bob Bundgaard for small subtropical cyclones which formed from mesoscale features. His friend Bob Simpson, director of NHC, adopted the term and began using it in seasonal summaries. However, when he began issuing public statements during the 1972 Atlantic hurricane season with the term 'neutercane', newspapers protested the term was "sexist". Bob White, administrator of NOAA at that time, ordered Simpson to cease use of the term and it fell into disuse.^[1] Subtropical cyclones were initially named from the NATO phonetic alphabet list in the early to mid-1970s. In the intervening years of 1975-2001, subtropical storms were either named from the traditional list and considered tropical in real-time, 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, Hurricane Karen in 2001 was initially known as Subtropical Storm One as well as AL1301 (or 13L for short).^[2] In 2002, the NHC began giving numbers to subtropical depressions and names to subtropical storms from the same sequence as tropical cyclones. From 2002 onwards, Subtropical Depression 13L would 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.^[3] The third was Subtropical Storm Andrea from the 2007 season.

These storms can have maximum winds extending further from the centre than in a purely tropical cyclone and have no weather fronts linking directly to the center of circulation. 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 knots, or 39 mph are called subtropical depressions, while those at or above this velocity are referred to as subtropical storms.^[4]

Subtropical cyclones are also more likely than tropical cyclones to form outside of a region's designated hurricane season. Examples include Subtropical Storm Ana (which became Tropical Storm Ana) in mid-April of the 2003 hurricane season and Subtropical Storm Andrea in early May and Subtropical Storm Olga (which became Tropical Storm Olga) in mid-December of the 2007 Atlantic hurricane season.

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 40 °C of contrast in this roughly 20,000 foot/6000 meter layer of the lower atmosphere. The mode of the sea surface temperatures that subtropical cyclones form over is 23 degrees Celsius.^[5]

Subtropical cyclones form in a wide band of latitude, mainly between the 20th and 50th parallels. Most subtropical cyclones form when a deep cold-core extratropical cyclone drops down into the subtropics. The system becomes blocked by a high latitude ridge, and eventually sheds its frontal boundaries as its source of cool and dry air from the high latitudes diverts away from the system. Temperature differences between the 500 hPa pressure level, or 6000 meters/20000 ft above ground level, and the sea surface temperatures initially exceed the dry adiabatic lapse rate, which causes an initial round of thunderstorms at a distance east of the center. Due to the initial cold temperatures aloft, sea surface temperatures usually need to reach at least 20 °C for this initial round of thunderstorms. The initial thunderstorm activity moistens up the environment around the low, which destabilizes the atmosphere by reducing the lapse rate needed for convection. When the next shortwave or upper level jet streak moves nearby, convection reignites closer to the center and the system develops into a true subtropical cyclone. The average sea surface temperature that helps lead to subtropical cyclogenesis is 24 °C.^[5] If the thunderstorm activity becomes deep and persistent, allowing its initial low level warm core to deepen, tropical cyclogenesis is possible.

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.

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 as a "neutercane". As of 2006, the warm core variety were moved under the umbrella of the tropical cyclone definition, and removed from the subtropical cyclone definition. ^[6]

Main article: Kona low

Kona lows are deep cyclones that form during the cool season of the central Pacific ocean. Once termed as subtropical cyclones, a definition change in the term during the early 1970's makes categorization of the systems more complex, as many kona lows are extratropical cyclones. Kona is a Hawaiian term for leeward, which explains the change in wind direction for the Hawaiian Islands from easterly to southerly when this type of cyclone is present.^[7]

• Extratropical transition technique
• Hebert-Poteat technique

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Extratropical - Meso-scale - Polar - Polar low - Subtropical - Tropical

• NOAA FAQ on tropical cyclones.
• USA Today article on subtropical storms.