Subtropical cyclone

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Subtropical Storm Leslie in September 2018 Leslie 2018-09-29 1410Z.jpg
Subtropical Storm Leslie in September 2018

A subtropical cyclone is a weather system that has some characteristics of a tropical and an extratropical cyclone. [1]

Weather Short-term state of the atmosphere

Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the lowest level of the atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature and precipitation activity, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.

Tropical cyclone Is a rotating storm system

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

Extratropical cyclone type of cyclone

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.

Contents

As early as the 1950s, meteorologists were uncertain whether they should be characterized as tropical or extratropical cyclones. They were officially recognized by the National Hurricane Center in 1972. Beginning in 2002, subtropical cyclones received names from the official tropical cyclone lists in the North Atlantic, South-west Indian Ocean, and South Atlantic basins.

National Hurricane Center Division of the United States National Weather Service

The National Hurricane Center (NHC) is the division of the United States' National Weather Service responsible for tracking and predicting tropical weather systems between the Prime Meridian and the 140th meridian west poleward to the 30th parallel north in the northeast Pacific Ocean and the 31st parallel north in the northern Atlantic Ocean. The agency, which is co-located with the Miami branch of the National Weather Service, is situated on the campus of Florida International University in University Park, Florida.

South-West Indian Ocean tropical cyclone type of tropical cyclone located in South West Indian Ocean and measured by Météo-France La Reunion scale

In the south-west Indian Ocean, tropical cyclones form south of the equator and west of 90° E to the coast of Africa.

South Atlantic tropical cyclone unusual weather event originating in the South Atlantic Ocean

South Atlantic tropical cyclones are unusual weather events that occur in the Southern Hemisphere. Strong wind shear, which disrupts the formation of cyclones, as well as a lack of weather disturbances favorable for development in the South Atlantic Ocean make any strong tropical system extremely rare, and Hurricane Catarina in 2004 is the only recorded South Atlantic hurricane in history. South Atlantic storms have developed year-round, with activity peaking during the months from November through May in this basin. Since 2011, the Brazilian Navy Hydrographic Center has assigned names to tropical and subtropical systems in the western side of the basin, near Brazil, when they have sustained wind speeds of at least 65 km/h (40 mph), the generally accepted minimum sustained wind speed for a disturbance to be designated as a tropical storm in the North Atlantic basin. Below is a list of notable South Atlantic tropical and subtropical cyclones.

There are two definitions currently used for subtropical cyclones. Across the north Atlantic and southwest Indian Ocean, they require central convection fairly near the center and a warming core in the mid-levels of the troposphere. Across the eastern half of the northern Pacific, they require a mid-tropospheric cyclone to be cut off from the main belt of the westerlies and only a weak surface circulation. Subtropical cyclones 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. [2]

Troposphere The lowest layer of the atmosphere

The troposphere is the lowest layer of Earth's atmosphere, and is also where nearly all weather conditions take place. It contains approximately 75% of the atmosphere's mass and 99% of the total mass of water vapor and aerosols. The average height of the troposphere is 18 km in the tropics, 17 km in the middle latitudes, and 6 km in the polar regions in winter. The total average height of the troposphere is 13 km.

Westerlies

The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and trend towards the poles and steer extratropical cyclones in this general manner. Tropical cyclones which cross the subtropical ridge axis into the westerlies recurve due to the increased westerly flow. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.

The maximum sustained wind associated with a tropical cyclone is a common indicator of the intensity of the storm. Within a mature tropical cyclone, it is found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are determined via their sampling and averaging the sampled results over a period of time. Wind measuring has been standardized globally to reflect the winds at 10 metres (33 ft) above the Earth's surface, and the maximum sustained wind represents the highest average wind over either a one-minute (US) or ten-minute time span, anywhere within the tropical cyclone. Surface winds are highly variable due to friction between the atmosphere and the Earth's surface, as well as near hills and mountains over land.

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 3°C (5°F), lying around 23 °C (73 °F). This also means that subtropical cyclones are more likely to form outside the traditional bounds of the North Atlantic hurricane season. Subtropical cyclones are also observed to form in the South Atlantic; South Atlantic subtropical cyclones are observed in all months. [3]

Celsius Scale and unit of measurement for temperature

The Celsius scale, also known as the centigrade scale, is a temperature scale used by the International System of Units (SI). As an SI derived unit, it is used by all countries except the United States, the Bahamas, Belize, the Cayman Islands and Liberia. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty. Before being renamed to honor Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps.

Fahrenheit unit of temperature

The Fahrenheit scale is a temperature scale based on one proposed in 1724 by Dutch–German–Polish physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit as the unit. Several accounts of how he originally defined his scale exist. The lower defining point, 0 °F, was established as the freezing temperature of a solution of brine made from equal parts of ice, water and salt. Further limits were established as the melting point of ice (32 °F) and his best estimate of the average human body temperature. The scale is now usually defined by two fixed points: the temperature at which water freezes into ice is defined as 32 °F, and the boiling point of water is defined to be 212 °F, a 180 °F separation, as defined at sea level and standard atmospheric pressure.

Atlantic hurricane season tropical cyclone season

The Atlantic hurricane season is the period in a year when hurricanes usually form in the Atlantic Ocean. Tropical cyclones in the North Atlantic are called hurricanes, tropical storms, or tropical depressions. In addition, there have been several storms over the years that have not been fully tropical and are categorized as subtropical depressions and subtropical storms. Even though subtropical storms and subtropical depressions are not technically as strong as tropical cyclones, the damages can still be devastating.

History of term

Throughout the 1950s and 1960s, the term semi-tropical and quasi-tropical were used for what would become known as subtropical cyclones. [4] 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 hybrid cyclones formed in the Atlantic Basin. In 1972, the National Hurricane Center (NHC) finally designated these storms as subtropical cyclones in real-time, [5] and updated the hurricane database to include subtropical cyclones from 1968 through 1971.

Horse latitudes A lattitude of 25 deg to 35 degree of both hemisphere is called hourse lattitude.

Horse latitudes, subtropical ridges or subtropical highs are the subtropical latitudes between 30 and 35 degrees both north and south where Earth's atmosphere is dominated by the subtropical high, an area of high pressure, which suppresses precipitation and cloud formation, and has variable winds mixed with calm winds.

The term "neutercane" began to be used for small subtropical cyclones which formed from mesoscale features, and the NHC began issuing public statements during the 1972 Atlantic hurricane season employing that classification. This name was not noted as controversial in contemporary news reports, but it was dropped less than a year later. Recent articles, published after the year 2000, have suggested that the name "neutercane" was considered sexist in the 1970s, but there do not appear to be any published reports from that period making this claim. [6]

1972 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 1972 Atlantic hurricane season was a significantly below average season, having only four fully tropical named storms – the fewest since 1930. It officially began on June 1, 1972, and lasted until November 30, 1972. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. The first storm, Subtropical Storm Alpha, developed on May 23 off the Southeast United States and struck Florida, causing minor damage and two fatalities. Although several other tropical depressions developed, only Tropical Depression Five is known to have affected land.

Naming

Subtropical Storm Gustav in 2002, the first system to be given a name as a subtropical cyclone Gustav 09 sep 2002 1805Z.jpg
Subtropical Storm Gustav in 2002, the first system to be given a name as a subtropical cyclone

In the North Atlantic basin, subtropical cyclones were initially named from the NATO phonetic alphabet list in the early to mid-1970s. [5] 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). [7] In 2002, the NHC began giving numbers to subtropical depressions and names to subtropical storms from the same sequence as tropical cyclones. From 2002 onward, Subtropical Depression 13L would be known as Subtropical Depression Thirteen instead. Hurricane Gustav of 2002 was the first Subtropical Storm to receive a name but became tropical shortly after naming. 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. [8]

In the southern Indian Ocean, subtropical cyclones are also named once winds reach tropical storm, or gale, force. [9]

Since 2011, subtropical storms in the western South Atlantic Ocean are named by the Brazilian Navy Hydrographic Center. [10]

Formation

A water vapour loop showing the formation of Subtropical Storm Andrea Maystormvapor.gif
A water vapour loop showing the formation of Subtropical Storm Andrea

Subtropical cyclones form in a wide band of latitude, mainly south of the 50th parallel in the northern hemisphere. [11] Due to the increased frequency of cyclones which cut off from the main belt of the westerlies during the summer and fall, subtropical cyclones are significantly more frequent across the North Atlantic than the northwestern Pacific Ocean. [12] In the eastern half of the north Pacific Ocean and north Indian Ocean, the older subtropical cyclone definition term is still used, which requires a weak circulation forming underneath a mid to upper-tropospheric low which has cut off from the main belt of the westerlies during the cold season (winter). In the case of the north Indian Ocean, the formation of this type of vortex leads to the onset of monsoon rains during the wet season. [13] In the southern hemisphere, subtropical cyclones are regularly observed across southern portions of the Mozambique Channel. [9]

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 5,900 meters (19,400 ft) above ground level, and the sea surface temperatures initially exceed the dry adiabatic lapse rate, which causes an initial round of thunderstorms to form 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 (68 °F) 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 (wind maximum within the jet stream) 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 (75 °F). [1] [14] If the thunderstorm activity becomes deep and persistent, allowing its initial low level warm core to deepen, tropical cyclogenesis is possible. [11] The locus of formation for North Atlantic subtropical cyclones is out in the open ocean; the island of Bermuda is regularly impacted by these systems. [15]

The South Atlantic environment for formation of subtropical cyclones has both stronger vertical wind shear and lower sea surface temperatures, yet subtropical cyclogenesis is regularly observed in the open ocean in the South Atlantic. A second mechanism for formation has been diagnosed for South Atlantic subtropical cyclones: lee cyclogenesis in the region of the Brazil Current. [3]

Transition from extratropical

By gaining tropical characteristics, an extratropical low may transit into a subtropical depression or storm. A subtropical depression/storm may further gain tropical characteristics to become a pure tropical depression or storm, which may eventually develop into a hurricane, and there are at least ten cases of tropical cyclones transforming into a subtropical cyclone (Tropical Storm Gilda in 1973, Subtropical Storm Four in 1974, Tropical Storm Jose in 1981, Hurricane Klaus in 1984, Tropical Storm Allison in 2001, Tropical Storm Lee in 2011, Hurricane Humberto in 2013, Tropical Storm Ian in 2016, Typhoon Jelawat in 2018, and Tropical Storm Gaemi in 2018). There have also been two recorded cases of a storm transitioning from tropical to extratropical to a subtropical cyclone; as seen with Hurricane Georges in 1980 and Hurricane Beryl in 2018. 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 20 °C, or 72 °F, of contrast in this roughly 5,900 meters (19,400 ft) layer of the lower atmosphere. The mode of the sea surface temperatures that subtropical cyclones form over is 23 °C (73 °F). [14] Transition from subtropical cyclones into tropical cyclones occurs only in very rare cases over the South Atlantic Ocean, such as Hurricane Catarina in 2004. [3]

Characteristics

A remnant low transited to a subtropical storm in July 2018 Beryl 2018-07-14 1440Z.jpg
A remnant low transited to a subtropical storm in July 2018

These storms can have maximum winds extending farther from the center than in a purely tropical cyclone and have no weather fronts linking directly to the center of circulation. In the Atlantic Basin, the United States NOAA classifies subtropical cyclones similarly 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 speed are referred to as subtropical storms. [16]

Subtropical cyclones with hurricane-force winds of 33  m/s, (119 km/h, 64  knots, or 74 mph) or greater are not officially recognized by the National Hurricane Center. Once a subtropical storm intensifies enough to have hurricane-force winds, it is then automatically assumed to have become a fully tropical hurricane. [17] Despite this, however, prior to the start of modern policies, two subtropical cyclones in the Atlantic hurricane database attained hurricane-force winds; a subtropical storm in 1968, and a subtropical storm in 1979. [18]

Subtropical cyclones are also more likely than tropical cyclones to form outside of a region's designated hurricane season. Recent North Atlantic examples of this include the following storms:

Diagrams which depict a cyclone's phase depict subtropical cyclones with a shallow warm core and as asymmetric systems, similar to tropical cyclones which have begun the transition to an extratropical cyclone. [20] [2] [21]

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 160 kilometers (99 mi) or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection. [22]

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 than 50 kilometers (31 mi). The entire circulation may initially have a diameter of less than 160 kilometers (99 mi). These generally short-lived systems may be either cold core or warm core, and in 1972 this type of subtropical cyclone was referred to as a "neutercane". [23]

Kona storm

A subtropical storm in December 2010, originally a Kona storm Omeka 2010-12-19 0057Z.jpg
A subtropical storm in December 2010, originally a Kona storm

Kona storms (or Kona lows) are deep cyclones that form during the cool season of the central Pacific Ocean. A definition change in the term during the early 1970s makes categorization of the systems more complex, as many kona lows are extratropical cyclones, complete with their own weather fronts. Those across the northeast Pacific Ocean consider them subtropical cyclones as long as a weak surface circulation is present. [13] 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. [24]

Australian East Coast Lows

An Australian East Coast Low in June 2013 ECL25 06 13.jpg
An Australian East Coast Low in June 2013

Australian east coast lows (known locally as east coast lows and sometimes as east coast cyclones [25] ) are extratropical cyclones, [26] the most intense of these systems have many of the characteristics of subtropical cyclones. [27] They develop between 25˚south and 40˚south and within 5˚ of the Australian coastline, [25] typically during the winter months. [28] [29] Each year there are about ten "significant impact" maritime lows. [30] Explosive cyclogenesis is seen on average just once per year, but these storms cause significant wind and flood damage when they occur. [28] Australian east coast cyclones vary in size from mesoscale (approximately 10 km to 100 km) to synoptic scale (approximately 100 km to 1,000 km). [31] [32] These storms which mostly affect the south east coast should not be confused with Australian region tropical cyclones which typically affect the northern half of the continent.

See also

Related Research Articles

Cyclone large scale air mass that rotates around a strong center of low pressure

In meteorology, a cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure. Cyclones are characterized by inward spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars, Jupiter, and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream.

A low-pressure area, low, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies. A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.

2000 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 2000 Atlantic hurricane season was the first Atlantic hurricane season without a tropical cyclone in the month of July since 1993. The hurricane season officially began on June 1, and ended on November 30. It was slightly above average due to a La Niña weather pattern although most of the storms were weak. The first cyclone, Tropical Depression One, developed in the southern Gulf of Mexico on June 7 and dissipated after an uneventful duration. However, it would be almost two months before the first named storm, Alberto, formed near Cape Verde; Alberto also dissipated with no effects on land. Several other tropical cyclones—Tropical Depression Two, Tropical Depression Four, Chris, Ernesto, Nadine, and an unnamed subtropical storm—did not impact land. Five additional storms—Tropical Depression Nine, Florence, Isaac, Joyce, and Leslie—minimally affected land areas.

1975 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 1975 Atlantic hurricane season featured the first tropical storm to be upgraded to a hurricane based solely on satellite imagery – Hurricane Doris. The season officially began on June 1 and lasted until November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. The season was near average, with nine tropical storms forming, of which six became hurricanes. Three of those six became major hurricanes, which are Category 3 or higher on the Saffir–Simpson scale. The first system, Tropical Depression One, developed on June 24. Tropical Storm Amy in July caused minor beach erosion and coastal flooding from North Carolina to New Jersey, and killed one person when a ship capsized offshore North Carolina. Hurricane Blanche brought strong winds to portions of Atlantic Canada, leaving about $6.2 million (1975 USD) in damage. Hurricane Caroline brought high tides and flooding to northeastern Mexico and Texas, with two drownings in the latter.

1976 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 1976 Atlantic hurricane season featured only one fully tropical storm throughout both the Caribbean Sea and the Gulf of Mexico, a rare occurrence. The season officially began on June 1 and lasted until November 30. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. However, the first system, a subtropical storm, developed in the Gulf of Mexico on May 21, several days before the official start of the season. The system spawned nine tornadoes in Florida, resulting in about $628,000 (1976 USD) in damage, though impact was minor otherwise. The season was near average, with ten tropical storm forming, of which six became hurricanes. Two of those six became major hurricanes, which are Category 3 or higher on the Saffir–Simpson scale.

Pacific hurricane mature tropical cyclone that develops within the eastern and central Pacific Ocean

A Pacific hurricane is a mature tropical cyclone that develops within the eastern and central Pacific Ocean to the east of 180°W, north of the equator. For tropical cyclone warning purposes, the northern Pacific is divided into three regions: the eastern, central, and western, while the southern Pacific is divided into 2 sections, the Australian region and the southern Pacific basin between 160°E and 120°W. Identical phenomena in the western north Pacific are called typhoons. This separation between the two basins has a practical convenience, however, as tropical cyclones rarely form in the central north Pacific due to high vertical wind shear, and few cross the dateline.

Hurricane Charley (1986) Category 1 Atlantic hurricane in 1986

Hurricane Charley was the first hurricane to threaten the East Coast of the United States since Hurricane Gloria the previous year. The third tropical storm and second hurricane of the season, Charley formed as a subtropical low on August 13 along the Florida panhandle. After moving off the coast of South Carolina, the system transitioned into a tropical cyclone and intensified into a tropical storm on August 15. Charley later attained hurricane status before moving across eastern North Carolina. It gradually weakened over the north Atlantic Ocean before transitioning into an extratropical cyclone on August 20. Charley's remnants remained identifiable for over a week until after crossing the British Isles and dissipating on August 30.

Tropical Storm Gilda (1973) Atlantic tropical storm in 1973

Tropical Storm Gilda in 1973 was the first documented tropical cyclone on record to transition into a subtropical cyclone. It formed on October 16 in the western Caribbean Sea from a tropical wave, and strengthened to reach peak winds of 70 mph (110 km/h) before striking Cuba. It later moved slowly through the Bahamas before weakening to tropical depression status. On October 24, with the assistance of a cold front off the coast of the eastern United States, Gilda transformed into a subtropical storm, becoming very large and strong. The storm later accelerated northeastward and became extratropical, ultimately dissipating near Greenland.

Tropical Storm Nicholas Atlantic tropical storm in 2003

Tropical Storm Nicholas was a long-lived tropical storm in October and November of the 2003 Atlantic hurricane season. Forming from a tropical wave on October 13 in the central tropical Atlantic Ocean, Nicholas slowly developed due to moderate levels of wind shear throughout its lifetime. Deep convection slowly organized, and Nicholas attained a peak intensity of 70 mph (110 km/h) on October 17. After moving west-northwestward for much of its lifetime, it turned northward and weakened due to increasing shear. The storm again turned to the west and briefly restrengthened, but after turning again to the north Nicholas transitioned to an extratropical cyclone on October 24. As an extratropical storm, Nicholas executed a large loop to the west, and after moving erratically for a week and organizing into a tropical low, it was absorbed by a non-tropical low. The low continued westward, crossed Florida, and ultimately dissipated over the Gulf Coast of the United States on November 5.

Tropical cyclogenesis

Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.

2005 Azores subtropical storm Atlantic subtropical storm in 2005

The 2005 Azores subtropical storm was the 19th nameable storm and only subtropical storm of the record-breaking 2005 Atlantic hurricane season. It was not officially named by the US National Hurricane Center as it was operationally classified as a non-tropical low. The storm developed in the eastern Atlantic Ocean out of a low-pressure area that gained subtropical characteristics on October 4. The storm was short-lived, crossing over the Azores later on October 4 before becoming extratropical again on October 5. Neither damages nor fatalities were reported during that time. After being absorbed into a cold front, the system went on to become Hurricane Vince, which affected the Iberian Peninsula.

Cold-core low cyclone aloft which has an associated cold pool of air residing at high altitude within the Earths troposphere

A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere. It is a low pressure system that strengthens with height in accordance with the thermal wind relationship. If a weak surface circulation forms in response to such a feature at subtropical latitudes of the eastern north Pacific or north Indian oceans, it is called a subtropical cyclone. Cloud cover and rainfall mainly occurs with these systems during the day. Severe weather, such as tornadoes, can occur near the center of cold-core lows. Cold lows can help spawn cyclones with significant weather impacts, such as polar lows, and Kármán vortices. Cold lows can lead directly to the development of tropical cyclones, owing to their associated cold pool of air aloft or by acting as additional outflow channels to aid in further development.

2013 Atlantic hurricane season hurricane season in the Atlantic Ocean

The 2013 Atlantic hurricane season was a well below average Atlantic hurricane season and the first since 1994 with no major hurricanes. It was also the first season since 1968 with no storms of at least Category 2 intensity on the Saffir–Simpson hurricane wind scale. The first tropical cyclone of this hurricane season, Andrea, developed on June 5, while the final cyclone, an unnamed subtropical storm, dissipated on December 7. Throughout the year, only two storms—Humberto and Ingrid—reached hurricane intensity; this was the lowest seasonal total since 1982.

Hurricane Epsilon Category 1 Atlantic hurricane in 2005

Hurricane Epsilon was the final of fifteen hurricanes within the record-breaking 2005 Atlantic hurricane season. Originating from a cold front beneath an upper-level low, Epsilon formed on November 29 about 915 mi (1470 km) east of Bermuda. Initially, the National Hurricane Center (NHC) forecast the storm to transition into an extratropical cyclone within five days, due to conditions unfavorable for significant intensification. Epsilon continually defied forecasts, at first due to an unexpected loop to the southwest, and later due to retaining its strength despite cold waters and strong wind shear.

Tropical Storm Omeka Pacific tropical storm in 2010

Tropical Storm Omeka was the latest forming Eastern Pacific named storm since reliable records began in the 1960s. The storm was part of the 2010 Pacific typhoon and hurricane season. On December 18, 2010, the Central Pacific Hurricane Center (CPHC) began monitoring a subtropical cyclone near the International Dateline for possible tropical cyclogenesis. Over the following two days, the system tracked southwestward, entering the Western Pacific basin. It then began to transition into a tropical cyclone. Shortly before crossing the dateline on December 20, the CPHC assessed the system to have become a tropical storm. The storm was assigned the name Omeka several hours later as it moved into the CPHC's area of responsibility – which is from 140°W to the International Dateline. Upon doing so, Omeka attained its peak intensity with winds of 60 mph (100 km/h). Later on December 20, wind shear in the region increased, causing the system to weaken. By December 21, the center of Omeka was devoid of convection and dissipated on the next day. Omeka brushed Lisianski Island with no damage.

Glossary of tropical cyclone terms

The following is a glossary of tropical cyclone terms.

1951 Pacific hurricane season hurricane season in the Pacific ocean

The 1951 Pacific hurricane season ran through the summer and fall of 1951. Nine tropical systems were observed this season.

2018 Atlantic hurricane season Hurricane season in the Atlantic ocean

The 2018 Atlantic hurricane season was the third in a consecutive series of above-average and damaging Atlantic hurricane seasons, featuring 15 named storms, 8 hurricanes, and 2 major hurricanes, which caused a total of over $50.205 billion in damages. The season officially began on June 1, 2018, and ended on November 30, 2018. These dates historically describe the period each year when most tropical cyclones form in the Atlantic basin and are adopted by convention. The formation of Tropical Storm Alberto on May 25, marked the fourth consecutive year in which a storm developed before the official start of the season. The next storm, Beryl, became the first hurricane to form in the eastern Atlantic during the month of July since Bertha in 2008. Chris, upgraded to a hurricane on July 10, became the earliest second hurricane in a season since 2005. No hurricanes formed in the North Atlantic during the month of August, marking the first season since 2013, and the eighth season on record, to do so. On September 5, Florence became the first major hurricane of the season. On September 12, Joyce formed, making 2018 the first season since 2008 to feature four named storms active simultaneously. On October 9, Michael became the second major hurricane of the season, and a day later, it became the first Category 5 hurricane to make landfall in the continental United States since Hurricane Andrew in 1992. With the formation of Oscar on October 26, the season is the first on record to see seven storms that were subtropical at some point in their lifetimes.

References

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