The central dense overcast, or CDO, of a tropical cyclone or strong subtropical cyclone is the large central area of thunderstorms surrounding its circulation center, caused by the formation of its eyewall. It can be round, angular, oval, or irregular in shape. This feature shows up in tropical cyclones of tropical storm or hurricane strength. How far the center is embedded within the CDO, and the temperature difference between the cloud tops within the CDO and the cyclone's eye, can help determine a tropical cyclone's intensity. Locating the center within the CDO can be a problem for strong tropical storms and with systems of minimal hurricane strength as its location can be obscured by the CDO's high cloud canopy. This center location problem can be resolved through the use of microwave satellite imagery.
After a cyclone reaches hurricane intensity, an eye appears at the center of the CDO, defining its center of low pressure and its cyclonic wind field. Tropical cyclones with changing intensity have more lightning within their CDO than steady state storms. Tracking cloud features within the CDO, using frequently updated satellite imagery, can also be used to determine its intensity. The highest maximum sustained winds within a tropical cyclone, as well as its heaviest rainfall, are usually located under the coldest cloud tops in the CDO.
It is a large region of thunderstorms surrounding the center of stronger tropical and subtropical cyclones which shows up brightly (with cold cloud tops) on satellite imagery. The CDO forms due to the development of an eyewall within a tropical cyclone. Its shape can be round, oval, angular, or irregular. Its development can be preceded by a narrow, dense, C-shaped convective band. Early in its development, the CDO is often angular or oval in shape, which rounds out, increases in size, and appears more smooth as a tropical cyclone intensifies. Rounder CDO shapes occur in environments with low levels of vertical wind shear.
The strongest winds within tropical cyclones tend to be located under the deepest convection within the CDO, which is seen on satellite imagery as the coldest cloud tops. The radius of maximum wind is usually collocated with the coldest cloud tops within the CDO, which is also the area where a tropical cyclone's rainfall reaches its maximum intensity. For mature tropical cyclones that are steady state, the CDO contains nearly no lightning activity, though lightning is more common within weaker tropical cyclones and for systems fluctuating in intensity.
The eye is a region of mostly calm weather at the center of the CDO of strong tropical cyclones. The eye of a storm is a roughly circular area, typically 30–65 kilometres (19–40 mi) in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms surrounding its center of circulation. The cyclone's lowest barometric pressure occurs in the eye, and can be as much as 15% lower than the atmospheric pressure outside the storm. In weaker tropical cyclones, the eye is less well-defined, and can be covered by high cloudiness caused by cirrus cloud outflow from the surrounding central dense overcast.
Within the Dvorak satellite strength estimate for tropical cyclones, there are several visual patterns that a cyclone may take on which define the upper and lower bounds on its intensity. The central dense overcast (CDO) pattern is one of those patterns. The central dense overcast utilizes the size of the CDO. The CDO pattern intensities start at T2.5, equivalent to minimal tropical storm intensity, 40 mph (64 km/h). The shape of the central dense overcast is also considered. The farther the center is tucked into the CDO, the stronger it is deemed. Banding features can be utilized to objectively determine the tropical cyclone's center, using a ten degree logarithmic spiral. Using the 85–92 GHz channels of polar-orbiting microwave satellite imagery can definitively locate the center within the CDO.
Tropical cyclones with maximum sustained winds between 65 mph (105 km/h) and 100 mph (160 km/h) can have their center of circulations obscured by cloudiness within visible and infrared satellite imagery, which makes diagnosis of their intensity a challenge. Winds within tropical cyclones can also be estimated by tracking features within the CDO using rapid scan geostationary satellite imagery, whose pictures are taken minutes apart rather than every half-hour.
An annular tropical cyclone is a tropical cyclone that features a normal to large, symmetric eye surrounded by a thick and uniform ring of intense convection, often having a relative lack of discrete rainbands, and bearing a symmetric appearance in general. As a result, the appearance of an annular tropical cyclone can be referred to as akin to a tire or doughnut. Annular characteristics can be attained as tropical cyclones intensify; however, outside the processes that drive the transition from asymmetric systems to annular systems and the abnormal resistance to negative environmental factors found in storms with annular features, annular tropical cyclones behave similarly to asymmetric storms. Most research related to annular tropical cyclones is limited to satellite imagery and aircraft reconnaissance as the conditions thought to give rise to annular characteristics normally occur over water well removed from landmasses where surface observations are possible.Cyclone Alessia
Tropical Cyclone Alessia was the first tropical cyclone to affect the Northern Territory of Australia in November since Cyclone Joan in 1975. The storm was first identified as a tropical low on 20 November 2013 well to the northwest of Australia. Tracking generally west to west-southwest, the small system steadily organized into a tropical cyclone by 22 November. Maintaining a small central dense overcast, Alessia brushed the Kimberley region before making landfall in the Top End region with winds of 65 km/h (40 mph) on 23 and 24 November respectively. Some weakening took place as the system moved over land; however, reorganization occurred as it neared the Gulf of Carpentaria. After moving over water on 26 November, it redeveloped gale-force winds. Alessia reached its peak intensity on 27 November with winds of 85 km/h (50 mph) and a barometric pressure of 991 mbar (hPa; 29.26 inHg) and subsequently made its final landfall near Wollogorang. Weakening ensued once more as the storm traveled over land; though, Alessia's remnants looped eastward back over water before doubling back to the west. The system was last noted moving inland again over the Northern Territory on 1 December.
Throughout Alessia's existence, it caused only minimal damage. Several areas experienced gale-force winds, with gusts measured up to 109 km/h (68 mph) on Centre Island. Moderate to heavy rains accompanied the system as well, with a storm maxima of 290.4 mm (11.43 in) also occurring on Centre Island.Dvorak technique
The Dvorak technique (developed between 1969 and 1984 by Vernon Dvorak) is a widely used system to estimate tropical cyclone intensity (which includes tropical depression, tropical storm, and hurricane/typhoon/intense tropical cyclone intensities) based solely on visible and infrared satellite images. Within the Dvorak satellite strength estimate for tropical cyclones, there are several visual patterns that a cyclone may take on which define the upper and lower bounds on its intensity. The primary patterns used are curved band pattern (T1.0-T4.5), shear pattern (T1.5–T3.5), central dense overcast (CDO) pattern (T2.5–T5.0), central cold cover (CCC) pattern, banding eye pattern (T4.0–T4.5), and eye pattern (T4.5–T8.0).
Both the central dense overcast and embedded eye pattern use the size of the CDO. The CDO pattern intensities start at T2.5, equivalent to minimal tropical storm intensity (40 mph, 65 km/h). The shape of the central dense overcast is also considered. The eye pattern utilizes the coldness of the cloud tops within the surrounding mass of thunderstorms and contrasts it with the temperature within the eye itself. The larger the temperature difference is, the stronger the tropical cyclone. Once a pattern is identified, the storm features (such as length and curvature of banding features) are further analyzed to arrive at a particular T-number. The CCC pattern indicates little development is occurring, despite the cold cloud tops associated with the quickly evolving feature.
Several agencies issue Dvorak intensity numbers for tropical cyclones and their precursors, including the National Hurricane Center's Tropical Analysis and Forecast Branch (TAFB), the NOAA/NESDIS Satellite Analysis Branch (SAB), and the Joint Typhoon Warning Center at the Naval Meteorology and Oceanography Command in Pearl Harbor, Hawaii.Eye (cyclone)
The eye is a region of mostly calm weather at the center of strong tropical cyclones. The eye of a storm is a roughly circular area, typically 30–65 kilometres (19–40 mi) in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather and highest winds occur. The cyclone's lowest barometric pressure occurs in the eye and can be as much as 15 percent lower than the pressure outside the storm.In strong tropical cyclones, the eye is characterized by light winds and clear skies, surrounded on all sides by a towering, symmetric eyewall. In weaker tropical cyclones, the eye is less well defined and can be covered by the central dense overcast, an area of high, thick clouds that show up brightly on satellite imagery. Weaker or disorganized storms may also feature an eyewall that does not completely encircle the eye or have an eye that features heavy rain. In all storms, however, the eye is the location of the storm's minimum barometric pressure—where the atmospheric pressure at sea level is the lowest.Hurricane Andres (2009)
Hurricane Andres was the first named storm and hurricane of the 2009 Pacific hurricane season. Forming on June 21, Andres gradually intensified as it tracked along the Mexican coastline. Deep convection developed around the center of circulation and by June 23, the storm attained hurricane-status, peaking with winds of 80 mph (130 km/h). Upon attaining this intensity, the storm featured a developing eyewall within a central dense overcast. Within 36 hours, the storm rapidly degenerated, having most of the convection being displaced by high wind shear, becoming a non-tropical trough during the afternoon of June 24.
Prior to becoming a tropical depression, Andres produced heavy rainfall in Oaxaca and Honduras, resulting in two deaths. Rough seas off the coast of Guerrero resulted in one fatality. Inland, flooding caused by heavy rains killed two additional people. An additional 20 people were injured. Several dozen structures were damaged and a few were destroyed. Total losses from the hurricane reached MXN 3 million ($231,000 USD) in Colima. Following the storm, roughly 350 people were left homeless.Hurricane Elida (2002)
Hurricane Elida was the first hurricane of the 2002 Pacific hurricane season to reach Category 5 strength on the Saffir-Simpson Hurricane Scale. Forming on July 23 from a tropical wave, the storm rapidly intensified from a tropical depression into a Category 5 hurricane in two days, and lasted for only six hours at that intensity before weakening. It was one of only sixteen known hurricanes in the East Pacific east of the International Date Line to have reached such an intensity. Although heavy waves were able to reach the Mexican coastline, no damages or casualties were reported in relation to the hurricane.
The hurricane moved westward due to a high pressure ridge while undergoing two eyewall replacement cycles: the first was around peak intensity and was completed when the hurricane moved over cooler waters, and the second was a brief cycle shortly after the hurricane began to weaken. The last advisory was issued while the hurricane was west of Mexico, but it was not until the remnants were west of Los Angeles, California that they finally dissipated. Elida's rapid intensification and unsteady weakening after reaching its peak intensity caused large errors in the intensity forecasting of the hurricane. Although the intensity forecasts were off, the track forecasts were better than usual compared to the ten-year period prior to that year.Hurricane Linda (2015)
Hurricane Linda was a strong tropical cyclone in September 2015 that resulted in heavy rains across portions of Mexico and the Southwestern United States. The seventeenth named storm and eleventh hurricane of the season, Linda developed southwest of Mexico from a low pressure area on September 5. Under warm sea surface temperatures and low to moderate wind shear, the system intensified into Tropical Storm Linda by September 6 and a hurricane by the next day. A well-defined eye soon formed within the storm's central dense overcast and Linda reached its peak intensity as a 125 mph (205 km/h) Category 3 major hurricane on the Saffir–Simpson hurricane wind scale on September 8. Thereafter, the storm moved into a stable environment and an area of lower sea surface temperatures, causing rapid weakening. Convective activity dissipated and Linda degenerated into a remnant low on September 10. The lingering system persisted southwest of Baja California, ultimately opening up into a trough on September 14.
In Mexico, the storm brought rainfall to nine states, causing flooding, especially in Oaxaca, Sinaloa, and Zacatecas. In Oaxaca, mudslides resulted in the closure of multiple highways and damage to over a dozen homes. Flooding in Sinola affected approximately 1,000 homes with hundreds damaged, prompting dozens of families to evacuate. Several small communities were temporarily isolated after flood waters covered bridges. Localized flooding in Zacatecas damaged crops and 25 dwellings; damage reached approximately 500,000 pesos (US$30,000). Although Linda did not directly impact land, moisture from the storm was pulled northeast into the Southwestern United States and enhanced the local monsoon. Los Angeles received 2.39 in (61 mm) of rain, contributing to the city's second wettest September on record. One fatality in the state occurred from a drowning at San Bernardino National Forest. Utah was impacted by major flash flooding incidents—with rainfall amounting to 1-in-100 year levels—which left 21 deaths in the state: 14 near Hildale and 7 in Zion National Park. Damage across the Southwest amounted to US$3.6 million.Hurricane Sandra (2015)
Hurricane Sandra was the latest-forming major hurricane in the northeastern Pacific basin, the strongest November Pacific hurricane on record, and the record eleventh major hurricane of the 2015 Pacific hurricane season. Originating from a tropical wave, Sandra was first classified as a tropical depression on November 23 well south of Mexico. Environmental conditions, including high sea surface temperatures and low wind shear, were highly conducive to intensification and the storm quickly organized. A small central dense overcast developed atop the storm and Sandra reached hurricane status early on November 25 after the consolidation of an eye. Sandra reached its peak intensity as a Category 4 hurricane on the Saffir–Simpson hurricane wind scale with winds of 150 mph (240 km/h) and a pressure of 934 mbar (hPa; 27.58 inHg) early on November 26. This made Sandra the strongest November hurricane on record in the Northeastern Pacific. Thereafter, increasing shear degraded the hurricane's structure and weakening ensued. Rapid weakening took place on November 27 and Sandra's circulation became devoid of convection as it diminished to a tropical storm that evening. The cyclone degenerated into a remnant low soon thereafter and ultimately dissipated just off the coast of Sinaloa, Mexico, on November 29.
As the precursor to Sandra traversed Central America, it produced unseasonably heavy rainfall that triggered flooding and landslides. Four people died in various incidents related to the system: three in El Salvador and one in Honduras. Initially expecting a landfalling storm, officials in Northwestern Mexico prepared equipment for power outages, closed schools, and evacuated 180 residents. Sandra's effects largely consisted of light to moderate rainfall; some traffic accidents and landslides resulted from this, though the overall impacts were limited.Kalahari High
The Kalahari High is an anticyclone that forms in winter over the interior of southern Africa, replacing a summer trough. It is part of the subtropical ridge system and the reason the Kalahari is a desert. It is the descending limb of a Hadley cell.Mesoscale meteorology
Mesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers. Examples of mesoscale weather systems are sea breezes, squall lines, and mesoscale convective complexes.
Vertical velocity often equals or exceeds horizontal velocities in mesoscale meteorological systems due to nonhydrostatic processes such as buoyant acceleration of a rising thermal or acceleration through a narrow mountain pass.Meteorological history of Hurricane Dennis
The meteorological history of Hurricane Dennis spanned twenty-two days, beginning with its inception as a tropical wave over Africa on June 26, 2005, and terminating with its dissipation on July 18 over the Great Lakes of North America. The incipient wave that became Dennis emerged over the Atlantic Ocean on June 29 and moved briskly to the west. Dry air initially inhibited development, though once this abated the wave was able to consolidate into a tropical depression on July 4. The depression soon crossed Grenada before entering the Caribbean Sea whereupon increasingly favorable environmental factors, such as low wind shear and high sea surface temperatures, fueled intensification. Turning west-northwest, the system achieved tropical storm status on July 5 and hurricane status the following day.
Formation of a well-defined eye and central dense overcast signaled Dennis's intensification into a major hurricane. The powerful storm soon struck Granma Province, Cuba, as a Category 4 early on July 8; violent winds battered the province and caused extensive damage. Paralleling the western coast of Cuba, Dennis attained its peak winds of 150 mph (240 km/h) later that day before making a second landfall in the country, this time in Matanzas Province. Interaction with the mountains of Cuba caused significant weakening; however, once Dennis emerged over the Gulf of Mexico on July 9, it was able to quickly reorganize. The hurricane reached Category 4 strength for a third time on July 10 as it approached Florida, weakening somewhat before striking the state. Dramatic weakening ensued once the cyclone moved ashore. Dennis lingered as a tropical depression and remnant low for roughly a week, traversing the Mississippi River Valley and Ohio River Valley before finally dissipating over Ontario on July 18.Meteorological history of Hurricane Patricia
Hurricane Patricia was the most intense tropical cyclone ever recorded in the Western Hemisphere and the second-most intense worldwide in terms of barometric pressure. It also featured the highest one-minute maximum sustained winds ever recorded in a tropical cyclone. Originating from a sprawling disturbance near the Gulf of Tehuantepec in mid-October 2015, Patricia was first classified a tropical depression on October 20. Initial development was slow, with only modest strengthening within the first day of its classification. The system later became a tropical storm and was named Patricia, the twenty-fourth named storm of the annual hurricane season. Exceptionally favorable environmental conditions fueled explosive intensification on October 22. A well-defined eye developed within an intense central dense overcast and Patricia grew from a tropical storm to a Category 5 hurricane in just 24 hours—a near-record pace. The magnitude of intensification was poorly forecast and both forecast models and meteorologists suffered from record-high prediction errors.
On October 23, two Hurricane Hunter missions both revealed the storm to have acquired maximum sustained winds of 205 mph (335 km/h) and a pressure of 879 mbar (hPa; 25.96 inHg). Since the peak intensity was assessed to have occurred between the missions, the National Hurricane Center ultimately estimated Patricia to have acquired winds of 215 mph (345 km/h) and pressure of 872 mbar (hPa; 25.75 inHg). This ranked it just below Typhoon Tip of 1979 as the most intense tropical cyclone on record. Patricia's exceptional intensity prompted the retirement of its name in April 2016. Late on October 23, Patricia made landfall in a significantly weakened state near Cuixmala, Jalisco. Despite weakening greatly, it was the strongest landfalling Pacific hurricane with winds estimated at 150 mph (240 km/h). Interaction with the mountainous terrain of Mexico induced dramatic weakening, faster than the storm had intensified. Within 24 hours of moving ashore, Patricia degraded into a tropical depression and dissipated soon thereafter late on October 24.Rapid intensification
Rapid intensification is a meteorological condition that occurs when a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period.South Pacific High
The South Pacific High is a semi-permanent subtropical anticyclone located in the southeast Pacific Ocean. The area of high atmospheric pressure and the presence of the Humboldt Current in the underlying ocean make the west coast of Peru and northern Chile extremely arid. The Sechura and Atacama deserts, as the whole climate of Chile, are heavily influenced by this semi-permanent high-pressure area. This high-pressure system plays a major role in the El Niño–Southern Oscillation (ENSO), and it is also a major source of trade winds across the equatorial Pacific.Superstorm
A superstorm is a large, unusually-occurring, destructive storm without another distinct meteorological classification, such as hurricane or blizzard. As the term is of recent coinage and lacks a formal definition, there is some debate as to its usefulness.Tropical cyclone rainfall forecasting
Tropical cyclone rainfall forecasting involves using scientific models and other tools to predict the precipitation expected in tropical cyclones such as hurricanes and typhoons. Knowledge of tropical cyclone rainfall climatology is helpful in the determination of a tropical cyclone rainfall forecast. More rainfall falls in advance of the center of the cyclone than in its wake. The heaviest rainfall falls within its central dense overcast and eyewall. Slow moving tropical cyclones, like Hurricane Danny and Hurricane Wilma, can lead to the highest rainfall amounts due to prolonged heavy rains over a specific location. However, vertical wind shear leads to decreased rainfall amounts, as rainfall is favored downshear and slightly left of the center and the upshear side is left devoid of rainfall. The presence of hills or mountains near the coast, as is the case across much of Mexico, Haiti, the Dominican Republic, much of Central America, Madagascar, Réunion, China, and Japan act to magnify amounts on their windward side due to forced ascent causing heavy rainfall in the mountains. A strong system moving through the mid latitudes, such as a cold front, can lead to high amounts from tropical systems, occurring well in advance of its center. Movement of a tropical cyclone over cool water will also limit its rainfall potential. A combination of factors can lead to exceptionally high rainfall amounts, as was seen during Hurricane Mitch in Central America.Use of forecast models can help determine the magnitude and pattern of the rainfall expected. Climatology and persistence models, such as r-CLIPER, can create a baseline for tropical cyclone rainfall forecast skill. Simplified forecast models, such as the Kraft technique and the eight and sixteen-inch rules, can create quick and simple rainfall forecasts, but come with a variety of assumptions which may not be true, such as assuming average forward motion, average storm size, and a knowledge of the rainfall observing network the tropical cyclone is moving towards. The forecast method of TRaP assumes that the rainfall structure the tropical cyclone currently has changes little over the next 24 hours. The global forecast model which shows the most skill in forecasting tropical cyclone-related rainfall in the United States is the ECMWF IFS (Integrated Forecasting System) .Typhoon Nuri (2014)
Typhoon Nuri, known in the Philippines as Typhoon Paeng, was the third most intense tropical cyclone worldwide in 2014. Nuri developed into a tropical storm and received the name Paeng from the PAGASA on October 31, before it intensified into a typhoon on the next day. Under excellent conditions, especially the synoptic scale outflow, Nuri underwent rapid deepening and reached its peak intensity on November 2, forming a round eye in a symmetric Central dense overcast (CDO). Having maintained the impressive structure for over one day, the typhoon began to weaken on November 4, with a cloud-filled eye.Because of increasing vertical wind shear from the mid-latitude westerlies, Nuri lost the eye on November 5, and deep convection continued to diminish. The storm accelerated northeastward and completely became extratropical on November 6. However, on November 7, Nuri's circulation split, and the new center absorbed the storm.Typhoon Ofelia
Typhoon Ofelia, known as Typhoon Bising in the Philippines, was the first of two typhoons in 1990 to directly affect the Philippines within a week. Typhoon Ofelia originated from an area of disturbed weather embedded in the monsoon trough situated near the Caroline Islands. Slowly organizing, the disturbance tracked westward, and was designated a tropical depression on June 15. After an increase in convection, the depression was upgraded into a tropical storm on June 17. On June 19, Ofelia turned northwest and after development of a central dense overcast, Ofelia was upgraded into a typhoon late on June 20. After turning north, Ofelia obtained its maximum intensity following the development of an eye. The typhoon skirted past the northeastern tip of Luzon and near the east coast of Taiwan, commencing a rapid weakening trend. On the evening on June 23, Ofelia struck the southern portion of Zhejiang. The storm then began to track north, recurving towards the Korean Peninsula. The storm tracked through the province of Jiangsu, and at 00:00 UTC on June 24, transitioned into an extratropical cyclone, only to merge with a frontal zone on June 25.
Although the inner core avoided the Philippines, the storm's large size resulted in inundation across the northern Philippines. The province of La Union was the hardest hit by the typhoon, where 22 people were killed and 90 homes were crushed. Three children perished and six others sustained injuries in Pasig. Overall, 56 people were killed and over 85,000 individuals were forced to flee their homes. Taiwan bore a direct landfall from Ofelia, dropping up to 460 mm (18 in) of rain. Hualien City was the hardest hit by the typhoon, where five people were killed. In all, Ofelia was the worst to hit eastern Taiwan in 30 years. More than 200 houses were destroyed or damaged and roughly 8,500 ha (21,005 acres) of rice paddies and vegetables were flooded. Roads and highways were blocked by landslides and floods. Agricultural losses exceeded NT$2.55 billion (US$94.7 million). Seventeen people died and twenty-three were missing due to flooding and mudslides. Although during a weakening phase at the time, the typhoon drenched central China. In Wenzhou, 12 people were killed and monetary damage was estimated at about 205 million RMB (US$42.8 million). In the province of Zhejiang, 15 fatalities were reported and 21 others were injured. In the neighboring province of Fujian, 15 people perished and 9,044 houses were demolished. About 91,000 ha (224,865 acres) of farmland were inundated and damage was estimated at 338 million RMB (US$70.5 million). Nationwide, 40 people were killed by Ofelia.