Wind shear in the context of "Tropical cyclogenesis"

Hurricane Harvey was a powerful and devastating tropical cyclone that made landfall in Texas and Louisiana in August 2017, causing catastrophic flooding and more than 100 deaths. It is tied with 2005's Hurricane Katrina as the costliest tropical cyclone on record, inflicting $125 billion (2017 USD) in damage, primarily from catastrophic rainfall-triggered flooding in Greater Houston and Southeast Texas; this made the storm the costliest natural disaster recorded in Texas at the time. It was the first major hurricane to make landfall in the United States since Wilma in 2005, ending a record 12-year span in which no hurricanes made landfall at the intensity of a major hurricane throughout the country. In a four-day period, many areas received more than 40 inches (1,000 mm) of rain as the system slowly meandered over eastern Texas and adjacent waters, causing unprecedented flooding. With peak accumulations of 60.58 in (1,539 mm) in Nederland, Texas, Harvey was the wettest tropical cyclone on record in the United States. The resulting floods inundated hundreds of thousands of homes, which displaced more than 30,000 people and prompted more than 17,000 rescues.

The eighth named storm, third hurricane, and first major hurricane of the extremely active 2017 Atlantic hurricane season, Harvey developed from a tropical wave to the east of the Lesser Antilles, reaching tropical storm status on August 17. The storm crossed through the Windward Islands on the following day, making landfall on the southern end of Barbados and a second landfall on Saint Vincent. Upon entering the Caribbean, Harvey began to weaken due to moderate wind shear, and degenerated into a tropical wave north of Colombia, late on August 19. The remnants were monitored for regeneration as it continued west-northwestward across the Caribbean and the Yucatán Peninsula, before redeveloping over the Bay of Campeche on August 23. Harvey then began to rapidly intensify on August 24, regaining tropical storm status and becoming a hurricane later that day.

A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its location and strength, a tropical cyclone is called a hurricane (/ˈhʌrɪkən, -keɪn/), typhoon (/taɪˈfuːn/), tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean. A typhoon is the same thing which occurs in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, comparable storms are referred to as "tropical cyclones". In modern times, on average around 80 to 90 named tropical cyclones form each year around the world, over half of which develop hurricane-force winds of 65 kn (120 km/h; 75 mph) or more.

Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately condenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are powered primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) in diameter. The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, cyclones rarely form within 5° of the equator. South Atlantic tropical cyclones are very rare due to consistently strong wind shear and a weak Intertropical Convergence Zone. In contrast, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea.

A typhoon is a tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere and which produces sustained hurricane-force winds of at least 119 km/h (74 mph). This region is referred to as the Northwestern Pacific Basin, accounting for almost one third of the world's tropical cyclones. For organizational purposes, the northern Pacific Ocean is divided into three regions: the eastern (North America to 140°W), central (140°W to 180°), and western (180° to 100°E). The Regional Specialized Meteorological Center (RSMC) for tropical cyclone forecasts is in Japan, with other tropical cyclone warning centres for the northwest Pacific in Hawaii (the Joint Typhoon Warning Center), the Philippines, and Hong Kong. Although the RSMC names each system, the main name list itself is coordinated among 18 countries that have territories threatened by typhoons each year.

Within most of the northwestern Pacific, there are no official typhoon seasons as tropical cyclones form throughout the year. Like any tropical cyclone, there are several main requirements for typhoon formation and development. It must be in sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower-to-middle levels of the troposphere, have enough Coriolis effect to develop a low pressure centre, a pre-existing low level focus or disturbance, and a low vertical wind shear. Although the majority of storms form between June and November, a few storms may occur between December and May (although tropical cyclone formation is very rare during that time). On average, the northwestern Pacific features the most numerous and intense tropical cyclones globally. Like other basins, they are steered by the subtropical ridge towards the west or northwest, with some systems recurving near and east of Japan. The Philippines receive the brunt of the landfalls, with China and Japan being less often impacted. However, some of the deadliest typhoons in history have struck China. Southern China has the longest record of typhoon impacts for the region, with a thousand-year sample via documents within their archives. Taiwan has received the wettest known typhoon on record for the northwest Pacific tropical cyclone basins. However, Vietnam recognises its typhoon season as lasting from the beginning of June through to the end of November, with an average of four to six typhoons hitting the country annually.

A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and thunder. Relatively weak thunderstorms are sometimes called thundershowers. Thunderstorms occur in cumulonimbus clouds. They are usually accompanied by strong winds and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms can produce little or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.

Thunderstorms result from the rapid upward movement of warm, moist air, sometimes along a front. However, some kind of cloud forcing, whether it is a front, shortwave trough, or another system is needed for the air to rapidly accelerate upward. As the warm, moist air moves upward, it cools, condenses, and forms a cumulonimbus cloud that can reach heights of over 20 kilometres (12 mi). As the rising air reaches its dew point temperature, water vapor condenses into water droplets or ice, reducing pressure locally within the thunderstorm cell. Any precipitation falls the long distance through the clouds towards the Earth's surface. As the droplets fall, they collide with other droplets and become larger. The falling droplets create a downdraft as it pulls cold air with it, and this cold air spreads out at the Earth's surface, occasionally causing strong winds that are commonly associated with thunderstorms.

Hurricane Marie is tied as the seventh-most intense Pacific hurricane on record, attaining a barometric pressure of 918 mbar (hPa; 27.11 inHg) in August 2014. The fourteenth named storm, ninth hurricane, and sixth major hurricane of the season, Marie began as a tropical wave that emerged off the west coast of Africa over the Atlantic Ocean on August 10. Some organization of shower and thunderstorm activity initially took place, but dry air soon impinged upon the system and imparted weakening. The wave tracked westward across the Atlantic and Caribbean for several days. On August 19, an area of low pressure consolidated within the wave west of Central America. With favorable atmospheric conditions, convective activity and banding features increased around the system and by August 22, the system acquired enough organization to be classified as Tropical Depression Thirteen-E while situated about 370 mi (595 km) south-southeast of Acapulco, Mexico. Development was initially fast-paced, as the depression acquired tropical storm-force winds within six hours of formation and hurricane-force by August 23. However, due to some vertical wind shear its intensification rate stalled, and for a time it remained a Category 1 hurricane on the Saffir–Simpson hurricane wind scale.

On August 24, Marie developed an eye and rapidly intensified to a Category 5 hurricane with winds of 160 mph (260 km/h). At its peak, the hurricane's gale-force winds spanned an area 575 mi (925 km) across. Marie subsequently underwent an eyewall replacement cycle on August 25 which prompted steady weakening. Over the next several days, Marie progressively degraded to below hurricane strength as it moved into an increasingly hostile environment with cooler waters and a more stable atmosphere. On August 29, after having lost all signs of organized deep convection, Marie degenerated into a remnant low. The large system gradually wound down over the following several days, with winds subsiding below gale-force on August 30. The remnant cyclone eventually lost a well defined center and dissipated on September 2 about 1,200 mi (1,950 km) northeast of Hawaii.

Rapid intensification (RI) is any process wherein a tropical cyclone strengthens very dramatically in a short period of time. Tropical cyclone forecasting agencies utilize differing thresholds for designating rapid intensification events, though the most widely used definition stipulates an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (55 km/h; 35 mph) in a 24-hour period. However, periods of rapid intensification often last longer than a day. About 20–30% of all tropical cyclones undergo rapid intensification, including a majority of tropical cyclones with peak wind speeds exceeding 51 m/s (180 km/h; 110 mph).

Rapid intensification constitutes a major source of error for tropical cyclone forecasting, and its predictability is commonly cited as a key area for improvement. The specific physical mechanisms that underlie rapid intensification and the environmental conditions necessary to support rapid intensification are unclear due to the complex interactions between the environment surrounding tropical cyclones and internal processes within the storms. Rapid intensification events are typically associated with warm sea surface temperatures and the availability of moist and potentially unstable air. The effect of wind shear on tropical cyclones is highly variable and can both enable or prevent rapid intensification. Rapid intensification events are also linked to the appearance of hot towers and bursts of strong convection within the core region of tropical cyclones, but it is not known whether such convective bursts are a cause or a byproduct of rapid intensification.