Pacific hurricane

Cumulative average number of tropical cyclones in the north Pacific

A Pacific hurricane or tropical storm is a tropical cyclone that develops in the northeastern part of the Pacific Ocean. 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). A Pacific hurricane, then, is a tropical cyclone in the northern Pacific Ocean east of 180°. Identical phenomena in the western north Pacific are called typhoons. This separation between the two basins is convenient, however, as tropical cyclones rarely form in the central north Pacific and few cross the dateline.

History

Documentation of Pacific hurricanes date to the Spanish colonization of Mexico, when military and missions wrote about "tempestades". In 1730, such accounts indicated an understanding of the storms. After observing the rotating nature of tropical cyclones, meteorologist William Charles Redfield expanded his study to include storms in the eastern North Pacific Ocean in the middle of the 19th century. Between June and October of 1850, Redfield observed five tropical cyclones along "the southwestern coast of North America", along with one in each of the three subsequent years. In 1895, Cleveland Abbe reported the presence of many storms between 5° to 15°–N in the eastern Pacific, although many such storms dissipated before affecting the Mexican coast. Two years later, the German Hydrography Office Deutsche Seewarte documented 45 storms from 1832 to 1892 off the west coast of Mexico.

Despite the documentation of storms in the region, the official position from the United States Weather Bureau denied the existence of such storms. In 1910, the agency reported on global tropical cyclones, noting that "the occurrence of tropical storms is confined to the summer and autumn months of the respective hemispheres and to the western parts of the several oceans." In 1913, the Weather Bureau reinforced their position by excluding Pacific storms among five tropical cyclone basins; however, the agency acknowledged the existence of "certain cyclones that have been traced for a relatively short distance along a northwest course... west of Central America."

After California became a state and the discovery of gold there in 1848, shipping traffic began increasing steadily in the eastern Pacific. Such activity increased further after the Panama Canal opened in 1914, and the shipping lanes moved closer to the coast. By around 1920, Pacific hurricanes were officially recognized due to widespread ship observations, radio service, and a newly created weather network in western Mexico. Within 60 years, further studies of the region's tropical activity indicated that the eastern Pacific was the second most active basin in the world.

Eastern North Pacific

Hurricane season runs between May 15 and November 30 each year. These date encompass the vast majority of tropical cyclone activity in this region.

The Regional Specialized Meteorological Center for this basin is the United States' National Hurricane Center. Previous forecasters are the Eastern Pacific Hurricane Center and the Joint Hurricane Warning Center. The RSMC monitors the eastern Pacific and issues reports, watches and warnings about tropical weather systems and cyclones as defined by the World Meteorological Organization.

This area is, on average, the second-most active basin in the world. There are an average of 16 tropical storms annually, with 9 becoming hurricanes, and 4 becoming major hurricanes. Tropical cyclones in this region frequently affect mainland Mexico and the Revillagigedo Islands. Less often, a system will affect the Continental United States or Central America. Northbound hurricanes typically reduce to tropical storms or dissipate before reaching the United States: there is only one recorded case of a Pacific system reaching California as a hurricane in almost 200 years of observations—the 1858 San Diego Hurricane.

Most east Pacific hurricanes originate from a tropical wave that drifts westward across the intertropical convergence zone, and across northern parts of South America. Once it reaches the Pacific, a surface low begins to develop, however, with only little or no convection. After reaching the Pacific, it starts to move north-westward and eventually west. By that time, it develops convection and thunderstorm activity from the warm ocean temperatures but remains disorganized. Winter in the east Pacific is usually covered with strong shear, which limits development. Once it becomes organized, it becomes a tropical depression. Formation usually occurs from south of the Gulf of Tehuantepec to south of Baja California with a more westerly location earlier in the season. In the eastern Pacific, development is more centered than anywhere. Most tropical cyclones in the eastern Pacific reach their peak strength as a strong tropical storm due to strong shear. If wind shear is low, storms can undergo major intensification as a result of very warm oceans, becoming a major hurricane. Major hurricanes weaken once they reach unfavorable areas for a tropical cyclone formation, and most of them do an eyewall replacement and weaken. Their remnants sometimes reach Hawaii and cause showers there.

There are a few types of Pacific hurricane tracks: one is a westerly track, another moves north-westward along Baja California and another moves north. Sometimes storms can move north-east either across Central America or mainland Mexico and possibly enter the Caribbean Sea becoming a North Atlantic basin tropical cyclone, but these are rare.

Central Pacific

Hurricane season runs from June 1 to November 30, with a strong peak in August and September. However, tropical cyclones have formed outside those dates. Should a tropical cyclone enter the central north Pacific from the western north Pacific, where they occur year-round, or from the eastern north Pacific, where the season starts in May, it is not known if such a system will be considered out of season or not.

The Central Pacific Hurricane Center is the RSMC for this basin and monitors the storms that develop or move into the defined area of responsibility. A previous forecaster was the Joint Hurricane Warning Center.

Central Pacific hurricanes are rare and on average 3 or 4 storms form or move in this area per year. Most often, storms that occur in the area are weak and often decline in strength upon entry. The only land masses impacted by tropical cyclones in this region are Hawaii and Johnston Atoll. Due to the small size of the islands in relation to the Pacific Ocean, direct hits and landfalls are rare.

Steering factors

See also: Tropical cyclone track forecasting and Subtropical ridge

Hurricanes in the Eastern Pacific tend to move westward out to sea, harming no land. However, hurricanes can recurve to the north or northeast, hitting Central America or Mexico early and late in the hurricane season.

See also

• North Atlantic tropical cyclone
• Tropical cyclone
• Pacific typhoon