A Pacific hurricane or tropical storm is a tropical cyclone that develops in the eastern part of the Pacific Ocean in the Northern Hemisphere. 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°, or in the southern Pacific Ocean east of 160°E. 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.
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 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.
During the 1920s, a few documents in the Monthly Weather Review reported additional storms within 2,000 mi (3,200 km) off the Mexican coastline. The HURDAT database began keeping records in 1949. Between 1970 and 1975, advisories for systems in the eastern Pacific basins were initiated by the Eastern Pacific Hurricane Center (EPHC) as part of the National Weather Service (NWS) office in San Francisco, California. At that time, the advisories released were written in cooperation with the United States Navy Fleet Weather Center in Alameda and the Air Force Hurricane Liaison Officer at the McClellan Air Force Base. Following the move of the hurricane center to Redwood City in 1976, track files were created and altered by Arthur Pike and were later re-modified following the release of a study in 1980. The National Hurricane Center (NC) extended its authority to the EPHC in 1988, and subsequently began maintaining the tracks.
The presence of a semi-permanent high-pressure area known as the North Pacific High in the eastern Pacific is a dominant factor against formation of tropical cyclones in the winter, as the Pacific High results in wind shear that causes environmental conditions for tropical cyclone formation to be not conductive. Its effects in the central Pacific basin are usually related to keeping cyclones away from the Hawaiian Islands. Due to westward trade winds, hurricanes in the Pacific nearly never head eastward, although several storms have defied the odds and headed eastward. A second factor preventing tropical cyclones from forming during the winter is the occupation of a semi-permanent low-pressure area designated the Aleutian Low between January and April. Its presence over western Canada and the northwestern United States contributes to the area's occurrences of precipitation in that duration. In addition, its effects in the central Pacific near 160° W causes tropical waves that form in the area to drift northward into the Gulf of Alaska and dissipate. Its retreat in late-April allows the warmth of the Pacific High to meander in, bringing its powerful clockwise wind circulation with it. The Intertropical Convergence Zone departs southward in mid-May permitting the formation of the earliest tropical waves, coinciding with the start of the eastern Pacific hurricane season on May 15.
Eastern North Pacific 
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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 
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 
- Chris Landsea (2011-07-15). "Subject: A1) What is a hurricane, typhoon, or tropical cyclone?". Atlantic Oceanographic and Meteorological Laboratory. Retrieved 2012-07-02.
- Arnold Court (1980). Tropical Cyclone Effects on California. Northridge, California: California State University. pp. 2, 4, 6, 8, 34. Retrieved 2012-02-02.
- Tingley, F. G. (1922). "North Pacific Ocean" (PDF). Monthly Weather Review (American Meteorological Society) 50 (3): 99. doi:10.1175/1520-0493(1929)57<121:NPO>2.0.CO;2. ISSN 1520-0493. Retrieved 2010-12-20.
- Hurd, Willis Edwin (4-21-1929). "Eastern North Pacific tropical cyclones" (PDF). Monthly Weather Review (American Meteorological Society) 57. doi:10.1175/1520-0493(1922)50<98:NPO>2.0.CO;2. Retrieved 2010-12-21.
- National Hurricane Center; Hurricane Research Division; Central Pacific Hurricane Center (April 5, 2013). "The Northeast and North Central Pacific hurricane database 1949-2012". United States National Oceanic and Atmospheric Administration's National Weather Service. Retrieved April 26, 2013.
- Blake, Eric S., et. al. (June 2009). "Tropical Cyclones of the Eastern North Pacific Basin, 1949-2006". Historical Climatology Series 6-5. National Climatic Data Center. p. 6. Retrieved 26 December 2012.
- Longshore, David (2009). Encyclopedia of Hurricanes, Typhoons, and Cyclones, New Edition. Infobase Publishing. p. 333. ISBN 978-1-4381-1879-6.
- Dorst, Neal (2010-01-21). "TCFAQ G1) When is hurricane season?". Hurricane Research Division. United States: Atlantic Oceanographic and Meteorological Laboratory. Retrieved 2011-11-30.
- "Tropical Cyclone Climatology". FAQ. Central Pacific Hurricane Center. Retrieved 2007-12-31.
- National Hurricane Center. Tropical Cyclone Climatology. Retrieved on 2008-04-19.
- Michael Chenoweth and Chris Landsea. The San Diego Hurricane of 2 October 1858. Retrieved on 2008-04-19.