The development of Category 3 hurricanes in the Northeast Pacific basin is influenced by many factors. During the Northern Hemisphere winter and spring months of December to April, sea surface temperatures in the tropics are usually too low to support tropical cyclogenesis. Furthermore, from January to April, the North Pacific High and Aleutian Low induce strong vertical wind shear and unfavorable conditions that serve to prevent the development of hurricanes. These effects are reduced or even disappear during hurricane season from May to November, when sea surface temperatures are also high enough to support tropical cyclogenesis; the bulk of recorded Category 3 hurricanes developed during June to October. Global weather patterns may also influence hurricane development in the Northeast Pacific. El Niño events result in increased numbers of powerful hurricanes through weaker wind shear and higher sea surface temperatures within the basin, while La Niña events reduce the number of such hurricanes through the opposite.
Tracks of all known Category 3 Pacific hurricanes from 1949–2015 in the Northeast Pacific basin
On the Saffir–Simpson scale, a hurricane reaches Category 3 status when it attains maximum sustained winds of between 96 knots (110 mph; 178 km/h; 49 m/s) and 112 knots (129 mph; 207 km/h; 58 m/s). The National Hurricane Center (NHC) takes sustained winds to be the average wind speed measured over the period of one minute at the height of 10 metres (33 ft) above the ground. When a hurricane reaches Category 3 intensity, it is termed a "major hurricane" by the NHC, though this term is also used to describe hurricanes at Category 4 or 5 intensity. Should a Category 3 hurricane make landfall, its strongest winds can cause very severe damage to human infrastructure, with debris carried by the winds capable of bringing injury or death to humans and animals.
The Northeast Pacific tropical cyclone basin is defined as the region of the Pacific Ocean north of the equator and east of the International Date Line. The Northeast Pacific is further divided into two sub-basins, namely the east and central Pacific. The east Pacific runs east of the 140th meridian west, and tropical cyclones occurring there are warned upon by the National Hurricane Center, the current Regional Specialized Meteorological Center (RSMC) for that area. The central Pacific, running from the 140th meridian west to the International Date Line, currently has the Central Pacific Hurricane Center as its RSMC. Tropical cyclones are generally much rarer in the central Pacific than in the east Pacific, with an average of just four to five storms forming or moving into the central Pacific compared to around 15 for the east Pacific. All tropical cyclones recorded by past and present RSMCs of the Northeast Pacific basin since 1949 are listed in the Northeast and North Central Pacific hurricane database (HURDAT), which is compiled and maintained by the National Hurricane Center.
Before 1970, tropical cyclones within the Northeast Pacific were classified into three categories: tropical depression, tropical storm, and hurricane; these were assigned intensities of 30 mph (45 km/h), 50 mph (85 km/h), and 85 mph (140 km/h) respectively. Exceptions to these rules would be storms that affected humans and as such humans were able to measure or estimate wind speeds or pressure data. Due to this lack of specific intensity records, there has been only one confirmed Category 3 hurricane prior to 1970.