The eastern Pacific hurricane region covers the eastern Pacific Ocean east of 140°W north of the equator. This area is one of the most prolific tropical storm formation regions in the world. Eastern Pacific tropical storms most often track westward over open waters, sometimes reaching Hawaii and beyond. However, some storms occasionally head toward the northeast, bringing rainfall to the arid southwestern United States during the summer months. Also, during any given season, tropical storms can affect western Mexico or Central America, especially early and late in the season
The official eastern Pacific hurricane season runs from 15 May through 30 November. The peak activity typically occurs during July through September. During the period 1981-2010, the eastern Pacific seasonal averages were 15.4 named storms (maximum 1-minute surface winds between 39-73 mph), with 8.4 of those becoming hurricanes (maximum 1-minute surface winds of at least 74 mph) and 3.9 becoming major hurricanes (maximum 1-minute surface winds exceeding 111 mph, categories 3-5 on the Saffir-Simpson hurricane scale).
Eastern Pacific hurricane seasons exhibit long periods of above- normal and below-normal activity in response to large-scale climate patterns. Seasons also exhibit year-to-year variability in response to ENSO. El Niño contributes to decreased easterly vertical wind shear and favors above-normal hurricane activity in this region. Historically, El Niño is not associated with below-normal seasons. Conversely, La Niña contributes to increased vertical shear and less overall activity. Historically, 60% of La Niña episodes have been associated with below-normal hurricane seasons, and only 28% have produced an above-normal season. However, the ENSO impacts can be strongly influenced by the background climate patterns. As a result, NOAA accounts for the combined influences of both climate factors when making its seasonal hurricane outlooks.
Measuring overall activity: The Accumulated Cyclone Energy (ACE) Index
The phrase "total seasonal activity" refers to the collective intensity and duration of eastern Pacific named storms and hurricanes occurring during a given season. The measure of total seasonal activity used by NOAA is called the Accumulated Cyclone Energy (ACE) Index. The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all eastern Pacific named systems while they are at least tropical storm strength.
NOAA's eastern Pacific hurricane season classifications
Reliable tropical storm and hurricane data for the eastern Pacific began in 1971. The 1981-2010 mean value of the ACE index is 113.3 x 104 kt2, and the median value is 100.4 x 104 kt2. The following season classifications are based on an approximate 3-way partitioning of seasons based on the ACE value, combined with the seasonal number of tropical storms, hurricanes and major hurricanes.
Above-normal season: An ACE index above 115 x 104 kt2 (115% of the median) and at least two of the following three conditions: 17 or more named storms, 9 or more hurricanes, and 5 or more major hurricanes.s
Near-normal season: An ACE index in the range 80-115 x 104 kt2 (80%-115% of the median), or an ACE value higher than 115 x 104 kt2 but with less than two of the following three conditions being met: 17 or more named storms, 9 or more hurricanes, and 5 or more major hurricanes.
Below-normal season: An ACE index below 80 x 104 kt2 (80% of the median).
Seasonal means and ranges during 1981-2010 of tropical storms, hurricanes, and major hurricanes during above-normal, near-normal, below-normal, and all, eastern Pacific hurricane seasons.
Seasonal means and ranges during 1981-2010 of tropical storms, hurricanes, and major hurricanes forming in the eastern Pacific during La Niña (7 cases), ENSO-neutral (15 cases), and El Niño (8 cases). The ENSO classification is provided by NOAA's Climate Prediction Center, and is valid for the July-September climatological peak of the eastern Pacific hurricane season.