The Eastern Pacific hurricane region covers the Eastern Pacific Ocean east of 140^oW 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 occasionally head toward the northeast, sometimes bringing rainfall to the arid southwestern United States during the summer months. Also, during any given season, one or two tropical storms can affect western Mexico or Central America.

The official Eastern Pacific hurricane season runs from 15 May through 30 November. The peak activity typically occurs during July through September. An average hurricane season features 15-16 tropical storms, of which 9 become hurricanes and 4 to 5 become major hurricanes.

Eastern Pacific hurricane seasons exhibit long periods of above-normal and below-normal activity in association with the tropical multi-decadal signal. They 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, La Niña has been associated equally with near-normal and below-normal hurricane seasons, but never with an above-normal season. However, the ENSO impacts can be strongly influenced by the background multi-decadal signal. 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 East Pacific hurricane season classifications

Reliable tropical storm and hurricane data for the tropical eastern North Pacific began in 1971. The 1971-2005 mean value of the ACE index is 125 x 10⁴ kt², and the median value is 109 x 10⁴ kt². The following classification of seasons is 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 150 x 10⁴ kt² (138% of the median) combined with at least two of the three following parameters above the long-term average: number of tropical storms, hurricanes and major hurricanes.

Near-normal season: An ACE index in the range 100-150 x 10⁴ kt² (92%-138% of the median), or an ACE value higher than 150 x 10⁴ kt² but with less than two of the three following parameters above the long-term average: number of tropical storms, hurricanes and major hurricanes.

Below-normal season: An ACE index below 100 x 10⁴ kt² (92% of the median).

Seasonal means and ranges of tropical storms, hurricanes, and major hurricanes during above-normal, near-normal, below-normal, and all, tropical eastern North Pacific hurricane seasons for the period 1971-2005.

Seasonal means and ranges of tropical storms, hurricanes, and major hurricanes forming over the tropical eastern North Pacific during La Niña (7 cases), ENSO-neutral (16 cases), and El Niño (12 cases) episodes for the period 1971-2005. The ENSO classification is provided by NOAA’s Climate Prediction Center, and is valid for the July-September climatological peak in the tropical eastern North Pacific hurricane season.