The East Pacific hurricane region covers the tropical eastern North Pacific east of 140^oW. This area is one of the most prolific tropical storm formation regions in the world. Most of the named storms forming in this area track westward across open waters, and sometimes reach Hawaii and beyond. An additional one to two tropical storms either head northward or recurve toward western Mexico, and influence the summer precipitation amounts there. Regardless of their track, the East Pacific tropical storms and hurricanes can supply much needed moisture to the arid southwestern United States.

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

East 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 Nino contributes to decreased easterly vertical wind shear and favors above-normal hurricane activity in this region. Historically, El Nino is not associated with below-normal seasons. Conversely, La Nina contributes to increased vertical shear and less overall activity. Historically, La Nina 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

NOAA’s Accumulated Cyclone Energy (ACE) index for the tropical eastern North Pacific hurricane region is calculated identically to that already in use for the Atlantic hurricane region. The ACE index is a wind energy index, defined as the sum of the squares of the estimated 6-hourly maximum sustained surface wind speed (knots) for all tropical systems forming in the tropical eastern North Pacific hurricane region while they have 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. The median value of the ACE index is 113 x 10⁴ kt².

Above-normal season: An Ace index above 150 x 10⁴ kt² (130% 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 (85%-130% 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 95 x 10⁴ kt² (85% 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 Nina (7 cases), ENSO-neutral (16 cases), and El Nino (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.