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 these systems track westward into open waters, sometimes reaching 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.

The ENSO cycle is the dominant known climate factor influencing hurricane variability over the tropical eastern North Pacific. El Ni�o contributes to decreased vertical wind shear and favors above-normal hurricane activity in this region. Historically, El Ni�o is not associated with below-normal seasons. Conversely, La Ni�a contributes to increased vertical shear and less overall activity. Historically, La Ni�a has been associated equally with near-normal and below-normal hurricane seasons, but never with an above-normal season.

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 essentially a wind energy index, defined as the sum of the squares of the estimated 6-hourly maximum sustained wind speed (knots) for all tropical systems forming in the tropical eastern North Pacific hurricane region while they are at least tropical storm strength.

NOAA definitions of above-normal, near-normal, and below-normal East Pacific Hurricane seasons

Reliable tropical storm and hurricane data for the tropical eastern North Pacific began in 1971. The 1971-2003 mean value of the ACE index is 128 x 10⁴ kt², and the median value is 111 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 (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 1971-2003.

Seasonal means and ranges of tropical storms, hurricanes, and major hurricanes forming over the tropical eastern North Pacific during La Ni�a (7 cases), ENSO-neutral (15 cases), and El Ni�o (11 cases) episodes for the period 1971-2003. 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.