The East Pacific hurricane region covers the tropical Northeast 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. Regardless of their track, Northeast Pacific tropical storms and hurricanes can supply much needed moisture to the arid southwestern United States.

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

The ENSO cycle is the dominant known climate factor influencing Northeast Pacific hurricane variability. El Niño contributes to decreased vertical wind shear and favors above-normal hurricane activity. 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.

Measuring overall activity: The Accumulated Cyclone Energy (ACE) Index

NOAA's Accumulated Cyclone Energy (ACE) index for the northeastern Pacific hurricane region is calculated identical 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 Northeast Pacific hurricane region while they are at least tropical storm strength. Since the ACE index represents a continuous spectrum of both system duration and intensity, it does not suffer from the discontinuities inherent in more widely used measures of activity such as the number of tropical storms, hurricanes, or major hurricanes.

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

Reliable tropical storm and hurricane data for the northeastern Pacific region begins in 1971. The 1971-2002 mean value of the ACE index is 130 x 10⁴ kt, and the median value is 113 x 10⁴ kt. The following classification of seasons is based on an approximate 3-way partitioning of seasons based on the percentage of the median ACE value, combined with the seasonal number of tropical storms, hurricanes and major hurricanes.

Above-normal season: An Ace index value above 150 x 10⁴ kt (130% of the median) or an ACE value, 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 value in the range 100-150 x 10⁴ kt (85%-130% of the median), or an ACE value higher than 150 x 10⁴ kt 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 value below 95 x 10⁴ kt, corresponding to 85% of the median.

Seasonal means and ranges of East Pacific tropical storms, hurricanes, and major hurricanes during above-normal, near-normal, below-normal, and all, Northeast Pacific hurricane seasons during 1971-2002.

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.