Skip Navigation Links www.nws.noaa.gov 
NOAA logo - Click to go to the NOAA home page National Weather Service   NWS logo - Click to go to the NWS home page
Climate Prediction Center
 
 

CPC Search
About Us
   Our Mission
   Who We Are

Contact Us
   CPC Information
   CPC Web Team

 
 
HOME > Expert Assessments > East Pacific Hurricane Outlook > Background Information
 
 
BACKGROUND INFORMATION:
EAST PACIFIC HURRICANE SEASON

The eastern Pacific hurricane region covers the eastern Pacific Ocean east of 140oW 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.

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.

Season  Type Mean # of Tropical Storms Range of Tropical Storms Mean # of Hurricanes Range of Hurricanes Mean # of Major Hurricanes Range of Major Hurricanes
Below-normal 12.4 7 to 17 6.0 3 to 8 1.7 0 to 4
Near-Normal 15.4 10 to 18 7.6 6 to 9 3.2 2 to 5
Above-Normal 18.2 13 to 24 11.2 9 to 16 6.4 4 to 8
All Seasons 15.4 7 to 25 8.4 3 to 16 3.9 0 to 9
 

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.

ENSO Status Mean # of Tropical Storms Range of Tropical Storms Mean # of Hurricanes Range of Hurricanes Mean # of Major Hurricanes Range of Major Hurricanes
La Niña 13.1 7 to 22 6.4 3 to 11 3.0 1 to 7
Neutral 16.3 9 to 24 9.3 5 to 16 4.0 0 to 8
El Niño 15.4 12 to 19 8.4 6 to 11 4.4 3 to 7

NOAA/ National Weather Service
National Centers for Environmental Prediction
Climate Prediction Center
5200 Auth Road
Camp Springs, Maryland 20746
Page Author: Climate Prediction Center Internet Team
Page last modified: May 27, 2015
Disclaimer
Information Quality
Credits
Glossary
Privacy Policy
Freedom of Information Act (FOIA)
About Us
Career Opportunities