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The Eastern Pacific hurricane season outlook is an official product of the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC), and is produced in collaboration with scientists from NOAA’s National Hurricane Center (NHC).
Interpretation of NOAA’s Eastern Pacific seasonal hurricane outlook
1. Preparedness
- This outlook provides the public with a general guide to the expected overall activity for the upcoming hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular region.
- Hurricane disasters can occur whether the season is active or quiet. Residents, businesses, and government agencies of coastal and near-coastal regions should prepare for every hurricane season regardless of the seasonal outlook. NOAA, FEMA, the NHC, Small Business Administration, and the Red Cross all provide important hurricane preparedness information on their web sites. It only takes one hurricane (or even tropical storm) to cause a disaster.
2. NOAA does NOT make seasonal hurricane landfall predictions
- NOAA does not make hurricane landfall predictions on a seasonal timescale. Hurricane landfalls are largely determined by the weather patterns in place as the hurricane approaches, which are not predictable more than 5-7 days in advance.
3. Nature of this Outlook and the “likely” ranges of activity
- This outlook is probabilistic, not deterministic. New this year, we are providing probabilities for the stated likely ranges of named storms, hurricanes, major hurricanes, and Accumulated Cyclone Energy (ACE).
- These likely ranges do not represent the total ranges of activity seen in past seasons having similar climate conditions to those expected this year, but are simply the most likely.
- The outlook is based on predictions of large-scale climate factors known to be strong indicators of upcoming seasonal Eastern Pacific hurricane activity, and takes into account uncertainties inherent in such climate outlooks (see item 4 below).
4. Three major sources of uncertainty in the seasonal outlooks
- El Niño and La Niña forecasts are presently the biggest source of uncertainty for the hurricane outlook. The period between March - July is referred to as the “springtime forecast barrier,” a period when predicting these phenomena can be difficult because the atmosphere is in a state of transition.
- Many combinations of named storms and hurricanes can occur for the same set of climate conditions. One cannot know with certainty whether a given climate signal will be associated with several short-lived storms or fewer longer-lived storms with greater intensity.
- Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.
2008 Eastern Pacific Hurricane Outlook Summary
The Climate Prediction Center’s 2008 Eastern Pacific Hurricane Season Outlook calls for a 70% chance of a below-normal season, a 25% chance of a near-normal season, and a 5% chance of an above-normal season. See NOAA definitions of above, near-, and below-normal seasons.
This outlook is based on the analysis and prediction of two main climate signals:
1) The ongoing conditions that have been suppressing Eastern Pacific hurricane seasons since 1995 (called the multi-decadal signal), and
2) Possible lingering effects from La Niña or ENSO-neutral conditions.
Currently, La Niña seems to be waning, but its atmospheric impacts often persist even after Pacific Ocean temperatures have returned to normal. There is considerable uncertainty among the forecast models as to how strong the La Niña influence will be.
Climate patterns similar to those expected this year have historically produced a wide range of activity. Allowing for uncertainties, we estimate a 60%-70% chance of occurrence for each of the following ranges of activity:
- 11-16 named storms,
- 5-8 Hurricanes,
- 1-3 Major Hurricanes,
- An ACE range 40%-100% of the median
These likely ranges have been observed in about two-thirds of past seasons having similar climate conditions to those expected this year. They do not represent the total ranges of activity seen in those past seasons.
The Eastern Pacific hurricane region covers the Eastern Pacific Ocean east of 140oW north of the equator.
There will be no further updates to this outlook.
DISCUSSION
1. Expected 2008 Activity
This Outlook is a general guide to the expected overall activity for the 2008 Eastern Pacific hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular area.
The expected conditions during the 2008 Eastern Pacific hurricane season are related to two main climate signals: 1) the continuation of conditions (called the multi-decadal signal) that have been suppressing Eastern Pacific hurricane activity since 1995, and 2) a possible La Niña influence or ENSO-neutral conditions during the peak months (July-September) of the season.
Historically, seasons with climate patterns similar to those expected this year have produced a wide range of activity. This outlook considers the historical distribution of activity for these climate factors, uncertainties in the La Niña impacts, and the possibility of other unpredictable factors also influencing the season. Based on these factors, we estimate a 70% chance of a below-normal Eastern Pacific hurricane season in 2008, a 25% chance of a near-normal season, and a 5% chance of an above-normal season.
An important measure of total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) inde, which accounts for the collective strength and duration of tropical storms and hurricanes during the season (see Background Information). The ACE index is also used to define the above-, near-, and below-normal seasons. A value of 92% of the median (Median value is 109) corresponds to the upper boundary for a below-normal season.
Based on the above factors, we estimate a 60%-70% chance that the 2008 seasonal ACE range will be 40%-100% of the median. This range can be satisfied even if the numbers of named storms, hurricanes, or major hurricanes fall outside their likely ranges.
The likely (60%-70% chance) ranges of activity for 2008 (each of which is seen in about two-thirds of similar seasons in the historical record) are: 11-16 Named Storms, 5-8 Hurricanes, and 1-3 Major Hurricanes. Much of this activity is expected during July-September, the peak months of the hurricane season.
2. Expected La Niña influence
A key predictor for the 2008 Eastern Pacific hurricane season is the expectation of either La Niña or ENSO-neutral conditions. In combination with an inactive hurricane era, these conditions increase the probability of a below-normal Eastern Pacific hurricane season.
Presently, La Niña is indicated by below average sea-surface temperatures across the central and east-central equatorial Pacific. La Niña is dominating the atmospheric convection and low-level winds in these regions as well, with suppressed convection over the central and Eastern Pacific and enhanced convection over the western Pacific. There has been a tremendous tropics-wide response in the upper-level (200-hPa) atmospheric winds to this anomalous convection, with easterly anomalies extending across the eastern tropical Pacific Ocean, the tropical Atlantic Ocean, and northern Africa. If these anomalies persist through the summer, they would reinforce the multi-decadal signal and increase the probability of a below-normal season.
In the latest ENSO Diagnostics Discussion released 8 May 2008, NOAA forecasters stated that La Niña has weakened since February, and that a transition to ENSO- neutral conditions was possible during June-July just prior to the peak months of the season. This evolution is typical for La Niña, which often dissipates during the late spring or summer.
There is considerable spread and uncertainty among the climate models regarding how strong the La Nina influence will be on the Eastern Pacific hurricane season. Most models are predicting ENSO-neutral conditions (no El Niño or La Niña) during the summer, with sea-surface temperature anomalies in the central and east-central equatorial Pacific between -0.5oC to 0.5oC. However, most of these models have historically shown little-to-no skill at this time of the year.
Historically, we have almost never (once in 100 years) seen a La Niña transition from its current present strength to an El Niño during the summer. Therefore, there is a likelihood that the current La Niña patterns of tropical convection and winds will persist and affect the hurricane season, even if La Niña dissipates.
3. Uncertainties in the Outlook
The likely (60%-70% chance) ACE range for the 2008 Eastern Pacific hurricane season reflects three inter-related sources of uncertainty: 1) When La Niña will dissipate, 2) the likelihood that the current La Niña patterns of tropical convection and winds will persist through the summer, even if La Niña dissipates, and 3) the likely strength of those patterns. These reflect the considerable spread in forecasts from the available ENSO prediction models.
Promising new climate models that can explicitly predict seasonal tropical cyclone activity in the Eastern Pacific are also suggesting a below-normal season. However, these new methods tend to have limited skill at this time of year because of the large uncertainties in the ENSO predictions being utilized.
NOAA FORECASTERS
Climate Prediction Center
Dr. Gerald Bell, Meteorologist, Gerry.Bell@noaa.gov
Dr. Jae Schemm, Physical Scientist, Jae.Schemm@noaa.gov
National Hurricane Center
Eric Blake, Hurricane Specialist, Eric.S.Blake@noaa.gov
Todd Kimberlain, Meteorologist, Todd.Kimberlain@noaa.gov
Dr. Christopher Landsea, Meteorologist, Chris.Landsea@noaa.gov
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www.nws.noaa.gov 
