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HOME > Expert Assessments > East Pacific Hurricane Outlook
 
 
NOAA 2015 Eastern Pacific Hurricane Season Outlook

Issued: 27 May 2015

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The 2015 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 the NOAA National Hurricane Center (NHC). The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140oW north of the equator.

Interpretation of NOAA's eastern Pacific hurricane season outlook
This outlook is general guide to the expected overall activity during the upcoming hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular location.

Preparedness
Hurricane disasters can occur whether the season is active or relatively quiet. It only takes one hurricane (or even a tropical storm) to cause a disaster. Therefore, residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare for every hurricane season regardless of this, or any other, seasonal outlook. NOAA, the Federal Emergency Management Agency (FEMA), the NHC, the Small Business Administration, and the American Red Cross all provide important hurricane preparedness information on their web sites.

NOAA does not make seasonal hurricane landfall predictions
NOAA does not make seasonal hurricane landfall predictions. Hurricane landfalls are largely determined by the weather patterns in place as the hurricane approaches, which are only predictable when the storm is several days of making landfall.

Nature of this Outlook and the "likely" ranges of activity
This outlook is probabilistic, meaning the stated "likely" ranges of activity have a certain likelihood of occurring. The seasonal activity is expected to fall within these ranges in 7 out of 10 seasons with similar conditions and uncertainties to those expected this year. They do not represent the total possible range of activity seen in past similar years.

This outlook is based on predictions of large-scale climate factors and conditions known to strongly influence seasonal eastern Pacific hurricane activity, along with climate model forecasts. The outlook also takes into account uncertainties inherent in such climate outlooks.

Sources of uncertainty in this seasonal outlook

  1. Predicting El Niño and La Niña (also called ENSO) impacts is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have limited skill.
  2. Many combinations of named storms and hurricanes can occur for the same general set of climate conditions. For example, 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.
  3. Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.

2015 Eastern Pacific Hurricane Outlook Summary

NOAA's 2015 Eastern Pacific Hurricane Season Outlook indicates that an above-normal season is most likely, with a 70% chance of an above-normal season, a 25% chance of a near-normal season, and only a 5% chance of a below normal season. See NOAA definitions of above, near-, and below- normal seasons. The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140oW north of the equator.

The main climate factor expected to enhance the 2015 eastern Pacific hurricane season is El Niño, which is now present and is predicted to last throughout the hurricane season. Many models predict this El Niño to strengthen further as the season progresses.

El Niño decreases the vertical wind shear over the eastern tropical Pacific, which favors more and stronger tropical storms and hurricanes. El Niño is already affecting the wind and rainfall patterns across the equatorial and subtropical Pacific Ocean.

Also, the sea surface temperatures patterns (Bottom) that have been associated with decreased hurricane activity in the eastern Pacific since 1995 are not expected during the 2015 season, and are therefore not expected to compete with El Niño's conducive wind patterns.

For the 2015 hurricane season, we estimate a 70% chance of occurrence for each of the following ranges of activity:

  • 15-22 named storms,
  • 7-12 hurricanes,
  • 5-8 major hurricanes,
  • An ACE range 110%-190% of the median.

The seasonal activity is expected to fall within these ranges in 7 of 10 seasons with similar climate conditions and uncertainties to those expected this year. They do not represent the total possible ranges of activity seen in past similar years.

The predicted ranges are centered above the official NHC 1981-2010 seasonal averages of 15 named storms, 8 hurricanes, and 4 major hurricanes.

There will be no further updates to this outlook.

DISCUSSION

1. Expected 2015 Activity

Evolving climate signals, combined with dynamical and statistical model forecasts, indicate that an above-normal eastern Pacific hurricane season is likely during 2015. This outlook calls for a 70% chance of an above-normal season, a 25% chance of a near-normal season, and only a 5% chance of a below-normal season.

Predictions from NOAA's Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) model FLOR-FA, the European Centre for Medium Range Weather Forecasting (ECMWF), and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble are all suggesting above-normal tropical cyclone activity in the eastern Pacific this season, though their forecast skill for the region is limited at this lead time.

An important measure of the total seasonal activity is NOAA's Accumulated Cyclone Energy (ACE) index, which accounts for the combined strength and duration of tropical storms and hurricanes during the season. We estimate a 70% chance that the 2015 seasonal ACE index will be in the range of 110%-190% of the median. According to NOAA's definitions of season strength, an ACE value above 115% of the 1981-2010 median indicates an above-normal season, and a value of 80%-115% of the median indicates a near-normal season.

Consistent with the predicted ACE range, other likely (70% chance) ranges of activity for 2015 are 15-22 named storms, 7-12 hurricanes, and 5-8 major hurricanes. The upper bound of the forecast range for major hurricanes matches the most ever recorded (1983, 1992, 1993, and 2014).

2. Science behind the outlook

a. El Niño

The 2015 seasonal hurricane outlook reflects the persistence and possible strengthening of the current El Niño as the season progresses. El Niño helps to strengthen the eastern Pacific (and central Pacific) hurricane season by reducing the vertical wind shear across the eastern half of the tropical North Pacific.

At present, SSTs are above average across the eastern half of the equatorial Pacific, and the largest departures exceed +1.0oC. SST anomalies averaged across the Niño 3.4 region (which spans the east-central equatorial Pacific between 120oW-170oW) are currently +1.0oC. This value is at CPC's lower threshold for a moderate-strength El Niño. The Niño 3.4 region is a main region used by the CPC to help monitor and assess the strength of El Niño.

The current sub-surface thermal structure also reflects El Niño, with above-average temperatures present from the surface to 150m depth between the date line and the west coast of South America. This anomalous warmth was initially linked to a downwelling equatorial oceanic Kelvin wave that was triggered in February. Unlike previous downwelling Kelvin waves over the past twelve months, this most recent wave was associated with a strong ocean-atmosphere coupling that led to El Niño.

Key aspects of this coupling include anomalous upper-level divergence and enhanced convection since mid-March over the central equatorial Pacific. Additionally, anomalous westerly trade winds (i.e. lighter easterly trade winds) have been present since March across the central and east-central equatorial Pacific Ocean, contributing to the warmer SSTs and to the enhanced convection.

Additional periods with exceptionally weak trade winds could further strengthen El Niño this hurricane season. One such period is currently occurring in association with a westerly wind burst over the western equatorial Pacific.

The average forecast (orange line) of the dynamical models (closed markers) in the suite of IRI/ CPC Niño 3.4 SST forecasts suggests that El Niño could become a strong event (Niño 3.4 SST anomalies ≥ 1.5oC) as the hurricane season progresses. Some models (such as the CFS Hi-Res model and the ECMWF) are predicting an exceptionally strong El Niño while others are predicting a moderate-strength event (Niño 3.4 SST anomalies between 1.0-1.5oC). The statistical model forecasts (open markers) are generally cooler than the dynamical model predictions, and on average (green line) show a borderline moderate-strength El Niño during the hurricane season. These differing forecasts, combined with overall lower predictive skill during March-May, produce uncertainty as to exactly how strong El Niño will become.

b. Global sea surface temperatures: differences from the previous multi-decadal signal

The current and predicted global SST patterns, along with the dominant multi-decadal signals since 1995, are shown. The observed March-April 2015 SST anomaly pattern (top left) reflects 1) a projection of the Atlantic SST anomalies onto the cold phase of the AMO, and 2) a horseshoe-shaped pattern of below-average SSTs (blue arc) in the western Pacific. NOAA's CFS high-resolution model (Top Right) predicts these two patterns to persist through the hurricane season. These conditions differ from the global patterns of SST anomalies observed during much of the preceding 1995-2014 period of reduced activity in the eastern Pacific hurricane basin (Bottom).

This year, we do not expect the low-activity era SST anomaly patterns to develop, which means that we do not expect the recent multi-decadal signal to offset El Niño's conducive wind patterns. Instead, the predicted SST anomaly patterns this season are more typical of the 1981-1994 high-activity era for eastern Pacific hurricanes. During that period, 62% of the hurricane seasons were above normal, 23% were near normal, and only 15% were below normal. Seasons during the period averaged about 18 named storms, 10.5 hurricanes, and 5 major hurricanes, with an ACE value of 150% of the median. This average value falls within NOAA's definition for an above-normal season. This elevated activity is comparable to the predicted 2015 ranges.

In contrast, eastern Pacific hurricane seasons have been less active since 1995. During 1995-2014, 40% of the seasons were below normal, 35% were near normal, and only 25% were above normal. Seasons during this period averaged about 13 named storms, 7 hurricanes, and 3 major hurricanes, with an ACE value of 88% of the median.

Despite this recent low-activity era, the eastern Pacific did have an above-normal hurricane season last year. If the 2015 season is also above normal, it would mark the first time since 1997-98 that two consecutive seasons were above normal.

NOAA FORECASTERS

Climate Prediction Center
Dr. Gerald Bell, Meteorologist, Gerry.Bell@noaa.gov
Dr. Jae Schemm, Meteorologist, Jae.Schemm@noaa.gov

National Hurricane Center
Eric Blake, Hurricane Specialist, Eric.S.Blake@noaa.gov
Todd Kimberlain, Hurricane Specialist, Todd.Kimberlain@noaa.gov

Dr. Christopher Landsea, Meteorologist, Chris.Landsea@noaa.gov


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Page Author: Climate Prediction Center Internet Team
Page last modified: May 27, 2015
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