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HOME > Expert Assessments > East Pacific Hurricane Outlook
 
NOAA PRESS RELEASE
 
NOAA: 2011 Eastern Pacific Hurricane Season Outlook

Issued: 19 May 2011

Realtime monitoring of tropical East Pacific conditions can be obtained here
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The 2011 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 region.

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 within 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 climate conditions and uncertainties to those expected this year. They do not represent the total possible ranges of activity seen in past similar years.

This outlook is based on 1) predictions of large-scale climate factors and conditions known to be strong indicators of seasonal eastern Pacific hurricane activity. The outlook also takes into account uncertainties inherent in such climate outlooks.

Sources of uncertainty in the seasonal outlooks

  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.

2011 Eastern Pacific Hurricane Outlook Summary

NOAA’s 2011 Eastern Pacific Hurricane Season Outlook indicates a 70% chance of a below-normal season, a 25% chance of a near-normal season, and only a 5% chance of an above 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.

This outlook is based on the analysis and prediction of two main climate signals:

Climate patterns similar to those expected this year have historically produced a wide range of activity. Allowing for uncertainties, we estimate a 70% chance of occurrence for each of the following ranges of activity this season:

  • 9-15 named storms,
  • 5-8 hurricanes,
  • 1-3 major hurricanes,
  • An ACE range 45%-105% of the median.

The seasonal activity is expected to fall within these ranges in 7 out 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 1981-2010 averages for the eastern Pacific hurricane season are 15-16 named storms, 8-9 hurricanes, and 4 major hurricanes.

There will be no further updates to this outlook.

DISCUSSION

1. Expected 2011 Activity

This Outlook is a general guide to the expected overall activity for the 2011 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 climate factors expected to guide the 2011 eastern Pacific hurricane season are 1) a continuation of conditions that have been suppressing activity since 1995, and 2) A high likelihood of ENSO-neutral conditions (no El Niño or La Niña) during the peak months (July-September) of the season, but with lingering La Niña impacts perhaps into the summer.

This outlook considers the historical distribution of activity for these climate factors, uncertainties in ENSO, and the possibility of other unpredictable factors also influencing the season.

Historically, seasons with climate patterns similar to those expected this year have typically produced below normal activity, with some producing near-normal activity. Therefore, we estimate a 70% chance of a below-normal season, a 25% chance of a near-normal season, and only a 5% chance of an above normal season.

An important measure of 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. Based on the expected climate factors above, we estimate a 70% chance that the 2011 seasonal ACE range will be 45%-105% of the median. According to NOAA’s definitions of season strength, an ACE value below 94% of the 1981-2010 median indicates a below normal season, and a value of 94%-141% of the median indicates a near-normal season.

Consistent with the expected ACE range, other likely (70% chance) ranges of activity for 2011 are: 9-15 named storms, 5-8 hurricanes, and 1-3 major hurricanes.

We expect dominant features of the ongoing low activity era to prevail this season, including increased vertical wind shear, generally shorter-lived and weaker storms, and a reduced westward track to those storms. These conditions would likely be accentuated if the current La Niña-related circulation anomalies persist even after La Niña dissipates.

This outlook is generally consistent with newly emerging climate model forecasts. Predictions from the Climate Forecast System (CFS), the European Centre for Medium Range Weather Forecasting (ECMWF), and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble are all suggesting normal or below-normal tropical cyclone activity in the eastern Pacific this season, though their forecast skill for the region is quite limited at this lead time.

2. The ongoing low-activity hurricane era in the eastern Pacific

The eastern Pacific has experienced suppressed hurricane activity since 1995 (while the Atlantic Basin has experienced greatly enhanced activity). The expected continuation of this low-activity era, combined with ENSO-Neutral conditions and the possibility of lingering La Niña-related circulation anomalies, is the main reason we expect the 2011 hurricane season to be below-normal.

During 1995-2010, 63% of eastern Pacific hurricane seasons were below normal, 31% were near normal, only one (1997) was above normal due to an El Niño episode of near-record strength. Eastern Pacific seasons during this period averaged about 14 named storms, 7 hurricanes, and 3 major hurricanes, with an ACE value of 83% of the median. This value falls within NOAA’s definition for a below-normal season.

Several regional circulation patterns have been associated with this reduced activity. The Pacific ITCZ has shifted farther to the north, allowing for extensive southwesterly flow into the eastern tropical North Pacific. Also, the upper-level ridge over Mexico has been stronger than average, resulting in enhanced upper-level easterly winds. This combination of factors leads to increased vertical wind shear, which limits the number, intensity, and duration of the tropical storms and hurricanes. Also, these storms tend to form closer to Mexico and closer to cooler ocean temperatures, both of which limit their duration.

The levels of hurricane activity since 1995 in the eastern Pacific and the Atlantic basin contrast sharply with those of the preceding period 1982-1994. The eastern Pacific was much more active during this earlier period, while the Atlantic basin was more suppressed. This multi-decadal see-saw in hurricane season strength between the two regions is associated with the tropical multi-decadal signal (Chelliah and Bell 2004, Bell and Chelliah 2006). For the eastern Pacific, 62% of hurricane seasons during 1982-1994 were above normal, 31% were near normal, and only one season (1988) was below normal. Seasons during that period averaged about 18 named storms, 11 hurricanes, and 5-6 major hurricanes, with an ACE value of 162% of the median. This value falls within NOAA’s definition for an above-normal season.

3. The El Niño/ Southern Oscillation (ENSO)

ENSO is an important climate predictor for eastern Pacific hurricane activity. The three phases of ENSO are El Niño, La Niña, and Neutral. El Niño acts to reduce the vertical wind shear and is therefore more conducive to eastern Pacific hurricane activity, while Neutral and La Niña conditions increase the wind shear and suppresses the activity. These typical impacts can be strongly modulated by conditions associated with the tropical multi-decadal signal.

We expect ENSO-Neutral during the peak months of the 2011 eastern Pacific hurricane season, but with lingering La Niña impacts into the summer. Both ENSO-Neutral and La Niña conditions typically enhance the dominant features of the ongoing low-activity era. Currently, the 2010-11 La Niña episode is waning. Based on current observations and ENSO forecast models, NOAA’s CPC indicates that ENSO-Neutral conditions are likely during the peak months of the 2011 eastern Pacific hurricane season.

However, predicting the various phases of ENSO and their impacts is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have limited skill. The present uncertainty in the model forecasts is indicated by the large spread in predicted SST departures in the east-central equatorial Pacific during July-September 2011. Most models predict ENSO-Neutral conditions (defined by the CPC as SST departures between -0.5oC and +0.5oC) during this period. However, some models predict weak La Niña conditions (SST departures cooler than -0.5oC) while others predict weak El Niño conditions (SST departures above +0.5oC).

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

REFERENCES

Bell, G. D., and M. Chelliah, 2006: Leading tropical modes associated with interannual and multi-decadal fluctuations in North Atlantic hurricane activity. J. of Climate. 19, 590-612.

Chelliah, M., Bell, G. D., 2004: Tropical multi-decadal and interannual climate variations in the NCEP/ NCAR Reanalysis. J. Climate, 17, 1777-1803.


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Page last modified: May 19, 2011
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