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.

2012 Eastern Pacific Hurricane Outlook Summary

NOAA’s 2012 eastern Pacific Hurricane Season Outlook indicates a 50% chance of a near-normal season, a 30% chance of a below-normal season, and a 20% 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 140°W north of the equator.

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

• The ongoing conditions that have been associated with reduced eastern Pacific hurricane activity since 1995, and
• Expected above-average sea-surface temperatures (SSTs) in the eastern tropical Pacific Ocean, with the possible development of El Niño later in the season. Warmer waters in the eastern Pacific can produce decreased vertical wind shear and increased hurricane activity in the eastern Pacific hurricane region.

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

• 12-18 named storms,
• 5-9 hurricanes,
• 2-5 major hurricanes,
• An ACE range 70%-130% 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 official NHC seasonal averages for the 1981-2010 period are 15 named storms, 8 hurricanes, and 4 major hurricanes.

There will be no further updates to this outlook.

DISCUSSION

1. Expected 2012 Activity

This Outlook is a general guide to the expected overall activity for the 2012 eastern Pacific hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular area.

Known climate signals and evolving oceanic and atmospheric conditions, combined with dynamical model forecasts, indicate that a near-normal 2012 eastern Pacific hurricane season is most likely. This outlook calls for a 50% chance of a near-normal season, a 30% chance of a below-normal season, and a 20% 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. We estimate a 70% chance that the 2012 seasonal ACE will be in the range of 70%-130% of the median. According to NOAA’s definitions of season strength, an ACE value below 80% of the 1981-2010 median indicates a below normal season, and a value of 80%-115% of the median indicates a near-normal season. Note that these ACE thresholds have changed from previous years due to a prior error in calculating the seasonal ACE values.

Consistent with the expected ACE range, other likely (70% chance) ranges of activity for 2012 are: 12-18 named storms, 5-9 hurricanes, and 2-5 major hurricanes.

This outlook is consistent with 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 limited at this lead time.

2. Science behind the outlook

This outlook is based on the analysis and prediction of two competing climate signals. The first is a continuation of conditions that have been suppressing eastern Pacific activity since 1995. Expected to offset this signal is above-average sea-surface temperatures in the eastern tropical Pacific Ocean, with the possible development of El Niño later in the season. Warmer waters in the eastern Pacific can produce decreased vertical wind shear and increased hurricane activity in the eastern Pacific hurricane region.

a. Expected continuation of low-activity hurricane era in the eastern Pacific
The eastern Pacific has experienced reduced hurricane activity since 1995 (while the Atlantic Basin has experienced increased activity). During 1995-2011, 41% of eastern Pacific hurricane seasons were below normal, 35% were near normal, and only 24% 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.

Associated with the climate conditions that have contributed to this reduction in eastern Pacific hurricane activity, the Pacific Inter-Tropical Convergence Zone (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 typically 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. For the eastern Pacific, 62% of hurricane seasons during 1982-1994 were above normal, 23% were near normal, and only 15% were below normal. Seasons during this period averaged about 18 named storms, 10.5 hurricanes, and 5 major hurricanes, with an ACE value of 150% of the median. This value falls within NOAA’s definition for an above-normal season.

b. 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. La Niña conditions increase the wind shear and usually suppresses the activity. These typical impacts can be strongly modulated by conditions associated with the tropical multi-decadal signal.

The 2011-12 La Niña episode dissipated in April, as equatorial SSTs across the central and east-central Pacific returned to near-average and SSTs in the eastern Pacific became above average. Based on observations and ENSO model forecasts, the official CPC/IRI ENSO forecast issued in early May indicates that ENSO-Neutral conditions (defined by the CPC as Niño 3.4 SST departures between -0.5^oC and +0.5^oC) are likely through the summer with approximately equal chances of Neutral and El Niño conditions (Niño 3.4 SST departures above +0.5^oC) during the fall.

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-October 2012. The 2012 seasonal hurricane outlook takes into account this forecast spread.

NOAA FORECASTERS

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