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.

2010 Eastern Pacific Hurricane Outlook Summary

NOAA’s 2010 Eastern Pacific Hurricane Season Outlook indicates a 75% chance of a below-normal season, a 20% 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. The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140^oW north of the equator.

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

• The ongoing conditions that have been suppressing eastern Pacific hurricane seasons since 1995, and
• Either ENSO-neutral (meaning no El Niño or La Niña) or La Niña conditions during the peak months (July-September) of the season, with La Niña becoming increasingly likely.

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,
• 4-8 hurricanes,
• 1-3 major hurricanes,
• An ACE range 45%-95% 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 long-term (1971-2005) averages for the eastern Pacific hurricane season are 15 named storms, 9 hurricanes, and 4 major hurricanes.

There will be no further updates to this outlook.

DISCUSSION

1. Expected 2010 Activity

This Outlook is a general guide to the expected overall activity for the 2010 eastern Pacific hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular area.

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 La Niña develops.

The climate factors expected to guide the 2010 eastern Pacific hurricane season are 1) a continuation of conditions that have been suppressing activity since 1995, and 2) either ENSO-neutral or La Niña conditions during the peak months (July-September) of the season, with La Niña becoming increasingly likely. This outlook considers the historical distribution of activity for these climate factors, uncertainties in whether La Niña will develop, 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 only a few producing near-normal activity. Therefore, we estimate a 75% chance of a below-normal season, a 20% 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 2010 seasonal ACE range will be 45%-95% of the median. According to NOAA’s definitions of season strength, an ACE value below 92% of the 1971-2005 median indicates a below normal season, and a value of 92%-138% of the median indicates a near-normal season.

Consistent with the expected ACE range, other likely (70% chance) ranges of activity for 2010 are: 9-15 named storms, 4-8 hurricanes, and 1-3 major hurricanes.

This outlook is 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 reduced 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. During 1995-2009, 60% of eastern Pacific hurricane seasons were below normal, 33% 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 13.7 named storms, 6.9 hurricanes, and 3 major hurricanes, with an ACE value of 82% of the median. This value falls within NOAA’s definition for a below-normal season. This ongoing low-activity era, combined with either ENSO-Neutral or La Niña conditions, is the main reason we expect the 2010 hurricane season to be below-normal.

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 much 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 due to a strong La Niña) was below normal. Seasons during that period averaged 18.2 named storms, 10.8 hurricanes, and 5.6 major hurricanes, with an ACE value of 157% 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 either Neutral or La Niña conditions during the 2010 eastern Pacific hurricane season, with La Niña now becoming increasingly likely. Both Neutral and La Niña conditions typically enhance the dominant features of the ongoing low-activity era.

The recent El Niño episode, which contributed last year to a near-normal hurricane season in the eastern Pacific, has dissipated. Conditions in the equatorial Pacific Ocean are currently favorable for the development of La Niña. Also, in the upper atmosphere the pattern of circulation (i.e. streamfunction) anomalies during the last 30 days, and the last 60 days, indicates cyclonic anomalies in the central subtropical Pacific of both hemispheres (blue shading in NH, orange shading in SH). This pattern suggests that the atmosphere has already transitioned out of its El Niño state observed last winter and early spring.

All ENSO forecast models currently predict either Neutral or La Niña conditions during the Atlantic hurricane season. During the last few months, the models have been increasingly indicating the development of La Niña during the summer. NOAA’s high-resolution CFS model indicates the development of La Niña-like circulation and precipitation anomalies during July.

NOAA FORECASTERS

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