The 2011 Atlantic hurricane season outlook is an official product of the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC). The outlook is produced in collaboration with scientists from the National Hurricane Center (NHC) and the Hurricane Research Division (HRD). The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

Interpretation of NOAA's Atlantic hurricane season outlook
This outlook is a general guide to the expected overall activity during the upcoming hurricane season. It is not a seasonal hurricane landfall forecast, and it does not predict 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 tropical storm) to cause a disaster. 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 National Hurricane Center (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) current and evolving patterns of wind, air pressure, and ocean temperature, 2) predictions of large-scale climate factors known to influence seasonal hurricane activity, and 3) promising new climate models that directly predict seasonal hurricane activity.

Sources of uncertainty in this seasonal outlook
1. Predicting El Niño and La Niña (also called the El Niño-Southern Oscillation- ENSO) impacts is an ongoing scientific challenge facing climate scientists today.

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 shorter-lived storms or fewer longer-lived storms.

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 Updated Atlantic Hurricane Season Outlook: Summary

NOAA’s updated 2011 Atlantic Hurricane Season Outlook calls for an 85% chance of an above-normal season, and a 15% chance of a near-normal season. There is no expectation for a below-normal season. Therefore, 2011 is expected to become the twelfth above-normal season since 1995. This updated outlook reflects a higher likelihood of an above-normal season compared to the pre-season outlook issued in May, which indicated a 65% chance of an above-normal season. See NOAA definitions of above-, near-, and below-normal seasons. The Atlantic hurricane region includes the North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico.

The higher confidence of an above-normal season is based on several factors. First, as predicted in May, conducive atmospheric and oceanic conditions are now in place over the tropical Atlantic Ocean and Caribbean Sea (called the Main Development Region- MDR). Second, these conditions are expected to persist throughout the peak months (August-October) of the hurricane season in association with the tropical multi-decadal signal, which has contributed to the high activity era that began in 1995. Third sea-surface temperatures in the MDR are the third warmest on record, and models predict a continuation of very warm SSTs through the hurricane season. Fourth, there is a possibility of La Niña re-developing.

Historically, this combination of conditions produces an active Atlantic hurricane season. In addition, several dynamical model forecasts of the number and strength of tropical cyclones indicate that an above normal season is likely.

The 2011 season is expected to be comparable to a number of active seasons since 1995. We estimate a 70% probability for each of the following ranges of activity during 2011:

• 14-19 Named Storms,
• 7-10 Hurricanes
• 3-5 Major Hurricanes
• An ACE range of 135%-215% of the 1981-2010 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. These ranges do not represent the total possible ranges of activity seen in past similar years.

The official NHC seasonal averages are 11 named storms, 6 hurricanes, and 2 major hurricanes.

Activity to Date:
To date, five tropical storms (Tropical Storms Arlene, Bret, Cindy, Don, and Emily) have formed in the Atlantic basin. Significant activity is expected for the remainder of the season, with an additional 9-14 named storms likely, of which 7-10 are expected to become hurricanes with 3-5 reaching major hurricane status.

Hurricane Landfalls:
NOAA does not make an official seasonal hurricane landfall outlook. Predicting where and when hurricanes will strike is related to daily weather patterns, which are not reliably predictable weeks or months in advance. Therefore, it is currently not possible to reliably predict the number or intensity of landfalling hurricanes at these extended ranges, or whether a given locality will be impacted by a hurricane this season.

But remember that it only takes one storm hitting your area to cause a disaster, regardless of the overall activity predicted in this outlook. Therefore, residents, businesses, and government agencies, of coastal and near-coastal regions are urged to prepare every hurricane season regardless of this, or any other, seasonal outlook.

DISCUSSION

1. Expected 2011 activity

Evolving oceanic and atmospheric conditions and their links to known climate signals, combined with dynamical model forecasts, indicate that an above-normal Atlantic hurricane season is very likely. This updated outlook calls for an 85% chance of an above-normal season , and a 15% chance of a near-normal season. There is no expectation for a below-normal season. This updated outlook reflects a higher likelihood of an above-normal season compared to the pre-season outlook issued in May, which indicated a 65% chance of an above-normal season.

An important measure of the total overall seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the intensity and duration of named storms and hurricanes during the season. We estimate a 70% chance that the 2011 seasonal ACE range will be 135%-215% of the median. This range is well above NOAA’s lower threshold (111%) for classifying a season as above normal. It is also centered at 175% of the median, which is above NOAA’s lower threshold (165%) for a very active (or hyperactive) season.

Consistent with the expected ACE range, the 2011 Atlantic hurricane season is predicted (with 70% chance) to produce 14-19 named storms, of which 7-10 are expected to become hurricanes, and 3-5 are expected to become major hurricanes. Therefore, this season could see activity comparable to some of the other active seasons since 1995.

To date, five tropical storms (Tropical Storms Arlene, Bret, Cindy, Don, and Emily) have formed in the Atlantic basin. Significant activity is expected for the remainder of the season, with an additional 9-14 named storms likely, of which 7-10 are expected to become hurricanes with 3-5 reaching major hurricane status.

2. Science behind the 2011 Outlook

The high confidence for an above-normal season is based on several factors. First, conducive atmospheric and oceanic conditions are now present in the MDR, as was predicted in May. Second, these conditions are expected to persist throughout the peak months (August-October) of the hurricane season in association with the tropical multi-decadal signal, which has contributed to the high activity era that began in 1995. Third sea-surface temperatures in the MDR are the third warmest on record, and models predict a continuation of very warm SSTs through the hurricane season. Fourth, there is a possibility of La Niña re-developing.

The outlook also takes into account dynamical model predictions from NOAA’s Climate Forecast System (CFS), NOAA’s Geophysical Fluid Dynamics Lab (GFDL) model CM2.1, the European Centre for Medium Range Weather Forecasting (ECMWF), the United Kingdom Meteorology (UKMET) office, and the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble. These models are indicating a high likelihood of an above normal season.

a. Expected continuation of tropical multi-decadal signal
One primary factor guiding this outlook is the expected continuation of the tropical multi-decadal signal (Bell and Chelliah 2006), which has contributed to the current high-activity era in the Atlantic basin that began in 1995. This signal incorporates the warm phase of the Atlantic Multi-decadal Oscillation (AMO) and an enhanced west African monsoon system. It is associated with an inter-related set of atmospheric conditions, all of which are conducive to increased Atlantic hurricane activity.

During 1995-2010, some key aspects of the tropical multi-decadal signal within the MDR have included warmer than average SSTs, reduced vertical wind shear and weaker easterly trade winds, below-average sea-level pressure, and a configuration of the African easterly jet that is more conducive to hurricane development from tropical waves moving off the African coast.

These conditions are now present, as seen in the recent patterns of below-average sea-level pressure across the MDR, along with reduced vertical wind shear and weaker (i.e. anomalous westerly) trade winds. These current conditions are expected to persist through the hurricane season because of their link to the tropical multi-decadal signal, and also in association with the exceptionally warm Atlantic SSTs now in place.

It is impossible to know with certainty whether the tropical multi-decadal signal is indeed continuing during 2011, and current climate models cannot skillfully forecast the multi-decadal variability of the Atlantic climate system. However, given that key anomaly patterns now present have also been present for the past 16 years, and that those patterns have been previously linked to the tropical multi-decadal signal, it is reasonable to expect that they are again linked to this signal. If so, this would reflect a continuation of the active Atlantic phase of the tropical multi-decadal signal that began in 1995.

b. Above average SSTs in the Main Development Region (MDR)
The second factor guiding this outlook is the expected continuation of well above-average SSTs in the MDR during August-October. This expectation is consistent with model forecasts from NOAA’s Climate Forecast System (CFS). It is also consistent with the expected pattern of reduced trade winds across the southern MDR and tropical Atlantic.

SSTs in the MDR are currently well above average, with departures exceeding +0.5^oC in the central tropical Atlantic and eastern Caribbean Sea. During June 2011, the area-averaged SST departure within the MDR was +0.48^oC, which is the third largest value since 1950. This departure is much larger than that of the remainder of the global tropics, and is a further indication that climate conditions are favorable for an active Atlantic hurricane season.

c. Possible re-development of La Niña
A third climate factor guiding this outlook is the possible re-development of La Niña, which would likely further reduce the vertical wind shear in the western MDR, as is currently predicted by NOAA’s high-resolution CFS.

La Niña represents one phase of the climate phenomenon known as ENSO (i.e. El Niño/ Southern Oscillation). The three phases of ENSO are El Niño, La Niña, and ENSO-Neutral. El Niño tends to suppress Atlantic hurricane activity, while La Niña tends to enhance it (Gray 1984). These impacts can be strongly modulated by conditions associated with a low- or high-activity era.

The 2010-11 La Niña episode has dissipated, as indicated by 1) only weak SST anomalies across the central and east-central equatorial Pacific Ocean, 2) weak sub-surface temperature departures within the tropical Pacific Ocean, and, 3) the near-average oceanic heat content across the eastern half of the Pacific Ocean.

However, the patterns of tropical rainfall/ convection and atmospheric winds previously associated with La Niña have persisted, and are predicted by the CFS to continue throughout Aug-Oct. These conditions include suppressed convection (below-average rainfall) near the date line and enhanced convection (above-average rainfall) over the western Pacific Ocean. They also include stronger-than-average easterly trade winds (easterly wind anomalies) in the lower atmosphere centered near the date line, and westerly wind anomalies in the upper atmosphere. Consistent with these conditions, cyclonic circulation anomalies in the upper atmosphere persist over the central Pacific in the subtropics of both hemispheres.

Based on the expected continuation of these conditions and on ENSO forecast models, the CPC indicates that La Niña could re-develop during the fall. The combination of La Niña, a high activity era, and well above-average sea-surface temperatures in the MDR, has historically produced an active Atlantic hurricane season.

3. Further analysis of the Ongoing High Activity Era in the Atlantic Basin

Atlantic hurricane seasons exhibit extended periods lasting decades of generally above-normal or below-normal activity. These fluctuations in hurricane activity result almost entirely from differences in the number of hurricanes and major hurricanes forming from tropical storms that first develop in the MDR.

The current high-activity era has been in place since 1995. Hurricane seasons during 1995-2010 have averaged about 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE index of 153% of the median. NOAA classifies 11 of the 16 seasons since 1995 as above normal, with nine being very active (i.e., ACE > 165% of median). Only five seasons since 1995 have not been above normal. These include four El Niño years (1997, 2002, 2006, and 2009) and the 2007 season.

The high level of activity since 1995 contrasts sharply to the low-activity era 1971-1994 (Goldenberg et al. 2001), which averaged only 8.5 named storms, 5 hurricanes, and 1.5 major hurricanes, and had an average ACE index of only 74% of the median. One-half of the seasons during this low-activity era were below normal, only three were above normal (1980, 1988, 1989), and none were hyperactive.

Within the MDR, the atmospheric circulation anomalies that contribute to these long-period fluctuations in hurricane activity are strongly linked to the tropics-wide multi-decadal signal (Bell and Chelliah 2006), which incorporates the warm phase of the AMO and an enhanced west African monsoon system. A change in the phase of this signal coincided with the transition in 1995 from a low-activity era to the current high-activity era.

NOAA FORECASTERS

Climate Prediction Center
Dr. Gerry Bell, Lead Forecaster, 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. Chris Landsea, Meteorologist; Chris.Landsea@noaa.gov
Dr. Richard Pasch, Hurricane Specialist; Richard.J.Pasch@noaa.gov

Hurricane Research Division
Stanley Goldenberg, Meteorologist; Stanley.Goldenberg@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.

Goldenberg, S. B., C. W. Landsea, A. M. Mestas-Nuñez, and W. M. Gray, 2001: The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293, 474-479.

Gray, W. M., 1984: Atlantic seasonal hurricane frequency: Part I: El Niño and 30-mb quasi-bienniel oscillation influences. Mon. Wea. Rev., 112, 1649-1668.