Meteorology

2011 Atlantic Hurricane Season Outlook

June 2, 2011, 10:30 am
Source: National Oceanic and Atmospheric Administration.
Content Cover Image

GOES-East Water Vapor, June 1, 2011. Source: NOAA>

NOAA 2011 Atlantic Hurricane
Season Outlook

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) predictions of large-scale climate factors known to influence seasonal hurricane activity, and 2) 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. 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. Model predictions of sea-surface temperatures, vertical wind shear, moisture, and stability have limited skill this far in advance of the peak months (August-October) of the hurricane season.

4. Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.

2011 Atlantic Hurricane Season Outlook: Summary

NOAA's 2011 Atlantic Hurricane Season Outlook calls for a 65% chance of an above normal season, a 25% chance of a near-normal season, and a 10% chance of a below-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.

This outlook reflects an expected set of conditions that is conducive to above-normal Atlantic hurricane activity. These conditions are based on three climate factors:

    • The tropical multi-decadal signal, which has contributed to the high-activity era in the Atlantic basin that began in 1995,
    • A continuation of above-average sea surface temperatures in the tropical Atlantic Ocean and Caribbean Sea (called the Main Development Region),
    • ENSO-neutral conditions most likely (no El Niño or La Niña), with lingering La Niña impacts into the summer.

In addition, several dynamical model forecasts of the number and strength of tropical cyclones generally predict an above normal season.

The conditions expected this year have historically produced some active Atlantic hurricane seasons. Therefore, the 2011 season could see activity comparable to a number of active seasons since 1995. We estimate a 70% probability for each of the following ranges of activity during 2011:

  • 12-18 Named Storms,
  • 6-10 Hurricanes
  • 3-6 Major Hurricanes
  • An ACE range of 105%-200% 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.

Hurricane Landfalls:
It only takes one storm hitting your area to cause a disaster, regardless of the overall activity predicted in the seasonal 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.

While NOAA does not make an official seasonal hurricane landfall outlook, the historical probability for multiple U.S. hurricane strikes, and for multiple hurricane strikes in the region around the Caribbean Sea, increases sharply for exceptionally active (i.e. hyperactive) seasons (ACE > 165% of the median). However, 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.

DISCUSSION

1. Expected 2011 activity

Known climate signals and evolving oceanic and atmospheric conditions, combined with dynamical model forecasts, indicate that an above-normal 2011 Atlantic hurricane season is likely. This outlook calls for a 65% chance of an above-normal season, a 25% chance of a near-normal season, and only a 10% chance of a below 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 105%-200% of the median. According to NOAA's hurricane season classifications, an ACE value above 111% of the 1981-2010 median reflects an above-normal season. An ACE value above 165% of the median reflects an exceptionally active (or hyperactive) season.

Consistent with the expected ACE range, the 2011 Atlantic hurricane season is predicted (with 70% chance) to produce 12-18 named storms, of which 6-10 are expected to become hurricanes, and 3-6 are expected to become major hurricanes. Therefore, this season could see activity comparable to some of the other active seasons since 1995.

For the U.S. and the region around the Caribbean Sea, the historical probability of a hurricane strike generally increases with increasing seasonal activity. During exceptionally active seasons (ACE > 165% of the median), the historical probabilities increase markedly for multiple hurricane strikes in these regions. Nonetheless, predicting the location, number, timing, and strength, of hurricanes landfalls is ultimately related to the daily weather patterns, which are not predictable weeks or months in advance. As a result, 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. Therefore, NOAA does not make an official seasonal hurricane landfall outlook.

2. Science behind the 2011 Outlook

The 2011 Atlantic hurricane season outlook primarily reflects an expected set of conditions during the peak months (August-October) of the season that is known to be conducive to increased Atlantic hurricane activity. This expectation is based on the prediction of three climate factors. These climate factors are: 1) the tropical multi-decadal signal, which has contributed to the ongoing high-activity era for Atlantic hurricanes that began in 1995, 2) a continuation of above average sea-surface temperatures (SSTs) in the Main Development Region (MDR, which includes the Caribbean Sea and tropical Atlantic ocean between 9oN-21.5oN; Goldenberg et al. 2001), and 3) a high likelihood of ENSO-neutral conditions (i.e., no El Niño or La Niña).

The outlook also takes into account dynamical model predictions from the NOAA 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. Many of these atmospheric features typically become evident during late April and May, as the atmosphere across the tropical Atlantic and Africa begins to transition into its summertime monsoon state.

Several of these conditions are now present, and are expected to persist through the hurricane season because of their link to the tropical multi-decadal signal. These conditions include 1) weaker (i.e. anomalous westerly) trade winds and mid-level winds, 2) anomalous easterly winds in the upper atmosphere, 3) anticyclonic circulation (i.e. streamfunction) anomalies in the upper atmosphere in both hemispheres, and 4) reduced vertical wind shear. It should be noted that the 200-hPa circulation and vertical wind shear patterns also reflect an influence from La Niña at this time.

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 15 years, and that those anomalies 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 during the 2011 Atlantic hurricane season of the active Atlantic phase of the tropical multi-decadal signal that began in 1995.

b. Above average SSTs in the Main Development Region
The second factor guiding this outlook is the expectation of continued above-average SSTs in the MDR during August-October. This expectation is consistent with model forecasts, and with the likely dissipation of La Niña (which acts to reduce the springtime SSTs in the MDR). It is also consistent with the expected pattern of reduced trade winds.

SSTs are currently above average in the MDR, with departures exceeding +0.5oC in the central tropical Atlantic. This warmth is also larger than that observed in the remainder of the global tropics, and is further indication that climate conditions are favorable for an active Atlantic hurricane season.

c. ENSO-Neutral conditions likely
Another climate factor known to significantly impact Atlantic hurricane activity is ENSO. The three phases of ENSO are El Niño, La Niña, and ENSO-Neutral. El Niño events tend to suppress Atlantic hurricane activity, while La Niña events tend to enhance it (Gray 1984). These typical impacts can be strongly modulated by conditions associated with a low- or high-activity era.

Currently, the 2010-11 La Niña episode is dissipating. Based on observations and ENSO forecast models, the CPC indicates that ENSO-Neutral conditions are likely during the 2011 Atlantic 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 August-October 2011. Most models predict ENSO-Neutral conditions (defined by the CPC as SST departures between -0.5oC and +0.5oC) during this period. However, a few models predict weak La Niña conditions (SST departures cooler than -0.5oC), or even weak El Niño conditions (SST departures above +0.5oC).

This spread in the model forecasts, combined with the limited predictive skill exhibited by all such models at this time of the year, is a main reason why we are presently indicating only a 65% chance of an above-normal season. If El Niño does not develop, the probability of an above- normal Atlantic hurricane season will be even higher and the actual seasonal activity will more likely be toward the upper end of our predicted ranges.

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 151% of the median. NOAA classifies 11 of the 16 seasons since 1995 as above normal, with eight being extremely active (i.e., hyperactive defined by ACE > 165% of median). Only five seasons since 1995 have not been above normal, which include four El Niño years (1997, 2002, 2006, and 2009) and the 2007 season.

This high level of activity since 1995 contrasts sharply to the low-activity era of 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 the tropical multi-decadal signal coincides 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.

 

Issued: 19 May 2011

Realtime monitoring of tropical Atlantic conditions
Realtime monitoring of tropical East Pacific conditions

 

Atlantic Hurricane Outlook & Seasonal Climate Summary Archive


Glossary

Citation

(2011). 2011 Atlantic Hurricane Season Outlook. Retrieved from http://www.eoearth.org/view/news/51cbf14d7896bb431f6a5088

0 Comments

To add a comment, please Log In.