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
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
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
- 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.
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.
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
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
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
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
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
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
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
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
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.