NOAA’s 2007 Atlantic hurricane season outlook indicates a very high 75% chance of an above-normal hurricane season, a 20% chance of a near-normal season, and only a 5% chance of a below-normal season. This outlook is produced by scientists at the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC), National Hurricane Center (NHC), Hurricane Research Division (HRD), and Hydrometeorological Prediction Center (HPC). See NOAA definitions of above-, near-, and below-normal seasons.

The outlook calls for a very high likelihood of an above-normal hurricane season, with 13-17 named storms, 7-10 hurricanes, and 3-5 major hurricanes. The likely range of the ACE index is 125% to 210% of the median. This prediction signifies an expected sharp increase in activity from the near-normal season observed in 2006.

The prediction for an above-normal 2007 hurricane season reflects the expected combination of two main climate factors: 1) the continuation of conditions that have been conducive to above-normal Atlantic hurricane seasons since 1995, and 2) the strong likelihood of either ENSO-neutral or La Niña conditions in the tropical Pacific Ocean.

An updated Atlantic hurricane outlook will be issued in early August, which begins the peak (August-October) of the hurricane season.

DISCUSSION

1. Expected Activity - 75% chance above normal, 20% chance near normal, 5% chance below normal

An important measure of the total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the collective intensity and duration of Atlantic named storms and hurricanes during the hurricane season. The ACE index is also used to define above-, near-, and below-normal hurricane seasons (see Background Information). A value of 117% of the median (Median value is 87.5) corresponds to the lower boundary for an above-normal season.

For the 2007 Atlantic hurricane season, the ACE index is expected to be in the range of 125% to 210% of the median. The upper portion of this range is above the 175% baseline that Goldenberg et al. (2001) used to define a hyperactive season. Based on this predicted ACE range, and on the 75% probability of an above-normal season, we expect a likely range of 13-17 named storms, 7-10 hurricanes, and 3-5 major hurricanes [categories 3-4-5 on the Saffir-Simpson scale]. This predicted ACE range can be satisfied even if the numbers of named storms, hurricanes, or major hurricanes fall outside their expected ranges.

The vast majority of tropical storms and hurricanes are expected to form during August-October over the tropical Atlantic Ocean, which is typical for above-normal seasons. These systems generally track westward toward the Caribbean Sea and/or United States as they strengthen. Historically, above-normal seasons have averaged 2-4 hurricane strikes in the continental United States and 2-3 hurricanes in the region around the Caribbean Sea. However, it is currently not possible to confidently predict at these extended ranges the number or intensity of landfalling hurricanes, or whether a given locality will be impacted by a hurricane this season.

2. Expected Climate Conditions – Active multi-decadal signal, either ENSO-neutral or La Niña

The prediction for an above-normal 2007 hurricane season reflects the expected combination of two main climate factors: 1) the continuation of conditions that have been conducive to above-normal Atlantic hurricane seasons since 1995, and 2) the strong likelihood of either ENSO-neutral or La Niña conditions in the tropical Pacific Ocean. Each of these factors is discussed below.

a. Expected continuation of active Atlantic hurricane era

Atlantic hurricane seasons exhibit prolonged 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 first named in the main development region (MDR), which spans the tropical Atlantic Ocean and Caribbean Sea between 30^oW-87.5^oW and 10^oN-21.5^oN (Goldenberg et al. 2001).

Hurricane seasons during 1995-2005 have averaged 15 named storms, 8.5 hurricanes, and 4 major hurricanes, with an average ACE index of 179% of the median. NOAA classifies nine of the last twelve hurricane seasons as above normal, with seven being hyperactive. Only three seasons since 1995 have not been above normal. These are the El Niño years of 1997, 2002, and 2006.

This high level of activity contrasts sharply to the 1971-1994 period of generally below-normal hurricane seasons (Goldenberg et al. 2001), when seasons averaged 8.5 named storms, 5 hurricanes, and 1.5 major hurricanes, with an average ACE index of only 75% of the median. One-half of those seasons were below normal, only three were above normal (1980, 1988, 1989), and none were hyperactive. Time series’ of key atmospheric wind parameters highlight the dramatic differences between these above-normal and below-normal hurricane eras.

The regional atmospheric circulation contributing to these long-period fluctuations in hurricane activity is strongly linked to the tropics-wide multi-decadal signal (Bell and Chelliah, 2006). A change in phase of that signal accounts for the transition in 1995 from the below-normal era to the above normal era . The multi-decadal signal is again a major factor guiding the 2007 outlook. Three key features of this signal that are associated with the current active hurricane era are: 1) a stronger West African monsoon system, 2) below-average convection in the Amazon Basin, and 3) warmer than average SSTs across the tropical Atlantic. Atlantic SSTs are presently 0.6^oC above average in the MDR, and this warmth is likely to continue through the 2007 hurricane season.

Other ongoing regional aspects of the multi-decadal signal again expected during the 2007 hurricane season include 1) lower surface air pressure, and increased moisture across the tropical Atlantic, 2) an amplified ridge at upper levels across the central and eastern subtropical North Atlantic, 3) reduced vertical wind shear in the deep tropics over the central North Atlantic, which results from an expanded area of easterly winds in the upper atmosphere (green arrows) and weaker easterly trade winds in the lower atmosphere (dark blue arrows), and 4) weaker easterly winds in the middle and lower atmosphere, resulting in a configuration of the African easterly jet (wavy blue arrow) that favors hurricane development from tropical waves moving westward from the African coast.

b. Expectation of either ENSO-neutral or La Niña conditions in the tropical Pacific Ocean

The second key predictor for the 2007 hurricane season is the strong likelihood of either ENSO-neutral or La Niña conditions. The Climate Prediction Center is currently indicating that La Niña conditions could develop within the next 1-3 months. As discussed by Gray (1984), La Niña favors more Atlantic hurricanes and El Niño favors fewer hurricanes.

Presently, equatorial Pacific Ocean temperatures are below-average in the eastern Pacific and above average in the western Pacific. This pattern is already associated with a La Niña-like distribution of tropical convection, with suppressed convection over the central and eastern equatorial Pacific and enhanced convection over the western equatorial Pacific. Consistent with these conditions, the upper-level atmospheric circulation across the equatorial Pacific during March and April featured enhanced subtropical ridges at 200-hPa flanking the region of enhanced convection and enhanced mid-Pacific troughs flanking the region of suppressed convection. If these conditions are present during August-October, they would favor a stronger upper-level ridge and reduced vertical wind shear over the western tropical Atlantic.

The combination of an active Atlantic era and La Niña is known to produce very active Atlantic hurricane seasons. The high likelihood of a La Niña-like influence on the 2007 Atlantic hurricane season is in complete contrast to last year, when a rapidly developing El Niño during August-September contributed to a shut-down in hurricane activity during October and November (Bell et al. 2007).

Even if there is no La Niña-like forcing this season, the conditions associated with the ongoing active hurricane era will likely be sufficient to produce an above-normal season. Indeed, several seasons since 1995 have been hyperactive even with ENSO-neutral conditions. However, if La Niña does develop, the probability of a hyperactive season increases.

3. Uncertainties in the Outlook

The somewhat broad predicted ACE range for the 2007 Atlantic hurricane season largely reflects three inter-related sources of uncertainty: 1) the probability of ENSO-neutral versus La Niña conditions during August-October, 2) the likelihood of a La Niña-like pattern of tropical convection, even in the absence of La Niña oceanic conditions, and 3) the likely strength of La Niña if indeed such an event develops. These uncertainties reflect the considerable uncertainty in forecasts from the available ENSO prediction models, along with the exceptionally low predictive skill that all ENSO model forecasts have historically exhibited at this time of the year.

Even with ENSO-neutral conditions, the combination of an active hurricane era and ongoing above-average SSTs across the tropical Atlantic still favors an above-normal season. However, it is important to note that several seasons since 1995 have been hyperactive (ACE > 175% of the median), even with ENSO-neutral conditions. However, if La Niña does develop, the probability of a hyperactive season increases.

While NOAA does not make an official seasonal hurricane landfall forecast, the historical probability for multiple hurricane strikes in the United States increases sharply for hyperactive seasons. For the U.S., all hyperactive seasons since 1950 have had at least one hurricane strike, 92% have had at least 2 hurricane strikes, and 58% have had at least 3 hurricane strikes. For the eastern seaboard of the United States, 92% of hyperactive seasons have had at least one hurricane strike, and 42% have had at least two hurricane strikes. For the Gulf Coast region of the United States, 83% of hyperactive seasons have had at least one hurricane strike, and 58% have had at least two hurricane strikes.

NOAA scientists will closely monitor the rapidly evolving climate conditions. NOAA’s updated Atlantic hurricane outlook will be issued in early August, which begins the peak (August-October) of the hurricane season.

CAUTIONARY NOTES

1) It is currently not possible to confidently predict at these extended ranges the number or intensity of landfalling hurricanes, or whether a particular locality will be impacted by a hurricane this season. Therefore, residents and government agencies of coastal and near-coastal regions should always maintain hurricane preparedness efforts regardless of the overall seasonal outlook.

2) Far more damage can be done by one major hurricane hitting a heavily populated area than by several hurricanes hitting sparsely populated areas. Therefore, hurricane-spawned disasters can occur even in years with near-normal or below-normal levels of activity.  Examples of years with near-normal activity that featured extensive hurricane damage and numerous fatalities include 1960 (Hurricane Donna), 1979 (Hurricanes David and Frederic), and 1985 (Hurricanes Elena, Gloria and Juan). Moreover, the nation's second most damaging hurricane, Andrew in 1992, occurred during a season with otherwise below normal activity.

NOAA FORECASTERS

Climate Prediction Center
Dr. Gerald Bell, Meteorologist; Gerry.Bell@noaa.gov
Dr. Kingste Mo, Meteorologist; Kingste.Mo@noaa.gov

National Hurricane Center
Eric Blake, Hurricane Specialist; Eric.S.Blake@noaa.gov
Dr. Christopher 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

Hydrometeorological Prediction Center
Todd Kimberlain, Meteorologist; Todd Kimberlain@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. Climate. 19, 590-612.

Bell, G. D., and Co-authors 2004: The 2003 Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2003. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 85, S1-S68.

Bell, G. D., and Co-authors 2005: The 2004 Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2004. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 86, S1-S68.

Bell, G. D., and Co-authors 2006: The 2005 Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2005. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 87, S1-S78.

Bell, G. D., and Co-authors 2007: The 2006 Atlantic Hurricane Season: A Climate Perspective. State of the Climate in 2006. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 88, S1-S78.

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.