2006 Atlantic Hurricane Season Outlook
SUMMARY  August 2006 Update 
Information from NOAA and Department of Commerce

NOAA continues to predict a high likelihood (75% chance) of an above-normal 2006 Atlantic hurricane season and a 20% chance of a near-normal season, according to a consensus of scientists at National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center (CPC), National Hurricane Center (NHC), and Hurricane Research Division (HRD). Therefore, 2006 is forecast to be the tenth above-normal season in the last twelve years. See NOAA’s definitions of above-, near-, and below-normal seasons.

This updated outlook calls for a seasonal total of 12-15 named storms, with 7-9 becoming hurricanes, and 3-4 becoming major hurricanes (categories 3-5 on the Saffir-Simpson hurricane intensity scale). The likely range of NOAA’s Accumulated Cyclone Energy (ACE) index (Bell and Halpert, 2000) is 110%-170% of the median. These totals include the three tropical storms (Alberto, Beryl, and Chris) that have already occurred. Therefore, for the remainder of the season, we expect an additional 9-12 named storms, 7-9 hurricanes, and 3-4 major hurricanes.

The predicted 2006 activity mainly reflects a continuation of conditions associated with the multi-decadal signal, which has favored above-normal Atlantic hurricane seasons since 1995. These conditions include warmer than average sea surface temperatures (SSTs), lower vertical wind shear, reduced sea level pressure, and a more conducive structure of the African easterly jet.

While we are predicting an active season, a repeat of last year’s record season is unlikely. The season is also expected to be slightly less active than previously forecast on 22 May 2006, when 13-16 Named Storms, 8-10 hurricanes, and 4-6 major hurricanes were predicted. The expected activity is lower for three reasons: 1) atmospheric and oceanic conditions are not as conducive as previously forecast, 2) the transition away from La Niña-like rainfall patterns occurred more quickly than expected, and 3) the very persistent upper-level ridge pattern over the eastern U.S. and western Atlantic, which contributed to the extremely active 2003-2005 hurricane seasons, is not present.

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 ACE index, which accounts for the collective intensity and duration of Atlantic named storms and hurricanes during a given 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 2006 Atlantic hurricane season, the expected ACE range is 110%-170% of the median. Based on this range and on the 75% probability of an above-normal season, we expect a seasonal total of 12-15 named storms, 7-9 hurricanes, and 3-4 major hurricanes. 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 named 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. NOAA does not currently make seasonal forecasts for landfalling hurricanes. However, similar above-normal seasons have historically averaged 2-3 landfalling hurricanes in the continental United States and 2-3 hurricanes in the region around the Caribbean Sea.

The conditions that produce hurricane landfalls are very difficult to predict at these extended ranges. As a result, it is currently not possible as part of this outlook to predict the number or intensity of landfalling hurricanes, or whether a given locality will be impacted by a hurricane this season.  It is important that residents and government officials in hurricane-vulnerable communities have a hurricane preparedness plan in place.

2. Expected Climate Conditions – Active multi-decadal signal, above-average Atlantic Ocean temperatures

All of the Atlantic hurricane seasons since 1995 have been above normal, with the exception of two moderate to strong El Niño years (1997 and 2002). This contrasts sharply with the 1971-1994 period of generally below-normal activity (Goldenberg et al., Science, 2001). The regional atmospheric circulation anomalies contributing to these long-period fluctuations in hurricane activity is strongly linked to the tropics-wide multi-decadal signal (Bell and Chelliah, 2006). Since 1995 this signal has been very conducive to above-normal hurricane seasons and warmer Atlantic SSTs, and it is again the main factor guiding the 2006 outlook.

Over the North Atlantic, key aspects of the multi-decadal signal expected during the 2006 hurricane season include 1) warmer SSTs, 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 easterly wind anomalies 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.

This outlook calls for a lower level of activity than was predicted on 22 May 2006. The May forecast called for 13-16 Tropical Storms, 8-10 Hurricanes, and 4-6 Major Hurricanes. The chances of an extremely active season are now lower for three reasons: 1) neither the atmospheric wind and air pressure patterns, nor the tropical Atlantic SSTs, are as conducive as expected; 2) long periods of suppressed convection near the date line, which acts to lower the vertical wind shear over the tropical Atlantic, are no longer present, 3) the very persistent upper-level ridge pattern over the eastern U.S. and western Atlantic, which contributed to the extremely active 2003-2005 hurricane seasons, is not present.

One factor known to significantly impact Atlantic hurricane seasons is ENSO (Gray, 1984). El Niño favors fewer hurricanes and La Niña favors more hurricanes. Based on the most recent ENSO outlook issued by NOAA’s Climate Prediction Center, ENSO-neutral conditions are expected in the tropical Pacific through much of the Atlantic hurricane season. Therefore, ENSO is not expected to impact this hurricane season.

3. Multi-decadal fluctuations in Atlantic hurricane activity

Atlantic hurricane seasons exhibit prolonged periods lasting several 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, which spans the tropical Atlantic Ocean and Caribbean Sea.

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 eleven hurricane seasons as above normal, and seven as hyperactive (ACE > 175% of median). In contrast, during the preceding 1971-1994 period, hurricane 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 these seasons were below normal, only three were above normal (1980, 1988, 1989), and none were hyperactive.

4. Uncertainties in the Outlook

The main uncertainty in this outlook is related to the strong variability in atmospheric and oceanic conditions across the tropical Atlantic in recent months. This variability is partly related to strong intraseasonal fluctuations in convection and upper-level divergence over the central equatorial Pacific. Current conditions are only modestly conducive to an above-normal season, although they may become even more conducive as impacts fade from the enhanced convection over the central equatorial Pacific during mid-June through early-July.

Another uncertainty is the upper-level circulation anomalies over the eastern U.S. and western North Atlantic. The last three hyperactive hurricane seasons (2003-2005) featured a persistent upper-level ridge in these regions. This ridge has been notably absent so far this season. Our only seasonal predictor for the circulation in this area is a significant El Niño or La Niña, neither of which is expected this season. A persistent ridge over the eastern U.S. would favor increased activity and more hurricane landfalls.

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. Muthuvel Chelliah, Physical Scientist; Muthuvel.Chelliah@noaa.gov
Dr. Kingste Mo, Meteorologist; Kingste.Mo@noaa.gov

National Hurricane Center
Eric Blake, Meteorologist; Eric.S.Blake@noaa.gov
Dr. Christopher Landsea, Meteorologist; Chris.Landsea@noaa.gov
Dr. Richard Pasch, Meteorologist; 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. 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 2004. A. M. Waple and J. H. Lawrimore, Eds. Bull. Amer. Meteor. Soc., 87, S1-S78.

Bell, G. D., and M. S. Halpert, 2000: Climate Assessment for 1999. Bull. Amer. Meteor. Soc., 81, S1-S51.

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.