NOAA is predicting a very high likelihood (85% chance) of an above-normal 2007 Atlantic hurricane season, a 10% chance of a near-normal season, and only a 5% chance of a below-normal season, according to a consensus of scientists at the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center, National Hurricane Center, Hurricane Research Division, and Hydrometeorological Prediction Center.
The outlook calls for an even higher probability of an above-normal season than was predicted in May (75%), and reiterates the expectation for a sharp increase in activity from the near-normal season observed last year. The 2007 season is expected to become the tenth above-normal season since the current active hurricane era began twelve years ago (in 1995). See NOAA’s definitions of above-, near-, and below-normal seasons.
The 2007 outlook calls for a likely range of 13-16 named storms, 7-9 hurricanes, and 3-5 major hurricanes. The likely range of the ACE index is 140%-200% of the median. These ranges are slightly tighter than those predicted in May (13-17 named storms, 7-10 hurricanes, 3-5 major hurricanes, and an ACE range of 125%-210%). The tighter ranges reflect not only an increased confidence for an above normal season, but also a reduced likelihood of seeing as many as 10 hurricanes and 17 named storms.
The prediction for an above-normal 2007 hurricane season reflects the 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 continued La Niña-like pattern of tropical convection. In addition, temperatures in the western tropical Atlantic and Caribbean Sea remain well above average (0.56oC). This combination of conditions is known to produce high levels of Atlantic hurricane activity.
So far this season, there have been three Atlantic named storms (Andrea, Barry, and Chantal), which is slightly above average for June and July. The Atlantic hurricane season runs from June 1 through November 30. However, the vast majority of the activity in 2007 is expected during the peak months of the season- August through October (ASO).
1. Expected Activity - 85% chance above normal, 10% 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 2007, the ACE index is expected to be in the range of 140% to 200% of the median. The upper portion of the range is above the 175% baseline used to define a hyperactive season (Goldenberg et al. 2001). Based on this predicted ACE range, and on the 85% probability of an above-normal season, we expect a likely range of 13-16 named storms, 7-9 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 majority of tropical storms and hurricanes during 2007 are expected to form 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, similar conditions have typically produced 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, La Niña-like pattern of tropical convection, warmer western tropical Atlantic and Caribbean Sea
The prediction for an above-normal 2007 hurricane season reflects the 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 continued La Niña-like pattern of tropical convection. All features of the multi-decadal signal are already in place across the tropical Atlantic, as is a persistent La Niña-like pattern of tropical convection across the eastern half of the equatorial Pacific. In addition, temperatures in the western tropical Atlantic and Caribbean Sea remain well above average (0.56oC). This combination of conditions is known to produce very active Atlantic hurricane seasons. Therefore, the probability of an above-normal season has increased to 85% (from 75% in May), and the lower bound for the ACE range has increased to 140% (from 125% in May).
At the same time, the upper bound for ACE has dropped slightly to 200% (from 210% in May), and the maximum expected number of hurricanes has dropped slightly to 9 (from 10 in May). These changes reflect: 1) A reduced chance of a moderate to strong La Niña episode, and 2) Cooler SSTs in the eastern tropical Atlantic Ocean compared to previous record levels.
a. 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 30oW-87.5oW and 10oN-21.5oN (Goldenberg et al. 2001).
Hurricane seasons during 1995-2006 have averaged 14.4 named storms, 8.2 hurricanes, and 4 major hurricanes, with an average ACE index of 172% 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, 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 in the tropical Atlantic and Caribbean Sea.
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 MDR, 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, which produce a configuration of the African easterly jet (wavy blue arrow) that favors hurricane development from tropical waves moving westward from the African coast.
b. Continuation of a La Niña-like pattern of convection in the tropical Pacific Ocean
The second key predictor for the 2007 hurricane season is the possibility of a La Niña episode in the tropical Pacific during ASO. The Climate Prediction Center is currently indicating a slightly greater than 50% chance that La Niña will develop during the peak of the hurricane season. As discussed by Gray (1984), La Niña favors more Atlantic hurricanes and El Niño favors fewer hurricanes.
Equatorial Pacific SSTs remain below-average in the east and above average in the west. This anomaly pattern has been associated with suppressed convection over the central and eastern equatorial Pacific and enhanced convection over the western equatorial Pacific. This La Niña-like pattern of tropical convection has already acted to suppress the East Pacific hurricane season. This pattern is expected to continue even if La Niña does not develop.
There is a strong inverse relationship between eastern Pacific and Atlantic hurricane activity, primarily because similar circulation anomalies impact the basins oppositely through differing climatological flow patterns. The significantly below-average eastern Pacific activity to date increases our confidence for an active Atlantic hurricane season.
La Niña also helps to extend the Atlantic activity into November. The high likelihood of a La Niña-like influence on the 2007 Atlantic hurricane season contrasts to last year, when a rapidly developing El Niño during August-September helped to shut-down Atlantic hurricanes in October and November (Bell et al. 2007).
c. Tropical Atlantic sea-surface temperatures (SSTs)
In the western tropical Atlantic/ Caribbean Sea, sea-surface temperatures remained well above average (0.56oC) during June and July, and departures of approximately +0.5oC are expected in that region during ASO 2007. In the eastern tropical Atlantic, SSTs fell back to only slightly above-average levels (0.18oC) during June-July, following record warm ASO temperatures in that region since 2004. This cooling was associated with a persistent high pressure pattern over the Atlantic Ocean, which brought strong northeasterly winds into the tropics that cooled the SSTs through increased upwelling and evaporation.
During the last few weeks this wind pattern disappeared and an enhanced southerly flow of moist tropical air typical of above-normal seasons became established. Associated with this evolution, the moist static stability has decreased, the African Easterly Jet has shifted to well north of normal, the easterly trade winds have weakened, and portions of the eastern tropical Atlantic have warmed to more than 0.5oC above average.
3. Uncertainties in the Outlook
There are two unrelated sources of uncertainty in this forecast. The first is whether or not La Niña will develop and if so, how strong it will become. Even with ENSO-neutral conditions, the combination of an active hurricane era with the ongoing La Niña-like pattern of tropical convection and winds produces a very high probability of an above-normal season. The development of La Niña increases the probability for a hyperactive season with activity in the upper end of the predicted range.
The second source of uncertainty is whether conditions over the eastern tropical Atlantic will continue to become increasingly conducive for hurricane formation. Atmospheric conditions in that region are already consistent with other above-normal seasons, and the associated SSTs have returned to above-average in many areas. However, the latest Climate Forecast System (CFS) forecast from the NOAA Environmental Monitoring Center indicates a continuation during ASO of near-average temperatures in the eastern tropical Atlantic, suggesting atmospheric conditions might be slightly less conducive to hurricane formation than current trends indicate.
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.
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
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.