NOAA's 2003 Atlantic hurricane season outlook indicates a 55% probability of an above-normal Atlantic hurricane season in 2003, a 35% probability of a near-normal season, and only a 10% chance of a below-normal season, according to a consensus of scientists at the National Oceanic and Atmospheric Administration's (NOAA) Climate Prediction Center (CPC), the Hurricane Research Division (HRD), and the National Hurricane Center (NHC). See NOAA’s definitions of above-, near-, and below-normal seasons.
The 2003 outlook calls for 11-15 tropical storms, with 6-9 becoming hurricanes, and 2-4 becoming major hurricanes. It also calls for an ACE index value in the range 110%-180% of the median. This expected activity is considerably more than the four hurricanes and ACE value of 74% of the median observed during 2002. It is also much larger than the seasonal average of five hurricanes and ACE value of 75% of the median observed during the relatively quiet period 1970-1994.
This outlook reflects the ongoing multi-decadal conditions, combined with a 70% chance of La Niña, both of which are conducive to increased activity during the normal peak (August-October) of the hurricane season.
1. Expected Activity- 55% chance above normal, 35% chance near normal, 10% chance below normal
An important measure of the overall seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the collective strength and duration of Atlantic tropical storms and hurricanes during a given hurricane season (see Background Information). The ACE index is also used to define the above-, near-, and below-normal seasons.
For the 2003 hurricane season the ACE index is expected to be in the range of 110%-180% of the median. (A value of 120% corresponds to the lower boundary for an above-normal season.) This comparatively wide range of ACE values reflects our uncertainty in whether or not La Niña will develop sufficiently to impact key atmospheric circulation features over the tropical Atlantic during the height of the hurricane season. NOAA’s Climate Prediction Center is indicating a 70% chance of La Niña conditions by August, and a 30% chance of neutral conditions (i.e., no El Niño or La Niña).
If La Niña conditions do not develop, the 2003 seasonal ACE value is expected to be near the border (120% of median) between a near-normal and an above-normal season in response to the ongoing active multi-decadal signal (see section 2 below). If La Niña conditions do develop, then the likelihood increases for an above-normal season, with expected levels of activity approaching the middle to upper portion of the predicted ACE range, or possibly even higher.
The expected climate conditions indicate a likely range of 11-15 tropical storms this season, with 6-9 becoming hurricanes, and 2-4 becoming major hurricanes [categories 3-4-5 on the Saffir-Simpson scale]. While it is reasonable to expect this range of tropical storms and hurricanes, the total seasonal activity measured by the ACE index can certainly be in the expected range without all three of these criteria being met.
Based on past historical data similar seasons have also averaged 2-3 landfalling hurricanes in the continental United States and 1-2 hurricanes in the region around the Caribbean Sea. However, it is important to recognize that 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.
2. Expected Climate Conditions – Active multi-decadal conditions and likely La Niña
The overall activity during 2003 is expected to depend primarily on the combined influence of the active multi-decadal signal and the state of the tropical Pacific Ocean (Gray 1984, Monthly Weather Review).
El Niño has dissipated, and a transition to La Niña is already under way in the equatorial Pacific. Based on present conditions and observed trends, it is likely that La Niña conditions will develop by August. La Niña favors increased hurricane activity by reducing the vertical wind shear over the Atlantic hurricane basin’s main development region. This La Niña influence can be greatly accentuated when it occurs in combination with the active multi-decadal signal.
The period 1995-2002 is the most active in the reliable historical record (dating back to 1944). Since 1995 North Atlantic sea-surface temperatures have been above normal, and the West African monsoon circulation has been stronger than normal (see Goldenberg et al. 2001, Science). These conditions have been associated with reduced vertical wind shear and weaker trade winds in the main hurricane development region, and with a configuration of the African easterly jet that is more conducive to hurricane development from tropical disturbances moving westward from the African coast. These key aspects of the active multi-decadal signal also contributed to the above-normal Atlantic hurricane decades of the 1950s and 1960s.
The combination of La Niña and the active multi-decadal signal produces conditions most conducive to hurricane and major hurricane formation. This combination is known to favor very active hurricane seasons with potential levels of activity in the upper portion of our predicted range, or even higher.
For neutral conditions the active multi-decadal signal favors seasonal activity in the lower part of our predicted ACE range, which is near the border between a near-normal and above-normal season.
A secondary climate factor, anomalously easterly winds in the lower stratosphere, is expected to provide a slightly suppressing influence on overall Atlantic hurricane activity this season.
3. Uncertainties in the Outlook
The main uncertainty in this outlook is whether or not La Niña will develop sufficiently to impact key atmospheric circulation features over the tropical Atlantic during the height of the hurricane season. Because of the high levels of hurricane activity that can result when La Niña conditions occur in combination with the active multi-decadal signal, this uncertainty is particularly relevant to the present hurricane outlook. NOAA scientists will closely monitor these evolving climate conditions and issue an updated hurricane outlook in early August, which is prior to the normal active portion of the Atlantic hurricane season.
1) It is important to recognize that 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 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 or, of course, not making landfall at all. Therefore, hurricane-spawned disasters can occur even in years with near-normal or below-normal levels of activity. Examples of years with near-normal levels of 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 most damaging hurricane, Andrew in 1992, occurred during a season with below normal overall activity.
Dr. Gerald Bell, Meteorologist, Climate Prediction
Mr. Eric Blake, Meteorologist, Tropical Prediction Center/National Hurricane Center;
Dr. Muthuvel Chelliah, Physical Scientist, Climate
Prediction Center; firstname.lastname@example.org
Mr. Stanley Goldenberg, Meteorologist, Hurricane
Research Division; email@example.com
Dr. Christopher Landsea, Meteorologist, Hurricane
Research Division; firstname.lastname@example.org
Dr. Richard Pasch, Meteorologist, Tropical Prediction Center/National
Hurricane Center; email@example.com