NOAA’s updated outlook for the 2003 Atlantic hurricane season indicates a high likelihood (60%) of an above-normal season, a 35% probability 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'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.
This update predicts a seasonal total of 12-15 tropical storms, with 7-9 becoming hurricanes, and 3-4 becoming major hurricanes. It also anticipates a seasonal Accumulated Cyclone Energy (ACE) index in the range of 120%-170% of the median.
Consistent with past above-normal hurricane seasons it is likely that many of these hurricanes will develop over the tropical Atlantic and move generally westward as they strengthen. Depending on future steering currents that cannot be predicted at extended ranges these hurricanes could pose a threat to the United States and/or the Caribbean Islands.
COMPARISON WITH MAY 19, 2003 OUTLOOK
These levels of expected activity are consistent with the ranges stated in our pre-season outlook issued May 19, which also indicated a high likelihood (55%) of an above-normal season and a 35% chance of a near-normal season. The May outlook called for 11-15 tropical storms, 6-9 hurricanes, and 2-4 major hurricanes, and an ACE index range of 110%-180% of the median.
In May we indicated an increased likelihood of an above-normal season based in part on the expectation that La Niña conditions would develop during the summer. La Nina reduces the vertical wind shear (i.e., the change in winds with height) over the tropical Atlantic Ocean, making it easier for tropical disturbances to become hurricanes (Gray 1984).
La Nina has not developed as expected. Nonetheless, the expected conditions combined with the active phase of the Atlantic multi-decadal signal are conducive to an above-normal Atlantic hurricane season.
1. Expected Activity- 60% chance above normal, 35% chance near normal, 5% 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). We expect the 2003 seasonal ACE index to be in the range of 120%-170% of the median. An above-normal season is defined by an ACE index above 120% of the median.
There have already been four tropical storms this Atlantic hurricane season, with two becoming hurricanes (Claudette and Danny). We are forecasting a seasonal total of 12-15 tropical storms, with 7-9 becoming hurricanes, and 3-4 becoming major hurricanes [categories 3-4-5 on the Saffir-Simpson scale]. This means that for the remainder of the season we expect an additional 8-11 tropical storms, with 5-7 becoming hurricanes, and 3-4 becoming major hurricanes. However, the total activity as 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 averaged 2-3 landfalling hurricanes [TABLE 2] 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 Atlantic multi-decadal signal and continuation of favorable atmospheric conditions despite absence of La Niña
The combination of the active Atlantic multi-decadal signal and La Niña typically produces conditions most conducive to an extremely active Atlantic hurricane season. The active Atlantic multi-decadal signals are in place. However, contrary to earlier forecasts La Niña has not developed, and ENSO-neutral conditions are expected to persist through October.
Several aspects of the atmospheric circulation are conducive to an above-normal season, even more so than can be attributed to the multi-decadal signal alone. These favorable conditions include 1) an enhanced Tropical Easterly Jet in the upper troposphere across equatorial Africa and the Atlantic Ocean, 2) weaker tropical easterly trade winds, 3) an exceptionally favorable African Easterly Jet in the lower atmosphere, 4) a more northward shift of deep tropical moisture into the main hurricane development region, and 5) generally warmer-than-average Atlantic sea-surface temperatures.
These overall conditions have prevailed since 1995 in association with the active phase of the multi-decadal signal (Goldenberg et al. 2001), making 1995-2002 the most active period in the reliable historical record dating back to 1944. Similar conditions also contributed to the above-normal Atlantic hurricane decades of the 1950s and 1960s.
3. Uncertainties in the Outlook
The current atmospheric conditions, combined with the active phase of the multi-decadal signal, indicate a high probability of an above-normal hurricane season. However, the persistence of these anomalies at their current strength and spatial extent is somewhat uncertain due to ongoing ENSO-neutral conditions. This is especially true across the western part of the Main Development Region, where the multi-decadal signal is weak (Chelliah and Bell 2003).
At present below-average vertical wind shear over the central tropical Atlantic extends only into the lower portion of the Main hurricane development region. While we expect the area of below-average shear to continue expanding northward in association with the active multi-decadal signal, the ENSO-neutral conditions raise some uncertainty as to whether the resulting area will become extensive enough to support an above-normal hurricane season.
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 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 costliest 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. Kingste Mo, Meteorologist, Climate Prediction Center; email@example.com
Dr. Richard Pasch, Meteorologist, Tropical Prediction Center/National
Hurricane Center; firstname.lastname@example.org
Chelliah, M., and G. D. Bell, 2003: Tropical multi-decadal and interannual climate variability in the NCEP/ NCAR Reanalysis. Accepted for Publication in Journal of Climate, July 2003.
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.