Issued 27 May 1999
There is a strong likelihood of above-average tropical storm and
hurricane activity over the North Atlantic basin during the June-November 1999 hurricane
season, in response to an expected continuation of ongoing La Niņa conditions and its
associated atmospheric circulation and tropical rainfall patterns, according to a
consensus reached by scientists at the National Oceanic and Atmospheric Administration's
(NOAA) Climate Prediction Center (CPC), National Hurricane Center (NHC) and Hurricane
Research Division (HRD). There is also a strong likelihood of at least three major
hurricanes this year. Most of this activity is expected to occur during the peak three
months (August-October) of the season. An updated outlook will be issued in early August.
This Outlook should be used as a guide of overall expected activity for
the Atlantic basin. The Outlook does not give any indication of whether a particular
locality will be impacted by a tropical storm or hurricane during 1999. Residents and
government agencies of coastal or near-coastal regions should always maintain normal
hurricane preparedness efforts regardless of the overall outlook for a given year.
La Niņa refers to cooler than average sea-surface temperatures across
the central and eastern tropical Pacific. In combination with these oceanic
conditions, the present pattern of tropical rainfall is dominated by above-normal rainfall
across Indonesia and the eastern Indian Ocean, and by a near-absence of rainfall across
the eastern equatorial Pacific. This pattern of tropical rainfall is associated with
atmospheric circulation features which often favor an active hurricane season. It appears
at this time that the ongoing La Niņa episode, and associated tropical rainfall and
atmospheric circulation patterns, will persist through the current hurricane season.
If, as expected, these atmospheric conditions persist through the
season, then they will likely favor an active hurricane season by influencing conditions
over the North Atlantic in two fundamental ways: 1) by reducing the vertical wind shear
across the tropical Atlantic and Caribbean Sea, and 2) by producing a structure and
location of the African easterly jet which is thought to be more efficient for providing
energy to developing tropical systems as they propagate westward from the African coast.
During active hurricane seasons, the continental United States usually
experiences a higher number of landfalling hurricanes than average. The Caribbean region
is also far more at risk of experiencing a tropical storm or hurricane during active
seasons than at other times, as was recently observed during the 1995, 1996, and 1998
seasons. In an average season the United States experiences 1-2 land-falling hurricanes,
while the Caribbean Islands experience 1 hurricane.
The hurricane season for the North Atlantic basin (which includes the
North Atlantic Ocean, Caribbean Sea and Gulf of Mexico) officially runs from June 1st
through November 30th. During this period the average number of systems reaching tropical
storm status (maximum sustained winds between 39-73 mph), hurricane status (maximum
sustained winds of at least 74 mph) and major hurricane status (maximum sustained winds
exceeding 110 mph, and corresponding to categories 3-4-5 on the Saffir-Simpson scale ) are
nine, six, and two, respectively. However, the vast majority of tropical storm and
hurricane activity typically occurs during August - October, which is considered the peak
of the hurricane season.
This outlook is meant to provide users with a consensus statement from
NOAA scientists regarding the upcoming Atlantic basin hurricane activity based on current
and expected climate conditions as described above. The expectation of above-average
activity during 1999 is not based on global warming associated with elevated carbon
This outlook is not designed to compete with other existing hurricane
forecasts and outlooks issues by persons or groups outside of NOAA. We gratefully
acknowledge the pioneering research provided by Dr. William Gray and others, which has
significantly increased scientific understanding of the association between the various
climate factors (particularly the ENSO cycle) and the atmospheric circulation features
that affect Atlantic basin hurricane activity. We also acknowledge the leading role that
Dr. Gray and colleagues at the Colorado State University have played in developing and
providing seasonal forecasts of Atlantic basin tropical storm and hurricane activity.
1) Far more damage can be done by one major hurricane hitting a heavily
populated area than by several major hurricanes hitting sparsely populated areas or, of
course, not making landfall at all.
Because of this, hurricane-spawned disasters can occur even in
relatively inactive years from landfalling systems.
2) Increased tropical storm and hurricane activity during a particular
year does not automatically mean increased storm-related damage. For example, in 1992
Hurricane Andrew, the only major hurricane to develop in that relatively inactive year,
caused over $25 billion in damage to the continental United States. In contrast there was
less than $4 billion in damage to the continental United States during 1995, one of the
most active seasons on record.
3) Although major hurricanes tend to be the deadliest and costliest
tropical systems, disasters can and indeed do occur due to flooding from less intense
hurricanes and tropical storms.