1. Overall activity: Definition
The term "overall activity" reflects a combination of frequency, intensity,
and duration of Atlantic tropical storms and hurricanes. Measures of overall activity are
a much better indicator, compared to the number of tropical storms or hurricanes alone, of
whether a particular season is above-average. We use a formulation of overall activity
known as the "Accumulated Cyclone Energy [ACE] index", discussed in the
Background Information section, as the basis for defining overall activity in a given
season, and for verifying our hurricane outlooks. Using the ACE index as the basis for
measuring overall activity, the historical record indicates that 94% of active seasons
have featured at least eleven tropical storms, while 70% of active seasons have featured
seven or more hurricanes, and 82% of active seasons have featured three or more major
hurricanes [categories 3-4-5 on the Saffir-Simpson scale]. Only 60% of all active seasons
feature the combination of at least 11 tropical storms, 7 hurricanes and 3 major
hurricanes. Thus, 40% of active years do not satisfy this 11-7-3 criterion, indicating
that this criterion is not the sole indicator of whether a season is defined as
"active."
2. Expected Overall Activity - above-average
Historically, when the existing global pattern of atmospheric anomalies was present at
this time of the year, nearly 80% of the Atlantic hurricane seasons featured average or
above-average activity, with just over 30% of the seasons being above-average. However,
when compared to decades in which global ocean temperatures also favored an active
hurricane era (such as the 1950's and 1960's and the period 1995 to the present), this
probability increases to a 60% chance of an above-average hurricane season. With this
forecast of an above-average hurricane season, it is reasonable to expect 11 or more
tropical storms, 7 or more hurricanes and 3 or more major hurricanes. However, the season
can certainly feature above-average activity without all three of these criteria being
met. Also, given the expected variability in the weather patterns over the North Atlantic,
we do not anticipate that the 2000 hurricane season to be as active as either of the
extremely busy 1998 or 1999 seasons. While the likelihood of an above-average season is
considerable, there is still a 30% chance of near-average activity during the upcoming
August - November period. However, there is a comparatively small chance (10%) of a
relatively inactive season.
3. Expected Preferred region of storm formation: the tropical Atlantic Ocean and the
Caribbean Sea
Current conditions remain conducive to considerable activity in the area from 10-20N
between the west coast of Africa and central America: this region encompasses the tropical
Atlantic and the Caribbean Sea, and is referred to as the main development region. On
average 7 tropical storms form in the main development region during active years,
compared to an average of 3 in near-average years and 2 in inactive years. Systems which
develop in the tropical Atlantic generally form from African easterly waves, and are
sometimes referred to as "Cape Verde" storms. These systems are much more likely
to be long-lived and to become major hurricanes, especially in active years, since they
remain in a very conducive environment for prolonged periods. The 2000 hurricane season is
expected to feature several Cape Verde storms, which generally move westward and pose a
significant threat to the Caribbean Islands and the coastal United States.
4. Expected Hurricane tracks and landfall: Increased threat to the countries of the
Caribbean and coastal United States
Storms that form in the main development region (see item 3 above) tend to move
westward toward the Caribbean Sea as they strengthen. Many of these systems then head
northwestward and threaten the coastal United States. Thus, the countries of the Caribbean
(particularly the northern islands) and both the Gulf and Atlantic coasts of the United
States are at an above-average risk of experiencing a hurricane in the upcoming months. In
a typical active year, the Caribbean and the United States each experience an average of
2-3 hurricane strikes. For the Caribbean, this frequency of hurricane landfall is much
larger than that observed in inactive years.
5. Expected Atmospheric conditions:
Many of the atmospheric and oceanic factors predicted in the 10 May 2000 Outlook to be
in place at this time are well established and are expected to persist throughout the
season. Other factors are not yet in place, and thus represent an important source of
uncertainty for this outlook. The factors that are currently suggesting an active season
are 1) a structure and location of the African easterly jet which is more efficient for
providing energy to developing tropical systems as they propagate westward from the
African coast, 2) low surface air pressure across the tropical Atlantic and Caribbean, and
3) a moist and convectively unstable atmosphere over the tropical Atlantic Ocean. There
are two key factors that represent the primary sources of uncertainty for this Outlook,
since they will ultimately help to determine whether the season is active or near average.
These factors are the vertical wind shear (i.e. the change of wind speed and direction
from near the ocean surface up to an altitude of approximately 8 miles [12 km]) and
Atlantic sea-surface temperatures. The vertical wind shear is a fundamental component of
tropical storm and hurricane development. Tropical storms require low vertical wind shear
for development, and they do not form or intensify in regions of high vertical wind shear.
The vertical wind shear over the main development region is uncertain at this time because
of the increased variability of the global-scale circulation patterns, in response to a
reduced persistence and reduced strength of the La Nina-related tropical rainfall
patterns. The tropical rainfall establishes the global-scale circulation patterns that
subsequently control the winds and the vertical wind shear over the North Atlantic and
Caribbean. At present, the tropical rainfall patterns are not as strong or persistent as
was expected earlier in the year. As a result, there has been a modification of the
upper-level winds over the main development region, which has caused stronger vertical
wind shear at times across the tropical Atlantic and Caribbean Sea than was previously
anticipated. The second key uncertainty at this time is the sea-surface temperatures of
the tropical Atlantic Ocean and Caribbean Sea. Warmer ocean temperatures tend to enhance
hurricane frequency and intensity. Currently, the sea-surface temperatures in the main
development region are near-average. If these conditions persist, they would not
contribute to an active season.
6. Note on the lack of tropical storms and hurricanes during June and July
No tropical storms or hurricanes were observed over the Atlantic basin in June and
July. This lack of activity is not uncommon, since conditions are typically not conducive
for development at that time of the year. Historically, 85% of tropical storms, 90% of
hurricanes and 95% of major hurricanes occur after July. Moreover, the inactivity so far
this year is not a reliable indicator of conditions during the remainder of the season.
For example, the 1999 hurricane season featured only one tropical storm before mid-August,
while the remaining portion of the season was very active. Conversely, an active June and
July can often be associated with an inactive season. This was last observed in the
inactive 1997 season, when four tropical storms (two of which became hurricanes) formed by
1 August, followed by only three tropical storms (one of which became a hurricane) during
the traditional peak in activity.
1) This Outlook should be used as a guide of overall expected activity for the Atlantic
basin. No outlook can give certainty as to whether a particular locality will be impacted
by a tropical storm or hurricane in any given year. Residents and government agencies of
coastal or near-coastal regions should always maintain hurricane preparedness
efforts regardless of the overall outlook for a given year. 2) 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.
Also, increased activity in a given year does not automatically mean increased
storm-related damage. 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. 4) Our expectation of above-average activity during 2000
is not based on global warming associated with elevated carbon dioxide levels. 5) If the
hurricane season, instead, is near-average overall, one should be reminded that there
could still be several potentially damaging hurricanes. Examples of
"near-average" years that caused extensive damage and numerous fatalities
include 1960 (Hurricane Donna), 1979 (Hurricanes David and Frederic) and 1985 (Hurricanes
Elena, Gloria and Juan).
ACKNOWLEDGMENTS
This outlook is not designed to compete with hurricane outlooks issued by groups
outside of NOAA. We gratefully acknowledge the pioneering research of Dr. William Gray and
others, which has significantly increased scientific understanding of the association
between the various climate factors (particularly the El Nino/ La Nina 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.
CONTACTS
Dr. Lixion Avila, Meteorologist, National Hurricane Center; ph: 305-229-4470; lixion@nhc.noaa.gov
Dr. Gerald Bell, Meteorologist, Climate Prediction Center: ph: 301-763-8000 x 7536; gbell@ncep.noaa.gov
Dr. Muthuvel Chelliah, Physical Scientist, Climate Prediction Center: ph: 301-763-8000
x 7546; mchelliah@ncep.noaa.gov
Mr. Stanley Goldenberg, Meteorologist, Hurricane Research Division: ph: 305-361-4362;goldenberg@aoml.noaa.gov
Dr. Vernon Kousky, Meteorologist, Climate Prediction Center:
ph: 301-763-8000 x 7539; vkousky@ncep.noaa.gov
Dr. Christopher Landsea, Meteorologist, Hurricane Research Division: ph: 305-361-4357; landsea@aoml.noaa.gov
Dr. Richard Pasch, Meteorologist, National Hurricane Center; ph: 305-229-4411; richard@nhc.noaa.gov
www.nws.noaa.gov 
