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) indicates an equal 40% probability of either an above-normal or
near-normal 2001 Atlantic hurricane season, and a 20% chance of a below-normal season. This
prediction calls for slightly more hurricane activity than was called for in the pre-season
NOAA Outlook issued in May. The current outlook represents the mixed combination of
ENSO-neutral conditions and ongoing decadal-scale anomalies that are more conducive to an
The 2001 hurricane season is expected to
feature normal to slightly above-normal levels of activity. On average, similar
seasons have featured 2-3 landfalling hurricanes in the continental United
States, and 1-2 landfalling hurricanes in the region around the Caribbean Sea.
The 2001 season is not expected to be hyper-active, as was observed during four
of the last six seasons (1995, 1996, 1998, 1999). However, it is expected to be
more active than most of the relatively quiet 1970-1994 period.
1. Expected Level of Overall Activity - Normal to slightly above normal
Given the current atmospheric and oceanic
conditions (see also section 2 below) we expect a 40% chance of an above-normal hurricane
season, a 40% chance of a near-normal season, and a 20% chance of a below-normal season.
These probabilities have changed from the May Outlook, which indicated a 50% chance of a
near-normal season, and a 25% chance of either an above-normal or below-normal season.
Based on the “Accumulate Cyclone Energy” [ACE] index, which measures overall seasonal
activity (see Background Information), we expect the total 2001 seasonal activity to fall
between 100% -140% of the long-term median value. This range is higher than the 76% - 120%
of median called for in the May outlook.
The historical record indicates that 58% of
comparable seasons featured 9-12 tropical storms, 67% featured 6-8 hurricanes, and 67%
featured 2-4 major hurricanes [categories 3-4-5 on the Saffir-Simpson scale]. Only 47%
of the comparable seasons featured the combination of 9-12 tropical storms, 6-8 hurricanes
and 2-4 major hurricanes. Thus, while it is reasonable to expect 9-12 tropical storms, 6-8
hurricanes and 2-4 major hurricanes this season, the season can certainly feature slightly
above-normal levels of activity without all three of these criteria being met.
Based on past historical data similar
seasons have averaged 2-3 landfalling hurricanes in the continental United States
and 1-2 hurricanes in the region around the Caribbean Sea.
2. Expected Climate Conditions- Decadal-scale
global climate patterns expected to dominate portions of the tropical North
Atlantic: La Niña/ El Niño not expected to play important role
The prominent climate signals guiding the
August Outlook are the expected continuation of ENSO-neutral conditions and
ongoing decadal anomalies. During the peak (August-October) months of the
hurricane season the regional decadal anomalies feature reduced vertical wind
shear and above-average SSTs over key portions of the main development region,
and increased cyclonic vorticity along the equatorward flank of the African
Easterly Jet. All of these conditions are now in place, and all favor
above-average hurricane activity by contributing to enhanced African easterly
wave development. Similar anomalies contributed to the active decades of the
1950's and 1960's, and to the strong upturn in hurricane activity since 1995.
These regional anomalies are associated
with global decadal patterns of enhanced tropical rainfall, and warmer-than-average
sea-surface temperatures (SST) over the tropical North Atlantic in association
with the Atlantic multi-decadal mode (Goldenberg et al. 2001; Science).
The above regional anomalies have strengthened considerably since mid-May and have become more
conducive to hurricane formation and intensification, thus suggesting a higher probability of
an above-normal season than was indicated in the May NOAA Atlantic Hurricane Outlook. Conversely, the persistence of ENSO-neutral conditions since mid-May, and the correspondingly
lower probability that an El Niño episode will develop and adversely affect 2001 hurricane
activity, suggests a reduced likelihood of a below-normal season compared to that indicated in
the May outlook.
A secondary climate factor, easterly winds
in the lower stratosphere, is also expected to provide a slightly suppressing
influence on overall Atlantic hurricane activity this season.
Based on the historical record, this
combination of climate conditions indicates a 40% chance of above-normal activity
for the 2001 hurricane season, a 40% chance of normal activity, and a 20% chance
of below-normal activity.
1) This Outlook represents our best
estimate for the expected overall level of activity for the Atlantic basin.
No outlook can give certainty as to whether or not a particular locality will be
impacted by a 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 hurricanes hitting sparsely populated areas or, of course, not making landfall at
all. Because of this, hurricane-spawned disasters can occur even in years with normal (or
below-normal) levels of activity. Examples of years with just 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 levels of activity.
This outlook is not designed to compete with
hurricane outlooks issued by groups outside NOAA. We gratefully acknowledge the
pioneering research of Dr. William Gray and others, which have significantly
increased scientific understanding of the links between various climate factors
(particularly the El Niño/ La Niña cycle) and the atmospheric circulation features
that affect Atlantic 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 hurricane activity.
Dr. Lixion Avila, Meteorologist, National Hurricane Center; ph: 305-229-4410; email@example.com
Dr. Gerald Bell, Meteorologist, Climate Prediction Center; ph: 301-763-8000 x7536; firstname.lastname@example.org
Dr. Muthuvel Chelliah, Physical Scientist, Climate Prediction Center; ph: 301-763-8000 x 7546; email@example.com
Mr. Stanley Goldenberg, Meteorologist, Hurricane Research Division; ph: 305-361-4362; firstname.lastname@example.org
Dr. Christopher Landsea, Meteorologist, Hurricane Research Division; ph: 305-361-4357; email@example.com
Dr. Richard Pasch, Meteorologist, National Hurricane Center; ph: 305-229-4411; firstname.lastname@example.org