**NOAA 2008 Atlantic Hurricane Season Outlook**

**Issued 7 August 2008**

The 2008 Atlantic hurricane season outlook is an official product of the
National Oceanic and Atmospheric Administration (NOAA) Climate
Prediction Center (CPC), and is produced in collaboration with
scientists from the NOAA National Hurricane Center (NHC) and Hurricane
Research Division (HRD). The Atlantic hurricane region includes the
North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico.

**Interpretation of NOAA's Atlantic seasonal hurricane outlook**

-   The August outlook coincides with the onset of the peak months
    (August-October) of the Atlantic hurricane season.

-   The outlook provides an updated forecast for the entire hurricane
    season, and is also a guide for the remainder of the season.

-   The outlook is [not]{.underline} a seasonal hurricane landfall
    forecast, and it [does not]{.underline} imply levels of activity for
    any particular region.

*Preparedness*

> Hurricane disasters can occur regardless of the overall
> activity. Residents, businesses, and government agencies of coastal
> and near-coastal regions should prepare for every hurricane season
> regardless of the seasonal outlook. NOAA, FEMA, the NHC, Small
> Business Administration, and the American Red Cross all provide
> important hurricane preparedness information on their web sites. It
> only takes one hurricane (or even a tropical storm) to cause a
> disaster.

*NOAA does not make seasonal hurricane landfall predictions*

> NOAA does not make seasonal hurricane landfall predictions. Hurricane
> landfalls are largely determined by the weather patterns in place as
> the hurricane approaches, which are not predictable more than 5-7 days
> in advance.

*Nature of this Outlook and the "likely" ranges of activity*

-   This outlook is probabilistic, not deterministic. New this year, we
    are providing probabilities for the stated likely ranges of named
    storms, hurricanes, major hurricanes, and Accumulated Cyclone Energy
    (ACE).

-   The outlook is based on 1) predictions of large-scale climate
    factors known to be strong indicators of seasonal Atlantic hurricane
    activity, and 2) an analysis of past seasons with similar
    early-season activity to this year.

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-   These likely ranges do not encompass the total ranges of activity
    seen in past seasons having similar climate conditions to those
    expected this year, but are simply the most likely ranges.

*Three general sources of uncertainty in the seasonal outlook*

-   The prediction of El Niño and La Niña impacts is an ongoing
    scientific challenge facing climate scientists today.

-   The numbers of named storms and hurricanes can sometimes vary
    considerably for the same set of climate conditions. One cannot know
    with certainty whether a given climate signal will be associated
    with several short-lived storms or fewer longer-lived storms with
    greater intensity.

-   Weather patterns that are unpredictable on seasonal time scales can
    sometimes develop and last for weeks or months, possibly affecting
    seasonal hurricane activity.

**2008 Updated Atlantic Hurricane Season Outlook: Summary**

-   NOAA's 2008 Atlantic Hurricane Season Outlook update \[FIG. 1\]
    indicates an 85% probability of an above-normal hurricane season, a
    10% chance of a near-normal season, and a 5% chance of a
    below-normal season. See NOAA definitions \[Joe: link to "background
    information"\] of above-, near-, and below-normal seasons. The
    Atlantic hurricane region includes the North Atlantic Ocean, the
    Caribbean Sea, and the Gulf of Mexico.

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-   The conducive atmospheric and oceanic conditions \[Fig. 2\]
    predicted in our May outlook are now in place over the tropical
    Atlantic. These conditions, combined with strong early-season
    activity \[Fig. 3\], yield increased confidence that the 2008 season
    will be above normal. We expect 2008 to be the tenth above-normal
    season since the current active hurricane era began in 1995.

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-   Although seasons with similar conditions to those now in place and
    seasons with similar early-season activity have generally been
    above-normal, they have featured a wide range of activity. Allowing
    for uncertainties, we estimate a 67% probability for each of the
    following seasonal (June-November) ranges, all of which are above
    average:

    -   14-18 named storms,

    -   7-10 Hurricanes,

    -   3-6 Major Hurricanes,

    -   An ACE range \[Fig. 4\] of 140%-230% of the median

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-   These likely seasonal ranges have been observed in about two-thirds
    (67%) of past seasons having similar climate conditions and similar
    early-season activity to this year. They do not encompass the total
    range of activity seen in those past seasons.

-   To date, the 2008 Atlantic hurricane season has produced five named
    storms, two hurricanes, and one major hurricane.

-   This means that we expect considerable activity during the remainder
    of the 2008 season \[Fig. 3\], with an additional 9-13 named storms,
    5-8 hurricanes and 2-5 major hurricanes likely during the remainder
    of the season (August 7^th^ through November 30^th^).

**DISCUSSION**

1\. Expected 2008 Activity

This Outlook is a general guide to the expected overall activity for the
2008 Atlantic hurricane season, and for the remainder (August-November)
of the season. It is [not]{.underline} a seasonal hurricane landfall
forecast, and it [does not]{.underline} imply levels of activity for any
particular area.

The 2008 outlook largely reflects the historical distribution of
activity for the dominant climate factors now present, as well as the
historical distribution of activity for seasons having significant
June-July activity. Two uncertainties in the outlook are the
contribution from lingering La Niña impacts, and the possibility that
other unpredictable factors could also influence the season.

As predicted in our May outlook, the atmospheric and oceanic conditions
typically associated with an above-normal season are now in place \[Fig.
2\], and are expected to produce considerable tropical storm and
hurricane activity during the remainder of the 2008 season. These
conditions are related to two main climate factors: 1) The multi-decadal
signal, which has been conducive to above-normal Atlantic hurricane
activity since 1995 \[Fig. 4\], and which includes above-normal Atlantic
sea-surface temperatures \[Fig. 5\], and 2) a lingering La Niña signal
as reflected in Pacific Ocean temperatures \[Fig. 6\], convection and
winds \[Fig. 7\].

Another factor guiding the outlook is the strong early-season activity
\[Fig. 3\] over the tropical Atlantic. This activity is a good indicator
of an above normal 2008 season.

An important measure of the seasonal activity is NOAA's
[A]{.underline}ccumulated [C]{.underline}yclone [E]{.underline}nergy
(ACE) index \[Fig. 4\], which accounts for the collective intensity and
duration of named storms and hurricanes during the season. The lower
threshold of ACE for an above-normal season is 117% of the median. The
outlook indicates a likely (67% chance) ACE range of between 140% and
230% of the median. The early-season activity has already produced an
ACE value of 46% of the median.

The outlook calls for a 67% chance for each of the following seasonal
ranges \[Fig. 1\]: 14-18 Named Storms, 7-10 Hurricanes, and 3-6 Major
Hurricanes. All of these ranges are above the long-term averages of 11
named storms, six hurricanes, and two major hurricanes. Note that the
predicted range for ACE can be satisfied even if these predicted numbers
of named storms, hurricanes, and major hurricanes fall outside the
likely ranges.

The 2008 hurricane season has already produced 5 named storms, two
hurricanes, and one major hurricane. Therefore, for the remainder of the
season \[Fig. 3\] (August 7^th^ through November 30^th^), we expect (67%
chance) an additional 9-13 Named Storms, 5-8 Hurricanes, and 2-5 Major
Hurricanes.

Based on the ACE prediction, and on the expected above-average numbers
of named storms, hurricanes, and major hurricanes, there is an 85%
chance the 2008 season will be above normal.

Promising new climate models that predict seasonal tropical cyclone
activity in the Atlantic are also suggesting an active season. These
forecasts are consistent with our assessment of the current and evolving
oceanic and atmospheric conditions for the 2008 Atlantic hurricane
season.

2\. Expected Climate Conditions -- **Active hurricane era, lingering La
Niña impacts**

*a. Expected continuation of active Atlantic hurricane era*

Atlantic hurricane seasons exhibit prolonged periods lasting decades
\[Fig. 4\] of generally above-normal or below-normal activity. These
fluctuations in hurricane activity result almost entirely from
differences in the number of hurricanes and major hurricanes forming
from tropical storms first named in the Main Development Region (MDR),
which spans the tropical Atlantic Ocean and Caribbean Sea (Goldenberg et
al. 2001).

The current active hurricane era began in 1995. Hurricane seasons during
1995-2007 have averaged 14.5 named storms, 8 hurricanes, and 4 major
hurricanes, with an average ACE index of 167% of the median. NOAA
classifies nine of the thirteen seasons since 1995 as above normal, with
seven being hyperactive (ACE \> 175% of median). Only four seasons since
1995 have not been above normal. These include three El Niño years
(1997, 2002, and 2006) and the 2007 season.

This high level of activity since 1995 contrasts sharply to the
1971-1994 period of generally below-normal hurricane seasons (Goldenberg
et al. 2001), which averaged only 8.5 named storms, 5 hurricanes, and
1.5 major hurricanes with an average ACE index of only 75% of the
median. One-half of those seasons were below normal, only three were
above normal (1980, 1988, 1989), and none were hyperactive.

The regional atmospheric circulation anomalies that contribute to these
long-period fluctuations in hurricane activity are strongly linked to
the tropics-wide multi-decadal signal (Bell and Chelliah, 2006). A
change in phase of the multi-decadal signal \[Fig. 8\] accounts for the
transition in 1995 from the below-normal to the above normal era. The
conditions associated with the multi-decadal signal are a major factor
guiding the 2008 outlook. Three key features of this signal \[Fig. 9\]
now in place include: 1) a stronger West African monsoon system \[Fig.
10\], 2) below-average convection in the Amazon Basin, and 3) ongoing
warmer than average Atlantic Ocean temperatures \[Fig. 5\].

Other regional features \[Fig. 2\] of the multi-decadal signal currently
affecting the Atlantic include 1) an amplified ridge \[Fig. 11\] at
upper levels across the central and eastern subtropical North Atlantic,
2) reduced vertical wind shear \[Fig. 2\] in the deep tropics over the
central North Atlantic, which results from an expanded area of easterly
wind anomalies in the upper atmosphere (green arrows) and weaker
easterly trade winds (westerly anomalies) in the lower atmosphere (dark
blue arrows), and 3) weaker easterly winds also evident \[Fig. 12\] in
the middle atmosphere near Africa, resulting in a configuration of the
African easterly jet that favors hurricane development from tropical
waves moving westward from the African coast. These conditions have
already been responsible for the high levels of early season activity,
and they are expected to persist through the season.

*b. Lingering La Niña influence*

In the latest ENSO Diagnostics Discussion released 7 August 2008, NOAA
forecasters state that although La Niña has ended, lingering La Niña
impacts are evident in the patterns of tropical convection and winds
\[Fig. 7\] over the central and western Pacific. This evolution is
consistent with NOAA predictions in May. As discussed by Gray (1984), La
Niña conditions are typically associated with higher Atlantic hurricane
activity, and El Niño with lower hurricane activity. The combination of
lingering La Niña conditions and an active hurricane era increases the
probability of an above-normal season.

*c. Atlantic Sea-surface temperatures* (SSTs)

Above-average temperatures \[Fig. 5\] returned to the tropical Atlantic
during the summer. For the entire main development region, sea-surface
temperatures during July averaged +0.3°C, which is consistent with an
above-normal season, the ongoing active hurricane era, and longer-term
trends.

# NOAA FORECASTERS

### Climate Prediction Center

Dr. Gerry Bell, Meteorologist;
[Gerry.Bell@noaa.gov](mailto:gerry.bell@noaa.gov)

Dr. Jae Schemm, Meteorologist; <Jae.Schemm@noaa.gov>

Dr. Tingzhuang Yan, Meteorologist, [Tingzhuang.Yan@noaa.gov]{.underline}

### National Hurricane Center

Eric Blake, Hurricane Specialist; <Eric.S.Blake@noaa.gov>.

Todd Kimberlain, Meteorologist; <Todd.Kimberlain@noaa.gov>

Dr. Chris Landsea, Meteorologist;
[Chris.Landsea@noaa.gov](mailto:chris.Landsea@noaa.gov)

Dr. Richard Pasch, Hurricane Specialist; <Richard.J.Pasch@noaa.gov>

### 

### Hurricane Research Division

Stanley Goldenberg, Meteorologist; <Stanley.Goldenberg@noaa.gov>

## REFERENCES

Bell, G. D., and M. Chelliah, 2006: Leading tropical modes associated
with interannual and multi-decadal fluctuations in North Atlantic
hurricane activity. *J. Climate*. **19**, 590-612.

Bell, G. D., and Co-authors 2007: The 2006 Atlantic Hurricane Season: A
Climate Perspective. *State of the Climate in 2006.* A. M. Waple and J.
H. Lawrimore, Eds. *Bull. Amer. Meteor. Soc*., **88**, S1-S78.

Bell, G. D., and Co-authors 2008: The 2007 Atlantic Hurricane Season: A
Climate Perspective. *State of the Climate in 2007.* A. M. Waple and J.
H. Lawrimore, Eds. *Bull. Amer. Meteor. Soc*., **89**, S1-S78.

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.

## BACKGROUND INFORMATION: NORTH ATLANTIC HURRICANE SEASON

The North Atlantic hurricane season officially runs from 1 June to 30
November. The vast majority of tropical storm and hurricane activity
typically occurs during the August-October peak of the hurricane
season.  An average hurricane season features approximately 11 named
storms (maximum sustained surface winds between 39-73 mph), six
hurricanes (maximum sustained surface winds of at least 74 mph) and two
major hurricanes (maximum sustained surface winds exceeding 110 mph,
categories 3-5 on the Saffir-Simpson scale). 

***Measuring total seasonal activity: The [A]{.underline}ccumulated
[C]{.underline}yclone [E]{.underline}nergy (ACE) index***

The phrase \"total seasonal activity\" refers to the collective
intensity and duration of Atlantic named storms and hurricanes occurring
during a given season. The measure of total seasonal activity used by
NOAA is called the [A]{.underline}ccumulated [C]{.underline}yclone
[E]{.underline}nergy (ACE) index \[Fig 4\]. The ACE index is a wind
energy index, defined as the sum of the squares of the maximum sustained
surface wind speed (knots) measured every six hours for all named
systems while they are at least tropical storm strength.

NOAA uses the ACE index, combined with the numbers of named storms,
hurricanes, and major hurricanes, to categorize North Atlantic hurricane
seasons as being above normal, near normal, or below normal.

NOAA's Atlantic hurricane season classifications

*[Above-normal season]{.underline}*: An ACE index value well above 103 x
10^4^ kt^2^ (corresponding to 117% of the median ACE value or 110% of
the mean), or an ACE value slightly above 103 x 10^4^ kt^2^ combined
with at least two of the following three parameters being above the
long-term average: number of named storms, hurricanes, and major
hurricanes. 

*[Near-normal season]{.underline}*: An ACE index value in the range
66-103 x 10^4^ kt^2^ (corresponding to 75%-117% of the median or
71%-110% of the mean), or an ACE index value slightly above 103 x 10^4^
kt^2^ but with less than two of the following three parameters being
above the long-term average: numbers of named storms, hurricanes, and
major hurricanes.

*[Below-normal season]{.underline}*: An ACE index value below 66 x 10^4^
kt^2^, corresponding to less than 75% of the median or 71% of the mean.

The 1951-2000 base period is used in the above classifications, where
the *mean value* of the ACE index (93.2 x 10^4^ kt^2^), and the *median
value* is (87.5 x 10^4^ kt^2^).

The following table shows the 1951-2005 seasonal means and ranges for
named storms, hurricanes and major hurricanes during above normal, near
normal, below normal, and all Atlantic hurricane seasons. This table
highlights the marked differences in activity between the three season
types.

+--------+--------+-------+----------+----------+----------+---------+
| **Se   | **Mean | **    | **Mean   | **Range  | **Mean   | **Range |
| ason** | \# of  | Range | \# of    | of       | \# of    | of      |
|        | Tr     | of**  | Hurr     | Hurr     | Major    | Major   |
| **     | opical |       | icanes** | icanes** | Hurr     | Hurri   |
| Type** | st     | **    |          |          | icanes** | canes** |
|        | orms** | Named |          |          |          |         |
|        |        | sto   |          |          |          |         |
|        |        | rms** |          |          |          |         |
+--------+--------+-------+----------+----------+----------+---------+
| *A     | 13.7   | 10 to | 8.6      | 6 to 15  | 4.5      | 2 to 8  |
| bove-N |        | 28    |          |          |          |         |
| ormal* |        |       |          |          |          |         |
+--------+--------+-------+----------+----------+----------+---------+
| *      | 9.4    | 6 to  | 5.6      | 4 to 8   | 1.9      | 1 to 3  |
| Near-N |        | 14    |          |          |          |         |
| ormal* |        |       |          |          |          |         |
+--------+--------+-------+----------+----------+----------+---------+
| *B     | 6.9    | 4 to  | 3.7      | 2 to 5   | 1.1      | 0 to 2  |
| elow-N |        | 9     |          |          |          |         |
| ormal* |        |       |          |          |          |         |
+--------+--------+-------+----------+----------+----------+---------+
| *All*  | 10.3   | 4 to  | 6.2      | 2 to 15  | 2.7      | 0 to 8  |
|        |        | 28    |          |          |          |         |
| *Se    |        |       |          |          |          |         |
| asons* |        |       |          |          |          |         |
+--------+--------+-------+----------+----------+----------+---------+

The following table shows the 1951-2005 seasonal means and ranges for
landfalling hurricanes in the continental United States and Caribbean
Islands (including the Caribbean Sea and surrounding land areas) during
above normal, near normal, below normal hurricane seasons. This table
highlights the marked differences for landfalling hurricanes between the
three season types.

  ----------------------- ----------------------- -----------------------
  **Season Type**         **Continental United    **Caribbean Islands**
                          States**                

                          **Mean Range**          **Mean Range**

  *Above-Normal*          2.5 0-6                 2.5 0-6

  *Near-Normal*           1.8 0-6                 0.9 0-3

  *Below-Normal*          0.9 0-2                 0.3 0-1
  ----------------------- ----------------------- -----------------------