SUMMARY - Climate Prediction Center



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NOAA 2008 Atlantic Hurricane Season Outlook

To Be Issued 22 May7 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.This outlook is produced by scientists at the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC), National Hurricane Center (NHC), and Hurricane Research Division (HRD).

InterpretatingInterpretation of NOAA’s Atlantic seasonal hurricane outlook

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

• The outlook presents aprovides an updated forecast for general guide to the overall activity for the upcoming entire hurricane seasonseason, and is also. a guide for the remainder of the season.

• The outlookIt is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular region.

Preparedness

HA hurricane disasters can occur regardless of the overall activity can strike any area in an active year or in a slow year. 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.It only takes one hurricane (or even tropical storm) to cause a disaster.

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• This Outlook is based on predictions of large-scale climate factors known to be strong indicators of upcoming seasonal Atlantic hurricane activity.

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.

• 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.

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• The stated probabilities of season type and ranges of activity reflect the historical relationships between the predicted climate factors and seasonal activity, along with uncertainties inherent in the Outlookin this forecast product (see below).

• Because of the multiple aspects of this outlook (season type, total activity, tropical storms, hurricanes, major hurricanes), and the inherent limited predictability of seasonal Atlantic hurricane activity based on large-scale climate factors, one should not expect every predicted range to verify correctly. I am not sure why this is here. What are you trying to covey here?

Seasonal hurricane landfall predictions

NOAA does not make seasonal hurricane landfall predictions on a seasonal timescale. The climate system (can you dummy this down) does not allow one to confidently predict this far in advance the number or intensity of landfalling hurricanes, or whether a given locality or state will be impacted this seasonduring any given season. Hurricane landfalls are largely determined by the weather patterns in place as the hurricane approaches. Residents and government agencies of coastal and near coastal regions should always maintain hurricane preparedness efforts regardless of the overall seasonal outlook.

With that said I have a couple of questions. Then, why do you make these forecasts? What is the significance of these forecasts? What should people take from it? How should people use these forecasts?

Three major general sources of uncertainty in the seasonal seasonal outlooks

• 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.

1. One uncertainty reflects the basic fact that many combinations of named storms, hurricanes, and major hurricanes, can occur for a given set of climate conditions. For example, one cannot know with certainty whether a given climate signal will produce several shorter-lived hurricanes and major hurricanes, or fewer long-lived storms with greater intensity(?).

• The second uncertainty is that the climate system does not completely control the seasonal activity. Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.

• What does? Or give us a sampling of what does control or what can influence seasonal activity that you cannot predict on a seasonal timescale.

The third uncertainty is the inherent limited predictability of the climate system. Expand on this. What are you trying to say here?

2008 Updated Atlantic Hurricane Season Outlook: SummaryOUTLOOK SUMMARY

• NOAA’s The Climate Prediction Center’sNOAA’s 2008 Atlantic hurricane Hurricane sSeason oOutlook update [FIG. 1] callsindicates for 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.

• 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.

a

• Although seasons with 70% chance of an above-normal hurricane season, a 20% chance of a near-normal season, and a 10% chance of a below-normal season. See NOAA definitions of above-, near-, and below-normal seasons.

• The expected combination of climate factors this season are:

1) The continuation of conditions that have been conducive to above-normal Atlantic hurricane seasons since 1995 [Fig. 3]. Even though the last two seasons have produced near-normal Atlantic hurricane activity, there remains no indication the current active hurricane era has ended.

2) A reasonable expectation that La Niña will influence conditions during the peak months (August-October) of the hurricane season. However, there is considerable uncertainty amongst the forecast models as to how strongly La Nina will impact the Atlantic hurricane season.

• Historically, this combination of climate factors increases the likelihood of an above-normal season.

Similar climate patterns have historically produced an ACE range of 100%-200% of the median. This is the predicted ACE range for 2008. When were these similar years?

• 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:

o 14-18 named storms,

o 7-10 Hurricanes,

o 3-6 Major Hurricanes,

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

• 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 7th through November 30th).

• Historically, the expected climate patterns have been associated with a wide range of tropical storms (11-17), hurricanes (6-13), and major hurricanes (2-6) [categories 3-4-5 on the Saffir-Simpson scale]. These are the likely ranges for 2008.

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OR we can continue by stating

• Historically, 70% of similar seasons have produced 13-15 tropical storms. This is the forecasted likely range for 2008.

• Historically, 70% of similar seasons have produced 7-9 hurricanes. This is the forecasted likely range for 2008.

• Historically, 70% of similar seasons have produced 3-5 major hurricanes. This is the forecasted likely range for 2008.

• However, only XX% of the seasons have produced the combination of 13-15 TS, 7-9 H, and 3-5 MH, which is the probability of all three categories being predicted correctly this year.

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• As always the Climate Prediction Center will issue an n update to this forecastd Atlantic hurricane outlook will be issued in early August, which beginsjust prior to the peak of the Atlantic hurricane season.

Additionally, the Climate Prediction Center will issue a climate summary on the 2008 Atlantic hurricane season in late January.

DISCUSSION

1. Expected 2008 Activity- 70% chance above normal, 20% chance near normal, 10% chance below normal

This oOutlook presents is a general guide to the expected overall activity for the 2008 Atlantic hurricane seasonseason, and for the remainder (August-November) of the season. It is not a seasonal hurricane landfall forecast, and it does not 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 total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index [Fig. 42], which accounts for the collective intensity and duration of Atlantic named storms and hurricanes during the season. The ACE index is used to define above-, near-, and below-normal hurricane seasons (see Background Information).

Historically 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 464% 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 7th through November 30th), 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.

, seasons with similar climate patterns to those expected during 2008 have produced an ACE range of 100%-200% of the median. This is the predicted ACE range for 2008. This range can be satisfied even if the numbers of named storms, hurricanes, or major hurricanes fall outside their expected ranges.

Historically, XX% of seasons with similar climate patterns have been above-normal (ACE > 117% of the median), and 30% of these have been extremely active with ACE exceeding the175% baseline used to define a hyperactive season (Goldenberg et al. 2001). Conversely, yy% of those seasons have been near-normal (ACE between 65-117% of the median), and zz% have been below-normal (ACE below 65% of the median).

Accounting for uncertainties in the strength of La Niña, and the possibility of lesser (and unpredictable) factors also influencing the seasonal activity, this outlook indicates a 70% chance of an above-normal season, a 20% chance of a near-normal season, and a 10% chance of a below-normal season.

Historically, the expected climate conditions have been associated with a wide range of tropical storms (11-17), hurricanes (6-13), and major hurricanes (2-6) [categories 3-4-5 on the Saffir-Simpson scale], which is reflected in this outlook.

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Alternative wording:

Of these seasons, 70% have produced 13-15 tropical storms, while 70% have produced 7-9 hurricanes and 75% have produced 3-5 major hurricanes. These are the predicted likely ranges for 2008. However, only XX% of the seasons have produced the combination of 13-15 TS, 7-9 H, and 3-5 MH, which is the probability of all three categories being predicted correctly this year.

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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@

Dr. Jae Schemm, Meteorologist; Jae.Schemm@

Dr. Tingzhuang Yan, Meteorologist, Tingzhuang.Yan@

National Hurricane Center

Eric Blake, Hurricane Specialist; Eric.S.Blake@.

Todd Kimberlain, Meteorologist; Todd.Kimberlain@

Dr. Chris Landsea, Meteorologist; Chris.Landsea@

Dr. Richard Pasch, Hurricane Specialist; Richard.J.Pasch@

Hurricane Research Division

Stanley Goldenberg, Meteorologist; Stanley.Goldenberg@

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 Accumulated Cyclone Energy (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 Accumulated Cyclone Energy (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: An ACE index value well above 103 x 104 kt2 (corresponding to 117% of the median ACE value or 110% of the mean), or an ACE value slightly above 103 x 104 kt2 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: An ACE index value in the range 66-103 x 104 kt2 (corresponding to 75%-117% of the median or 71%-110% of the mean), or an ACE index value slightly above 103 x 104 kt2 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: An ACE index value below 66 x 104 kt2, 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 104 kt2), and the median value is (87.5 x 104 kt2).

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.

| | | | | | | |

|Season |Mean # of |Range of |Mean # of |Range of Hurricanes|Mean # of Major |Range of Major |

|Type |Tropical storms |Named storms |Hurricanes | |Hurricanes |Hurricanes |

| | | | | | | |

|Above-Normal |13.7 |10 to 28 |8.6 |6 to 15 |4.5 |2 to 8 |

| | | | | | | |

|Near-Normal |9.4 |6 to 14 |5.6 |4 to 8 |1.9 |1 to 3 |

| | | | | | | |

|Below-Normal |6.9 |4 to 9 |3.7 |2 to 5 |1.1 |0 to 2 |

| | | | | | | |

|All |10.3 |4 to 28 |6.2 |2 to 15 |2.7 |0 to 8 |

|Seasons | | | | | | |

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 States |Caribbean Islands |

| | | |

| |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 |

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