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NOAA Atlantic Hurricane Season Outlook Update

Issued: 8 August 2013

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Atlantic Hurricane Outlook & Seasonal Climate Summary Archive
 
 

The 2013 Atlantic hurricane season outlook is an official product of the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC). The outlook is produced in collaboration with hurricane experts from the NOAA’s National Hurricane Center (NHC) and Hurricane Research Division (HRD). The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

Interpretation of NOAA’s Atlantic hurricane season outlook
This outlook is a general guide to the expected overall activity for the entire hurricane season. It is not a hurricane landfall forecast, and does not predict levels of activity for any particular region.

Preparedness
Hurricane disasters can occur whether a season is active or relatively quiet. It only takes one hurricane (or tropical storm) to cause a disaster. Residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare for every hurricane season regardless of this, or any other, seasonal outlook. NOAA, the Federal Emergency Management Agency (FEMA), the National Hurricane Center (NHC), the Small Business Administration, and the American Red Cross all provide important hurricane preparedness information on their web sites.

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 only reasonably predictable when the storm is within several days of making landfall.

Nature of this Outlook and the “likely” ranges of activity
This outlook is probabilistic, meaning the stated “likely” ranges of activity have a certain likelihood of occurring. The seasonal activity is expected to fall within these ranges in 7 out of 10 seasons with similar climate conditions and uncertainties to those expected this year. They do not represent the total possible ranges of activity seen in past similar years.

This outlook is based on 1) observations and predictions of large-scale climate factors known to influence seasonal hurricane activity, and 2) climate models that directly predict seasonal hurricane activity.

Sources of uncertainty in this seasonal outlook
1. Predicting El Niño and La Niña (also called the El Niño-Southern Oscillation, or ENSO) events and their impacts is an ongoing scientific challenge facing climate scientists today.

2. Many combinations of named storms and hurricanes can occur for the same general set of climate conditions. For example, one cannot know with certainty in a given year whether the climate signals will be associated with more longer-lived but weaker storms, or with fewer shorter-lived but stronger storms.

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

2013 Update Atlantic Hurricane Season Outlook: Summary

NOAA’s updated 2013 Atlantic Hurricane Season Outlook continues to call for an above-normal season, with the possibility that the season could be very active. The outlook indicates a 70% chance of an above-normal season, a 25% chance of a near-normal season, and only a 5% chance for a below-normal season. See NOAA definitions of above-, near-, and below-normal seasons, which have been slightly modified from previous years. The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

As predicted in May, atmospheric and oceanic anomalies across the tropical Atlantic Ocean and Caribbean Sea (called the Main Development Region, MDR) are now conducive to an above-normal Atlantic hurricane season. These conditions are expected to persist throughout the peak months (August-October) of the hurricane season in association with the tropical multi-decadal signal, which has contributed to the high activity era that began in 1995. This signal is linked to above-average sea surface temperatures in the MDR and to an enhanced west African monsoon, both of which are now in place.

The presence of two named storms in the deep tropical Atlantic during June-July reinforces the expectation for an above-normal season. Historically, years with early-season activity in this region have a high likelihood of being above-normal, with many also being very active (i.e., hyper-active).

A third factor for the season is the likely continuation of ENSO-neutral conditions. There is only a low probability (19%) that La Niña will develop and further enhance the activity, and an even lower probability (8%) that El Niño will develop and suppress the activity.

Based on the current and expected conditions, combined with model forecasts, we estimate a 70% probability for each of the following ranges of activity for the entire 2013 Atlantic hurricane season:

  • 13-19 Named Storms
  • 6-9 Hurricanes
  • 3-5 Major Hurricanes
  • Accumulated Cyclone Energy (ACE) range of 120%-190% of the median.

The seasonal activity is expected to fall within these ranges in 70% of seasons with similar climate conditions and uncertainties to those expected this year. These ranges do not represent the total possible ranges of activity seen in past similar years.

The expected ranges are centered well above the official NHC 1981-2010 seasonal averages of 12 named storms, 6 hurricanes, and 3 major hurricanes.

Activity to Date:
Four tropical storms (Andrea, Barry, Chantal, and Dorian) have formed in the Atlantic basin to date, with Chantal and Dorian forming in the deep tropical Atlantic. Significant activity is expected for the remainder of the season, with an additional 9-15 named storms likely, of which 6-9 are expected to become hurricanes with 3-5 reaching major hurricane status.

Changes from the pre-season outlook issued May 23rd:
All of the predicted ranges of activity have been lowered and narrowed slightly from the May outlook. Three reasons for these changes to the ranges are: 1) No hurricanes or major hurricanes formed during June and July; 2) The probability of La Niña developing during August-October is now low; and 3) Many models now have more conservative predictions of hurricane activity.

Hurricane Landfalls:
While NOAA does not make an official seasonal hurricane landfall outlook, the historical likelihood for multiple U.S. hurricane strikes, and for multiple hurricane strikes in the region around the Caribbean Sea, increases sharply for very active (or hyperactive) seasons (ACE > 165% of median). However, regardless of the activity predicted in the seasonal outlook, it only takes one storm hitting an area to cause a disaster. Therefore, residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare every hurricane season regardless of this, or any other, seasonal outlook.

Predicting where and when hurricanes will strike is related to daily weather patterns, which are not reliably predictable weeks or months in advance. Therefore, it is currently not possible to accurately predict the number or intensity of landfalling hurricanes at these extended ranges, or whether a particular locality will be impacted by a hurricane this season.

DISCUSSION

1. Expected 2013 activity

Climate signals and evolving oceanic and atmospheric conditions, combined with dynamical and statistical model forecasts, continue to indicate a high likelihood (70% chance) for an above normal 2013 Atlantic hurricane season, with a reasonable possibility that the season could be very active (i.e., hyperactive). The outlook also indicates a 25% chance of a near-normal season, and only a 5% chance of a below-normal season. See NOAA definitions of above-, near-, and below-normal seasons, which have been slightly modified from previous years.

An important measure of the total seasonal activity is NOAA’s Accumulated Cyclone Energy (ACE) index, which accounts for the combined intensity and duration of tropical storms and hurricanes during the season. This outlook indicates a 70% chance that the 2013 seasonal ACE range will be 120%-190% of the median. According to NOAA’s hurricane season classifications, an ACE value above 120% of the 1981-2010 median reflects an above-normal season, and an ACE value above 165% of the median reflects a very active (or hyperactive) season.

The 2013 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 13-19 named storms, of which 6-9 are expected to become hurricanes, and 3-5 are expected to become major hurricanes. These ranges are centered well above the 1981-2010 seasonal averages of 12 named storms, 6 hurricanes and 3 major hurricanes.

Although this updated outlook is consistent with the pre-season outlook issued in May, all of the updated predicted ranges of activity have been lowered and narrowed slightly. These changes reflect: 1) The lack of hurricanes during June and July; 2) The latest CPC/ IRI ENSO forecast and many climate models which predict that La Niña is less likely to develop and further enhance the season; and 3) Some models are more conservative in their predictions of Atlantic hurricane activity.

To date, four tropical storms (Andrea, Barry, Chantal, and Dorian) have formed in the Atlantic basin, with Chantal and Dorian forming in the deep tropical Atlantic. For the remainder of the season, an additional 9-15 named storms are expected, of which 6-9 are likely to become hurricanes with 3-5 reaching major hurricane status.

For the U.S. and the region around the Caribbean Sea, the historical probability of a hurricane landfall generally increases with increasing seasonal activity. During very active seasons, the historical probabilities increase markedly for multiple hurricane strikes in these regions. Nonetheless, predicting the location, number, timing, and strength of hurricane landfalls is ultimately related to the daily weather patterns, which are not reasonably predictable weeks or months in advance. As a result, it is currently not possible to reliably predict the number or intensity of landfalling hurricanes at these extended ranges, or whether a given locality will be impacted by a hurricane this season. Therefore, NOAA does not make an official seasonal hurricane landfall outlook.

2. Science behind the 2013 Outlook

The continued high confidence for an above-normal season is based on several factors. First, conducive atmospheric and oceanic conditions have developed across the MDR, as was predicted in May. Second, these conditions are expected to persist throughout the peak months (August-October) of the hurricane season in association with the tropical multi-decadal signal, which has contributed to the Atlantic high activity era that began in 1995. This multi-decadal signal is linked to above-average sea-surface temperatures in the Main Development Region (MDR, which spans the Caribbean Sea and tropical Atlantic Ocean between 9oN-21.5oN; Goldenberg et al. 2001), and to an enhanced west African monsoon system. Recent SST analyses and tropical circulation analyses show that both of these conditions are now in place. Third, early-season activity in the deep tropics (Tropical Storms Chantal and Dorian) is generally indicative of an above-normal season. Fourth, ENSO-neutral conditions are expected to persist through August-October.

The outlook also takes into account dynamical model predictions from the NOAA Climate Forecast System (CFS), the European Centre for Medium Range Weather Forecasting (ECMWF), the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble, along with ENSO (El Niño/ Southern Oscillation) forecast models contained in the suite of Niño 3.4 SST forecasts compiled by the IRI (International Research Institute for Climate and Society).

a. Expected continuation of tropical multi-decadal signal

The main guiding factor behind this updated outlook is the expected continuation of the tropical multi-decadal signal (Goldenberg et al. 2001, Bell and Chelliah 2006), which has contributed to the current high-activity era in the Atlantic basin that began in 1995. This signal incorporates the warm phase of the Atlantic Multi-decadal Oscillation (AMO) and an enhanced west African monsoon system, both of which are in place again this summer. It is associated with inter-related atmospheric conditions that are conducive to increased Atlantic hurricane activity.

During 1995-2012, some key atmospheric aspects of the tropical multi-decadal signal have included weaker easterly trade winds and reduced vertical wind shear in the MDR. The weaker trade winds are already present. They are associated with a more northward location and more conducive structure of the African Easterly Jet (AEJ), including increased cyclonic shear along its equator-ward flank. This position and structure of the AEJ strengthens African Easterly waves moving westward from Africa, and directs them westward over progressively warmer ocean waters and weaker vertical wind shear. During August-October, these waves and their associated convective cloud systems are responsible for the vast majority of Atlantic hurricanes and major hurricanes.

During July, the weaker vertical wind shear was not yet well established across the MDR, and instead was variable in association with strong intra-seasonal variability partly linked to the Madden- Julian Oscillation (MJO). Therefore, although the climate signals point to an above-normal season, the shorter-term variability in the vertical wind shear lends uncertainty as to how strong the season will be. If the shear remains highly variable, then the activity could be near the lower end of the predicted ranges. If the shear weakens further, which would be consistent with the expected climate patterns, then the activity could be near the middle or higher end of the predicted ranges.

b. Expected above-average SSTs in the Main Development Region

The second factor guiding this outlook is the expected continuation of above-average SSTs across the MDR throughout August-October. This expectation is based on current observations, the ongoing warm phase of the AMO, and CFS T-382 model forecasts.

June-July sea surface temperatures measured over the entire MDR were 0.10oC above-average, and were also 0.09oC warmer than the remainder of the global tropics. While this relative warmth is somewhat less than in some recent years, it is consistent with the ongoing high-activity era for Atlantic hurricanes, and with the continued expectation for an above-normal Atlantic hurricane season.

c. ENSO-Neutral conditions

ENSO is another climate factor known to significantly impact Atlantic hurricane activity. The three phases of ENSO are El Niño, La Niña, and neutral. El Niño tends to suppress Atlantic hurricane activity, while La Niña tends to enhance it (Gray 1984). These typical impacts can be strongly modulated by conditions associated with a low- or high-activity hurricane era.

The combination of a high-activity era, above-average Atlantic SSTs, and ENSO-neutral conditions historically produces active or very active Atlantic hurricane seasons. ENSO-neutral conditions have been present throughout the summer. SSTs are currently slightly below average across the east-central and eastern equatorial Pacific, and the Niño 3.4 index is slightly negative. The equatorial Pacific sub-surface temperatures are near average.

The observations, ENSO model forecasts, and the latest CPC/IRI ENSO forecast issued August 8th, all suggest ENSO-Neutral conditions are likely to continue through August-October. For this period, the CPC/ IRI forecast indicates a 73% chance of ENSO-neutral, a 19% chance of La Niña, and only an 8% chance of El Niño.

3. Multi-decadal fluctuations in Atlantic hurricane activity

Atlantic hurricane seasons exhibit extended periods lasting decades (25-40 years) of generally above-normal or below-normal activity. These multi-decadal fluctuations in hurricane activity result almost entirely from differences in the number of hurricanes and major hurricanes forming from tropical storms that first develop in the MDR.

The current high-activity era began in 1995 (Goldenberg et al. 2001). Hurricane seasons during 1995-2012 have averaged about 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE value of 151% of the median. Also, the continental U.S. has averaged almost 2 landfalling hurricanes per year during this period. NOAA classifies 12 of the 18 seasons since 1995 as above normal, with eight being very active (i.e. hyperactive). Only two seasons since 1995 were below normal (1997 and 2009, which were both El Niño years).

This high level of activity contrasts sharply to the low-activity era of 1971-1994 (Goldenberg et al. 2001), which averaged only 9.5 named storms (which includes likely under-counts prior during 1971-1980, Landsea et al. 2010), 5 hurricanes, and 1.5 major hurricanes, with an ACE index of 74% of the median. One-half of the seasons during this period were below normal, only two were above normal (1980, 1989), and none were hyperactive. Also, the average number of landfalling hurricanes during this period (1.3) was almost 50% lower than that observed during 1995-2012 (1.9).

Within the MDR, the atmospheric circulation anomalies that contribute to these long-period fluctuations in hurricane activity are strongly linked to the Tropical multi-decadal signal (Bell and Chelliah 2006), which incorporates the warm phase of the AMO and an enhanced west African monsoon system. A change in the phase of the tropical multi-decadal signal coincides with the transition in 1995 from a low-activity era to the current high-activity era.

NOAA FORECASTERS

Climate Prediction Center/NWS
Dr. Gerry Bell, Lead Forecaster, Meteorologist; Gerry.Bell@noaa.gov
Dr. Jae Schemm, Meteorologist; Jae.Schemm@noaa.gov

National Hurricane Center/NWS
Eric Blake, Hurricane Specialist; Eric.S.Blake@noaa.gov
Todd Kimberlain, Hurricane Specialist; Todd Kimberlain@noaa.gov
Dr. Chris Landsea, Meteorologist; Chris.Landsea@noaa.gov
Dr. Richard Pasch, Hurricane Specialist; Richard.J.Pasch@noaa.gov

Hurricane Research Division/OAR
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. of Climate. 19, 590-612.

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.

Landsea,C.W., G.A. Vecchi, L. Bengtsson, and T. R. Knutson, 2010: Impact of duration thresholds on Atlantic tropical cyclone counts. J. Climate, 23, 2508-2519