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NOAA PRESS RELEASE
 
NOAA 2016 Atlantic Hurricane Season Outlook

Issued: 11 August 2016

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Atlantic Hurricane Outlook and Seasonal Climate Summary Archive
 
 

The 2016 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 National Hurricane Center (NHC) and the 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 during the upcoming hurricane season. It is not a seasonal hurricane landfall forecast, and it does not predict levels of activity for any particular region.

Preparedness
Hurricane disasters can occur whether the 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 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) 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 the seasonal outlooks
1. The possible changing phase of the Atlantic Multi-Decadal Oscillation (AMO)

2. Predicting El Niño and La Niña (also called the El Niño-Southern Oscillation, or ENSO) impacts is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have limited skill.

3. Many combinations of named storms and hurricanes can occur for the same general set of climate conditions. For example, 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.

4. Model predictions of SSTs, vertical wind shear, moisture, and stability have limited skill this far in advance of the peak months (August-October) of the hurricane season. 5. Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.

2016 Updated Atlantic Hurricane Season Outlook: Summary

a. Predicted Activity

NOAA's updated 2016 Atlantic Hurricane Season Outlook indicates an 85% chance that season will either be near-normal or above-normal. The outlook calls for a 50% chance of a near-season, a 35% chance of an above-normal season, and only a 15% chance of a below-normal season. See NOAA definitions of above-, near-, and below-normal seasons. The Atlantic hurricane region includes the North Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

Compared to the pre-season outlook issued 27 May, the probability of either a near-normal or above-normal season has increased to 85% (compared to 75% in May), and the probability of a below-normal season has decreased to 15% (from 25% in May). The season is still expected to be the strongest since 2012.

This is a more challenging hurricane season outlook than most because it is difficult to determine whether there will be reinforcing or competing climate influences on tropical storm development. The outlook calls for a 70% probability for each of the following ranges of activity during the 2016 hurricane season:

  • 12-17 Named Storms, which includes Alex in January
  • 5-8 Hurricanes, which includes Alex in January
  • 2-4 Major Hurricanes
  • Accumulated Cyclone Energy (ACE) range of 85%-150% of the median.

Each of these ranges is higher than was predicted in the May outlook (which indicated likely ranges of 10-16 named storms, 4-8 hurricanes, and 1-4 major hurricanes). The seasonal activity is expected to fall within the predicted 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.

During 1981-2010, Atlantic hurricane seasons averaged 12 named storms, 6 hurricanes, 3 major hurricanes, and 92.4% of the median ACE.

To date, the 2016 Atlantic hurricane season has produced five named storms, with two being hurricanes. Therefore, for the remainder of the season, we expect (with 70% probability for each range) an additional 7-12 named storms, of which 3-6 are expected to become hurricanes and 2-4 of those are expected to become major hurricanes.

b. Reasoning behind the updated outlook

The strength of the hurricane season is largely controlled by conditions and activity within the Main Development Region (MDR, which spans the Caribbean Sea and tropical Atlantic Ocean between 9°N-21.5°N; Goldenberg et al. 2001). This year, although competing factors are still likely within the MDR during the peak months (August-October) of the hurricane season, overall conditions within the MDR are now expected to favor a more active season than was predicted in May. However, there remains considerable uncertainty regarding when La Niña will develop and how strong it will become, which leads to uncertainty as to whether the Atlantic hurricane season will be near-normal or above normal.

The main factors that are conducive to a more active season now compared to the May outlook include 1) the disappearance of El Niño's suppressing influence on Atlantic hurricanes, 2) weaker vertical wind shear across the central MDR, 3) somewhat weaker trade winds across portions of the southern MDR, and 4) a stronger west African monsoon.

The main competing factors this season are 1) anomalous sinking motion and increased vertical wind shear in the western MDR (which are somewhat related to the active eastern Pacific hurricane season), and 2) less conducive global sea surface temperature (SST) patterns. A stronger La Niña could weaken or dissipate some of these competing factors and result in an above-normal season, while a weak or non-existent La Niña may allow these conditions to persist and thereby favor a near-normal season. Model predictions are now generally calling for only a border-line weak La Niña to form during August-October.

Activity to Date:

Five named storms (Alex, Bonnie, Colin, Danielle, and Earl) have formed in the Atlantic basin to date. Alex, which formed last January, and Earl became hurricanes. Four storms to date have made landfall, including Bonnie in South Carolina, Colin in western Florida, Danielle in eastern Mexico, and Earl in Belize and Mexico.

Preparedness for Tropical Storm and Hurricane Landfalls:

It only takes one storm hitting an area to cause a disaster, regardless of the overall activity. 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 2016 activity

NOAA's updated 2016 Atlantic Hurricane Season Outlook indicates an 85% chance that season will either be near-normal or above-normal. The outlook calls for a 50% chance of a near-season, a 35% chance of an above-normal season, and only a 15% chance of a below-normal season. The season is still expected to be the strongest since 2012. Compared to the pre-season outlook issued in May, the probabilities for both a near-normal and above-normal season have increased, and the likelihood of a below-normal season has decreased. See NOAA definitions of above-, near-, and below-normal seasons.

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 named storms and hurricanes during the season. This outlook indicates a 70% chance that the 2016 seasonal ACE range will be 85%-150% of the median. This range is centered at 117.5%, which is close to the upper bound for a near-normal season. According to NOAA's hurricane season classifications, an ACE value between 71.4% and 120% of the 1981-2010 median reflects a near-normal season. Values above this range reflect an above-normal season and values below this range reflect a below-normal season.

The 2016 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 12-17 named storms (which includes Hurricane Alex in January and Hurricane Earl, along with tropical storms Bonnie, Colin, and Danielle), of which 5-8 are expected to become hurricanes, and 2-4 of those are expected to become major hurricanes. The 1981-2010 period averages are 12 named storms, 6 hurricanes and 3 major hurricanes.

For the remainder of the season, we expect (with 70% probability for each range) an additional 7-12 named storms, of which 3-6 are expected to become hurricanes and 2-4 of those are expected to become major hurricanes.

Predicting the location, number, timing, and strength of hurricanes landfalls is ultimately related to the daily weather patterns including genesis locations and steering patterns, which are not 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 2016 Outlook

The 2016 Atlantic hurricane season is expected to be near-normal or above-normal, primarily because some conditions with the MDR are expected to be conducive to hurricane formation and intensification, while others might remain non-conducive. While La Niña is slightly favored to develop during the hurricane season (50%-60% chance), there is uncertainty as to how soon it will form and how strong its impacts will be. The average of the dynamical model forecasts now predicts only borderline weak La Niña conditions during ASO. If La Niña develops it will most likely be weak and have little impact on the hurricane season.

Also, during weak La Niña or ENSO-neutral conditions (no El Niño or La Niña), the atmospheric winds and pressure patterns can be more variable, and weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months and affect seasonal hurricane activity. The possibility of such a condition is also a source of uncertainty in this outlook.

The Atlantic hurricane season outlooks also takes into account dynamical model predictions from the NOAA Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL), the European Centre for Medium Range Weather Forecasting (ECMWF), and the United Kingdom Meteorology (UKMET) office, along with ENSO (El Niño/ Southern Oscillation) forecasts from statistical and dynamical models contained in the suite of Niño 3.4 SST forecasts compiled by the IRI (International Research Institute for Climate and Society) and the NOAA Climate Prediction Center.

a. Current and expected atmospheric conditions

Some features of the atmospheric circulation are now conducive to a more active season than was indicated in May. These include 1) the disappearance of El Niño's suppressing influence on Atlantic hurricanes, 2) the expected persistence of weaker vertical wind shear across the central MDR, 3) somewhat weaker trade winds across portions of the southern MDR, and 4) the continuation of an enhanced west African monsoon system.

An extensive area of anomalously weak vertical wind shear is already present across the central and northern MDR. This reduced shear is related to a combination of anomalous upper-level easterly winds across the northern MDR and anomalously weak easterly trade winds in the southern MDR. All of these conditions are typical of an above-normal season.

The substantial vertical extent (i.e. extending upward to the 700-hPa level) of the weaker easterly trade winds is also typical of an above-normal season. Weaker easterlies at this level are typically associated with increased cyclonic shear along the equatorward flank of the African easterly Jet, which is more conducive for African easterly waves to develop into tropical storms as they move westward from the west coast of Africa.

An enhanced west African monsoon system is also now in place. A stronger monsoon promotes an entire set of conducive conditions in the MDR, including weaker trade winds, weaker vertical wind shear, a more conducive African easterly jet, an enhanced upper-level ridge, an enhanced inflow of deep tropical moisture, and warmer sea surface temperatures.

Competing with these more conducive conditions are the possibility that the vertical wind shear could remain stronger than average over the Caribbean Sea, and anomalous sinking motion could persist across the Caribbean Sea and portions of the central MDR. As discussed below, these conditions have been in place since 2014 and appear to have links to persistent global SST anomaly patterns. They also have links to the active eastern Pacific hurricane season now underway. Active hurricane seasons in the eastern Pacific are typically associated with near- or below-normal hurricane seasons in the Atlantic basin.

b. SSTs in the Atlantic MDR and the central/ eastern subtropical North Pacific

In the Atlantic MDR and across the central and eastern subtropical North Pacific, the current SST anomaly patterns are not conducive to an above-normal Atlantic hurricane season. These patterns include 1) cooler SSTs in the MDR compared to the remainder of the global Tropics, 2) cooler SSTs in the eastern MDR compared to the Caribbean Sea, and 3) exceptionally warm waters across the central and eastern subtropical North Pacific.

In the MDR, although area-averaged SSTs were above-average (+0.33oC) during June-July, this value was slightly less than the remainder of the global Tropics. This condition is more typical of a near-normal Atlantic hurricane season because it does not favor anomalous rising motion or increased atmospheric instability in the MDR. Also in the MDR, SSTs were below average in the east and above average in the central/ west. This signal is also more typical of a near-normal season because it tends to suppress tropical storm formation in the eastern MDR, thereby limiting/delaying the intensity and duration of some tropical storms and hurricanes.

Across the central and eastern subtropical North Pacific, the exceptionally warm waters seen during June and July have been present since 2014. This warmth has contributed to more active hurricane seasons in the eastern Pacific hurricane basin, including a very active start to the 2016 season. This combination of warmer waters and stronger hurricane activity in the eastern Pacific tends to produce increased vertical wind shear and anomalous sinking motion in the western portion of the Atlantic MDR. It remains to be seen whether La Niña will develop with sufficient strength to offset these non-conducive conditions in the western MDR.

c. Possible La Niña

One uncertainty in this outlook is whether or not La Niña will develop and offset the less conducive conditions currently seen in the western MDR. SSTs are currently below average across the east-central and eastern equatorial Pacific. The weekly indices of area-averaged SST anomalies show the Niño 3.4 region index is at -0.5°C and the Niño-3 region index is at -0.6°C. Consistent with this cooling, subsurface temperatures are below average across the region. These conditions are consistent with an evolution toward La Niña.

However, a time-longitude section of equatorial oceanic heat content anomalies (which reflects the average temperature departure in the upper 300 meters of the ocean) shows that the recent evolution toward La Niña has not been continuous. Instead, a rapid cooling occurred during March-May as El Niño dissipated and the upwelling phase of an equatorial oceanic Kelvin wave propagated across the Pacific basin. Since that time, there have been no further upwelling waves and the negative heat content anomalies have slowly lessened.

For operational purposes, the Climate Prediction Center declares a La Niña when the monthly Niño 3.4 index meets or exceeds -0.5°C, along with consistent atmospheric features. These anomalies must also be forecasted to persist for 3 consecutive months. By historical standards, to be classified as a full-fledged La Niña episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons.

Earlier in the year, the average of the dynamical models contained in the suite of ENSO forecasts had predicted a moderate strength La Niña during ASO, and the Climate Prediction Center had indicated about a 70% chance of La Niña during ASO. These forecasts are now much less bullish on La Niña, with the average of the dynamical models now predicting only a borderline weak La Niña, and the Climate Prediction Center now predicting about a 50%-60% chance of La Niña during the hurricane season.

The high-resolution Climate Forecast System (CFS) is an important model used to predict La Niña and El Niño, and also to predict environmental conditions and hurricane activity within the MDR. This CFS model is now calling for weak La Niña conditions to develop in September and for border-line moderate La Niña conditions to develop in October. This strength is much weaker than the strong La Niña predicted by the CFS in April.

La Niña reduces the vertical wind shear and produces anomalous rising motion in the atmosphere across the western MDR, thus favoring more, stronger, and longer-lived hurricanes. However, because of the reduction in both the likelihood and strength of La Niña compared to May, it is now more likely that the non-conducive patterns of anomalous sinking motion and increased vertical wind shear currently in the western MDR will persist, thereby limiting the overall strength of the Atlantic hurricane season. Ongoing uncertainty regarding when La Niña will develop and how strong it will become, leads to uncertainty as to whether the Atlantic hurricane season will be near-normal or above normal.

NOAA FORECASTERS

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

National Hurricane Center
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
Stanley Goldenberg, Meteorologist; Stanley.Goldenberg@noaa.gov

REFERENCES

Bell, G. D., and co-authors, 2014: [The Tropics] The 2013 North Atlantic Hurricane Season: A Climate Perspective [in "State of the Climate in 2013"]. Bull. Amer. Meteor. Soc., 95 (7), S86-S90.

Bell, G. D., and co-authors, 2015: [The Tropics] The 2014 North Atlantic Hurricane Season: A Climate Perspective [in "State of the Climate in 2014"]. Bull. Amer. Meteor. Soc., 96 (7), S101-S107.

Bell, G. D., and co-authors, 2016: [The Tropics] The 2015 North Atlantic Hurricane Season: A Climate Perspective [in "State of the Climate in 2015"]. Bull. Amer. Meteor. Soc., 97th, In Press

Bell, G. D., and co-authors, 1999: [The Tropics] The 1998 North Atlantic Hurricane Season: A Climate Perspective [in "State of the Climate in 1998"]. Bull. Amer. Meteor. Soc., 80, 824-828

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.

Goldenberg, S. B. and L. J. Shapiro, 1996: Physical mechanisms for the association of El Niño and west African rainfall with Atlantic major hurricane activityJ. Climate, 9, 1169-1187.

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.