The 2015 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. 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.

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 whether a given climate signal will be associated with several short-lived storms or fewer longer-lived storms with greater intensity.

3. Model predictions of sea-surface temperatures, vertical wind shear, moisture, and stability have limited skill this far in advance of the peak months (August-October) of the hurricane season.

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

2015 Atlantic Hurricane Season Outlook: Summary

NOAA's 2015 Atlantic Hurricane Season Outlook indicates that a below-normal hurricane season is most likely this year. The outlook calls for a 70% chance of a below-normal season, a 20% chance of a near-normal season, and only a 10% chance of an above-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.

The main climate factor expected to suppress this hurricane season is El Niño, which is now present and is expected to last through the hurricane season. Many models predict this El Niño to further intensify as the season progresses. The current El Niño is already affecting the wind and rainfall patterns across the equatorial Pacific Ocean.

El Niño is expected to impact the wind and rainfall patterns in the Atlantic hurricane Main Development Region (MDR) throughout the hurricane season, but its suppressing influence will be most noticeable during the season's peak months of August-October (ASO). The MDR spans the Caribbean Sea and tropical Atlantic Ocean between 9^oN-21.5^oN (Goldenberg et al. 2001).

El Niño's expected atmospheric impacts within the MDR include a continuation of stronger upper-level westerly winds, increased vertical wind shear, and anomalous sinking motion. These conditions are already occurring, and are known to suppress the hurricane activity that typically develops from African easterly waves during August-October. Also this season, sea surface temperature departures in the MDR are expected to be small (Fig. 4), meaning they are not expected to have a major impact on the overall seasonal 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 during the 2015 hurricane season:

• 6-11 Named Storms, which includes TS Ana that formed in May
• 3-6 Hurricanes
• 0-2 Major Hurricanes
• Accumulated Cyclone Energy (ACE) range of 40%-85% 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.

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

Uncertainties
One uncertainty in this outlook is how strong the current El Niño will be during the peak months (ASO) of the season. The average of the dynamical model forecasts suggests El Niño could become a strong event as the summer progresses, while the average of the statistical model forecasts suggests El Niño will be a borderline weak-moderate strength event.

This Atlantic hurricane season outlook will be updated in early August, which coincides with the onset of the peak months of the hurricane season.

Tropical Storm and Hurricane Landfalls:
Tropical storms and hurricanes can and do strike the U.S., even during seasons with El Niño. One tropical storm (Ana) has already made landfall in the U.S. this season, striking South Carolina with heavy rains and 45 mph winds.

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 2015 activity

Climate signals and evolving oceanic and atmospheric conditions, combined with model forecasts, indicate that a below-normal Atlantic hurricane season is likely in 2015. This outlook calls for a 70% chance of a below-normal season, a 20% chance of a near-normal season, and only a 10% chance of an above-normal season. See NOAA definitions of above-, near-, and below-normal seasons.

An important measure of the total overall 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 2015 seasonal ACE range will be 40%-85% of the median. According to NOAA's hurricane season classifications, an ACE value below 71.4% of the 1981-2010 median reflects a below-normal season, and an ACE value of 71.4%-120% reflects a near-normal season.

The 2015 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 6-11 named storms (which includes TS Ana that formed in May), of which 3-6 are expected to become hurricanes, and 0-2 are expected to become major hurricanes. These ranges are centered below the 1981-2010 period averages of 12 named storms, 6 hurricanes and 3 major hurricanes.

For the U.S. and the region around the Caribbean Sea, tropical storms and hurricanes can and do strike even during seasons with El Niño. One tropical storm (Ana) has already made landfall in the U.S. this season, striking South Carolina with heavy rains and 45 mph winds.

Predicting the location, number, timing, and strength of hurricanes landfalls is ultimately related to the daily weather 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 2015 Outlook

The 2015 seasonal hurricane outlook reflects the persistence and possible strengthening of the already developed El Niño during the hurricane season, combined with an expectation for relatively small SST departures in the Atlantic hurricane MDR. Overall, the predicted oceanic and atmospheric conditions for 2015 across the MDR are those which typically produce a below-normal hurricane season.

The outlook takes into account dynamical model predictions from the NOAA Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) model FLOR-FA, the European Centre for Medium Range Weather Forecasting (ECMWF), the United Kingdom Meteorology (UKMET) office, the EUROpean Seasonal to Inter-annual Prediction (EUROSIP) ensemble, along with ENSO (El Niño/ Southern Oscillation) forecasts from statistical and other 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. El Niño

El Niño suppresses Atlantic hurricane activity (Gray 1984) by producing a set of non-conducive atmospheric conditions within the MDR, including 1) stronger vertical wind shear resulting primarily from stronger upper-level westerly winds, 2) increased sinking motion, and 3) increased stability.

At present, SSTs are above average across the eastern half of the equatorial Pacific, with the largest departures exceeding +1.0^oC. SST anomalies averaged across the Niño 3.4 region, which spans the central and east-central equatorial Pacific between 120^oW-170^oW, are currently +1.0^oC. This value is at CPC's lower threshold for a moderate-strength El Niño. The Niño 3.4 region is a main region used by the CPC to help monitor and assess the strength of El Niño.

The current sub-surface thermal structure of the equatorial Pacific also reflects El Niño conditions, with above-average temperatures present from the surface to 150m depth between the date line and the west coast of South America. This anomalous warmth was initially linked to a downwelling equatorial oceanic Kelvin wave that was triggered in February. Unlike previous downwelling Kelvin waves over the past twelve months, this most recent wave was associated with a strong ocean-atmosphere coupling that led to El Niño.

Key aspects of this coupling include anomalous upper-level divergence and enhanced convection since mid-March over the central equatorial Pacific. Additionally, anomalous westerly trade winds (i.e. lighter easterly trade winds) have been present since March across the central and east-central equatorial Pacific Ocean, contributing to the warmer SSTs and to the enhanced convection.

Additional periods with exceptionally weak trade winds could further strengthen El Niño this hurricane season. One such period is currently occurring in association with a westerly wind burst over the western equatorial Pacific.

The average forecast (orange line) of the dynamical models (closed markers) in the suite of IRI/ CPC Niño 3.4 SST forecasts suggests that El Niño could become a strong event (Niño 3.4 SST anomalies ≥ 1.5^oC) as the hurricane season progresses. Some models (such as the CFS Hi-Res model and the ECMWF) are predicting an exceptionally strong El Niño while others are predicting a moderate-strength event (Niño 3.4 SST anomalies between 1.0-1.5^oC). The statistical model forecasts (open markers) are generally cooler than the dynamical model predictions, and on average (green line) show a borderline moderate-strength El Niño during ASO. These differing forecasts, combined with overall lower predictive skill during March-May, produce uncertainty as to exactly how strong El Niño will become.

b. Sea surface temperatures across the Main Development Region (MDR)

SSTs within the Atlantic hurricane MDR are currently below-average in the eastern tropical Atlantic and above-average in the Caribbean Sea. Both of these deviations from average are modest. SST departures averaged across the MDR are near zero, which is cooler (by 0.31^oC) than the average departure for the remainder of the global tropics.

One issue for this outlook is whether or not the SSTs in the eastern MDR will warm as the season progresses. Cooler waters in this region are typically associated with stronger northeasterly trade winds, which are now present. El Niño favors stronger easterly trade winds in the MDR, which suggests that a significant warming of the MDR is unlikely this season.

Key dynamical models are predicting that area-averaged SSTs for the MDR as a whole will be close to average during the hurricane season. The CFS high-resolution (T-382) model is predicting below-average SSTs across the Caribbean Sea during ASO, and near-average SSTs in the eastern MDR.

A configuration of near- or below-average SSTs in the MDR would likely result in a continuation of cooler ocean temperatures relative to the remainder of the tropics, a condition which is often quite hostile to Atlantic tropical cyclone development.

These expected conditions within the MDR contrast with those that have produced numerous active hurricane seasons since 1995. That increased hurricane activity has been linked to the warm phase of the Atlantic Multi-decadal Oscillation (AMO) (Goldenberg et al. 2001, Bell and Chelliah 2006), which produces above-average SSTs across the MDR in addition to atmospheric circulation patterns which are also more conducive to active seasons. The warm AMO also makes the MDR anomalously warm compared to the remainder of the global tropics. The current and predicted SST anomalies for 2015 suggest that the warm AMO signal might be absent this season.

c. Expected atmospheric conditions across the Main Development Region (MDR)

The main expected impact from the ongoing El Niño on the 2015 hurricane season is enhanced vertical wind shear across the western and southern MDR during ASO, mainly in response to stronger upper-level westerly winds. The vertical wind shear is already anomalously strong across this region, and the CFS hi-resolution model predicts the enhanced shear to persist during ASO.

El Niño also tends to favor anomalous sinking motion and increased atmospheric stability across the Caribbean Sea and central MDR. This combination of conditions suppresses the Atlantic hurricane activity during the peak months (ASO) of the season by reducing the number and strength of named storms and hurricanes that develop from African easterly waves.

In contrast to the MDR, we often see storms occurring in a belt extending from the Gulf of Mexico to the central Atlantic north of the MDR during El Niño years, in association with reduced vertical wind shear across the region (Goldenberg and Shapiro 1996). The CFS model predicts the vertical wind shear in this belt to be weaker than average during ASO 2015, which could allow activity near the higher end of the predicted ranges.

3. Multi-decadal fluctuations in Atlantic hurricane activity

Atlantic hurricane seasons exhibit extended periods lasting 25-40 years of generally above-normal or below-normal activity. For example, a high-activity era for Atlantic hurricanes began in 1995. Seasons during 1995-2014 averaged about 14.7 named storms, 7.6 hurricanes, and 3.5 major hurricanes, with an ACE index of 142% of the median. NOAA classifies 12 of the 20 seasons since 1995 as above normal, with eight being very active (i.e., hyperactive defined by ACE > 165% of median). Only three seasons since 1995 were below normal (1997, 2009, and 2013).

In contrast, the preceding low-activity era of 1971-1994 (Goldenberg et al. 2001) averaged 8.5 named storms, 5 hurricanes, and 1.5 major hurricanes, with an ACE index of only 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.

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 AMO, and its associated changes in the strength of the west African monsoon system, is the climate pattern responsible for this multi-decadal variability.

The high-activity era for Atlantic hurricanes that began in 1995 reflected a transition to the warm phase of the AMO and to an enhanced west African monsoon system as shown (Bell and Chelliah 2006). This climate pattern produces stronger hurricane seasons by creating conducive atmospheric conditions in the MDR, including 1) reduced vertical wind shear, 2) weaker easterly trade winds, 3) a more conducive configuration of the African easterly jet (i.e. increased cyclonic shear), 4) warm, moist, unstable air, and 5) reduced sinking motion.

However, the atmospheric conditions expected during ASO 2015 (i.e. stronger vertical wind shear, enhanced sinking motion, increased atmospheric stability) contrast with these active-era patterns. Following two relatively quiet hurricane seasons (2013 and 2014 (Bell et al. 2014, 2015), along with the current projection of Atlantic SST anomalies onto the cold phase of the AMO, debate has surfaced as to whether we are still in this high-activity era.

4. Uncertainties in the Outlook

The 2015 Atlantic hurricane season will likely be below-normal. Key indications for this outlook are 1) the persistence and possible strengthening of the ongoing El Niño and its associated atmospheric circulation patterns, and 2) expected near-average SSTs in the MDR during ASO.

Uncertainties in the outlook are related to the amount of strengthening that El Niño undergoes as the season progresses, and to what extent El Niño will suppress the hurricane season.

Another uncertainty is whether the warm phase of the AMO is still present. There have been two seasons in a row, 2013 and 2014, with below-normal and near-normal activity respectively and neither had an El Niño event responsible for the reduced activity. The current configuration of SSTs in the Atlantic Ocean, both in the MDR and the entire North Atlantic, are suggestive that the AMO may no longer be in the warm phase.

However, whether this current cool phase of the AMO continues during the 2015 season (and beyond) is unknown. Given that the AMO had been in the warm phase during the last two decades, it would be reasonable to expect a transition back to the cool phase at some point in the 2010s or 2020s. Whether that transition has now occurred cannot be addressed with any degree of certainty.

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, In press.

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 activity. J. 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.