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NOAA 2015 Atlantic Hurricane Season Outlook

Issued: 6 August 2015

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Atlantic Hurricane Outlook & Seasonal Climate Summary Archive
 
 

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 location.

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 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.

General sources of uncertainty in the seasonal outlooks
1. Predicting the extent and magnitude of El Niño and La Niña (also called the El Niño-Southern Oscillation, or ENSO) 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 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 atmospheric stability for the peak months (August-October) of the hurricane season are generally skillful, but can be substantially in error in some seasons.

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 Updated Atlantic Hurricane Season Outlook: Summary

NOAA's updated 2015 Atlantic Hurricane Season Outlook indicates that a below-normal hurricane season is very likely. The outlook calls for a 90% chance of a below-normal season and a 10% chance of a near-normal season, with no realistic expectation that the season will be above-normal. This 90% probability of a below-normal season is the highest given by NOAA for any such season since their seasonal hurricane outlooks began in August 1998. 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 difference between this updated outlook and the pre-season outlook issued 27 May is that there is now an even higher chance of a below-normal season (90% compared to 70%), along with lower chances of a near-normal (10% compared to 20%) or an above-normal (negligible compared to 10%) season.

The likelihood of a below-normal hurricane season has increased for three main reasons. First, El Niño has strengthened as predicted, and NOAA's latest prediction calls for a significant El Niño to continue through the remainder of the hurricane season. Second, atmospheric conditions that are exceptionally non-conducive to tropical storm and hurricane formation are now present in response to El Niño. These conditions, which include strong vertical wind shear and enhanced sinking motion, are predicted to continue through the peak months (August-October, ASO) of the hurricane season across the Atlantic hurricane Main Development Region (MDR, which spans the Caribbean Sea and tropical Atlantic Ocean between 9oN-21.5oN; Goldenberg et al. 2001). Third, sea-surface temperatures (SSTs) across the MDR are predicted to remain below average, and to also remain much cooler than the rest of the global tropics. Cooler Atlantic SSTs are associated with stronger trade winds, and further reduce the ability of storms to form and gain strength in the MDR.

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:

  • 6-10 Named Storms, which includes the three named storms to date
  • 1-4 Hurricanes
  • 0-1 Major Hurricanes
  • Accumulated Cyclone Energy (ACE) range of 25%-70% 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 well below the official NHC 1981-2010 seasonal averages of 12 named storms, 6 hurricanes, and 3 major hurricanes.

Compared to the pre-season May outlook, the predicted likely maximum number of named storms has been lowered to 10 (from 11 in May), the likely hurricane range has been lowered to 1-4 (from 3-6 in May), the likely maximum number of major hurricanes has been dropped to one (from 2 in May), and the ACE range has lowered to 25%-70% of the median (from 40%-80% in May).

For the remainder of the season, this updated outlook calls for a 70% probability of 3-7 named storms, of which 1-4 are expected to become hurricanes and 0-1 are expected to become major hurricanes.

Activity to Date:
Three named storms (Ana, Bill, and Claudette) have formed in the Atlantic basin to date. Ana made landfall in South Carolina, and Bill made landfall in Texas.

Uncertainties
One uncertainty in this outlook is how strong the current El Niño will be during ASO 2015. The average of the dynamical model forecasts suggests El Niño will be a strong event, while the average of the statistical model forecasts suggests El Niño will be a borderline moderate-strong event.

Tropical Storm and Hurricane Landfalls:
Tropical storms and hurricanes can and do strike the U.S., even during seasons with El Niño. Two tropical storms (Ana and Bill) have already made landfall in the U.S. this season, striking South Carolina and Texas, respectively.

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 model forecasts, along with current and predicted oceanic and atmospheric conditions, indicate that a below-normal Atlantic hurricane season is very likely in 2015. This outlook calls for a 90% chance of a below-normal season and 10% chance of a near-normal season, with no realistic expectation for an above-normal season. The stated 90% probability of a below-normal season is the highest given by NOAA since their seasonal hurricane outlooks began in 1998. 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 25%-70% of the median. According to NOAA's hurricane season classifications this predicted range is below the upper threshold (71.4% of the 1981-2010 median) for a below-normal season.

The 2015 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 6-10 named storms (which includes tropical storms Ana, Bill and Claudette), of which 1-4 are expected to become hurricanes, and 0-1 are expected to become major hurricanes. These ranges are centered well below the 1981-2010 period averages of 12 named storms, 6 hurricanes and 3 major hurricanes.

For the remainder of the season, the outlook calls for 3-7 named storms, of which 1-4 are expected to become hurricanes and 0-1 are expected to become major hurricanes. This outlook means that we could see a record low number of hurricanes (1), which would surpass the previous modern-era record low of two hurricanes seen during 1982 and 2013.

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. Two tropical storms (Ana and Bill) have already made landfall in the U.S. this season, striking South Carolina and Texas, respectively.

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 strength of the Atlantic hurricane season is largely controlled by conditions and activity within the MDR. The 2015 Atlantic hurricane season is expected to be below normal, primarily because of exceptionally non-conducive conditions and reduced hurricane activity within MDR.

Overall, the current and predicted oceanic and atmospheric conditions across the MDR during ASO 2015 are those which typically produce a below-normal hurricane season. These conditions reflect a strengthening El Niño, which NOAA's predicts will remain a significant event through the hurricane season. Also, below-average SSTs across the MDR are predicted to persist during ASO, and SST anomalies within the MDR are expected to remain colder than the rest of the global tropics.

The outlook takes into account dynamical model predictions from the NOAA Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) new high-resolution model HiFLOR-A07, the European Centre for Medium Range Weather Forecasting (ECMWF), and the United Kingdom Met Office (UKMET), 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 (CPC).

a. El Niño

At present, SSTs are well above average across the eastern half of the equatorial Pacific, with the largest departures exceeding +3oC (+6oF) in the eastern Pacific. SST anomalies averaged across the Niño 3.4 region, which spans the central and east-central equatorial Pacific between 120oW-170oW, are currently +1.7oC. This value is above CPC's lower threshold (+ 1.5oC) for a strong 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 Ocean also reflects El Niño, with above-average temperatures present from the surface to 150m depth between the date line and the west coast of South America.

NOAA's latest El Niño forecast calls for a significant El Niño to continue through the hurricane season. This prediction is consistent with the average forecast (orange line) of the dynamical models (closed markers) which predicts a strong El Niño this hurricane season.

Current observations show a substantial atmospheric coupling to El Niño. One key aspect of this coupling is an eastward extension of deep tropical convection to the region of anomalously warm waters in the central and eastern Pacific. The resulting extensive area of enhanced convection is seen across the eastern half of the equatorial Pacific in the patterns of enhanced Outgoing Longwave Radiation (OLR), anomalous upper-level divergence, and a core of negative velocity potential anomalies. A second aspect of the atmospheric coupling to El Niño is an extensive area of anomalous westerly trade winds (i.e. lighter easterly trade winds) across the equatorial Pacific Ocean, which contributes to the warmer SSTs and to the enhanced convection.

Consistent with these conditions, the upper-atmospheric response to El Niño is now substantial and extends around the globe. This response is seen by an El Niño-related zonal wave-1 pattern of 200-hPa streamfunction anomalies extending around the globe in the subtropics of both hemispheres. This pattern features anti-cyclonic streamfunction anomalies over the subtropical Pacific Ocean in both hemispheres, straddling the region of enhanced tropical convection. The pattern also features cyclonic streamfunction anomalies in each hemisphere extending from the America's to eastern Asia/ Australasia. Regionally, this pattern produces strong vertical wind shear and anomalous sinking motion over the Atlantic hurricane MDR.

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

SSTs within the Atlantic hurricane MDR have been below-average this summer. Departures averaged across the MDR during June-July 2015 were -0.14oC, which is 0.57oC cooler than the average departure for the remainder of the global tropics. This value is the coolest that the MDR has been relative to the remainder of the global tropics for any June-July period in the 1950-2015 record.

The CFS high-resolution (T-382) model is predicting a continuation of below-average SSTs during ASO in the MDR. The CFS is also predicting the MDR to remain anomalously cool compared to the global tropics throughout the season. These conditions are 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), makes the MDR anomalously warm compared to the remainder of the global tropics. The current SST anomaly pattern and the predicted SST anomaly pattern for ASO 2015 suggest that the warm AMO signal is absent this season.

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

El Niño produces a set of atmospheric conditions within the MDR that are not conducive to tropical storm and hurricane formation and intensification. These conditions include 1) stronger vertical wind shear, 2) increased sinking motion, and 3) increased stability (Gray 1984). These conditions suppress the Atlantic hurricane activity during ASO by reducing the number and strength of named storms and hurricanes that develop from African easterly waves. These non-conducive conditions for storm formation and intensification are now present and are expected to persist during ASO.

For example, the El Niño-related upper-level pattern of cyclonic streamfunction anomalies has acted to amplify the Tropical Upper-Tropospheric Trough (TUTT), which now extends well southward into the western MDR. This enhanced TUTT is contributing to anomalous upper-level convergence and anomalous sinking motion across large portions of the MDR. The TUTT is also associated with strong upper-level westerly winds and record-strength vertical wind shear across the Caribbean Sea and western tropical Atlantic Ocean. The CFS hi-resolution model predicts the enhanced shear to persist during ASO.

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 remain 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 primary climate pattern directly 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 (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.

The atmospheric conditions expected within the MDR during ASO 2015 (i.e. stronger vertical wind shear, enhanced sinking motion, increased atmospheric stability) contrast with these active-era patterns. Also, the current SST anomaly pattern and the predicted SST anomaly pattern for ASO 2015 suggest that the warm AMO signal is absent this season. The current suppressive conditions are expected to make 2013-2015 the only 3-year period since 1992-94 in which three consecutive Atlantic hurricane season were not above normal (Bell et al. 2014, 2015).

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 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.