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NOAA PRESS RELEASE
 

NOAA 2017 Atlantic Hurricane Season Outlook

Issued: 25 May 2017

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Atlantic Hurricane Outlook & Seasonal Climate Summary Archive

 

The 2017 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.

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 sea surface temperatures (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.

2017 Atlantic Hurricane Season Outlook: Summary

a. Predicted Activity

NOAA's 2017 Atlantic Hurricane Season Outlook indicates that an above-normal or near-normal hurricane season is most likely. The outlook indicates a 45% chance for an above-normal season, a 35% chance for a near-normal season, and a 20% chance for 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.

The outlook calls for a 70% probability for each of the following ranges of activity during the 2017 hurricane season, which runs from June 1st through November 30th:

  • 11-17 Named Storms, which includes Tropical Storm Arlene in April
  • 5-9 Hurricanes
  • 2-4 Major Hurricanes
  • Accumulated Cyclone Energy (ACE) range of 75%-155% of the median, which includes Arlene in April

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 near or above the 1981-2010 seasonal averages of 12 named storms, 6 hurricanes, and 3 major hurricanes. Most of the predicted activity is likely to occur during the peak months (August-October, ASO) of the hurricane season.

b. Reasoning behind the outlook

NOAA's 2017 Atlantic hurricane season outlook reflects three main factors during August-October:

  • (1) Either ENSO-neutral or weak El Niño conditions are expected over the tropical Pacific Ocean [ENSO refers to El Niño/ Southern Oscillation, which has three phases: El Niño, Neutral, and La Niña.],
  • (2) Near- or above-average sea-surface temperatures (SSTs) across much of the Atlantic hurricane Main Development Region (MDR, which includes the tropical North Atlantic Ocean and Caribbean Sea between 9.5°N and 21.5°N latitude), and
  • (3) Near-average or weaker-than-average vertical wind shear in the MDR.

There is currently large model forecast uncertainty regarding both oceanic and atmospheric predictions for August-October 2017. This uncertainty is reflected in the forecasts for ENSO, and for forecasts of SSTs and vertical wind shear in the MDR. The combination of ENSO-neutral, a warmer MDR, and weaker vertical wind shear within the MDR would likely yield levels of activity near the higher ends of the predicted ranges. The combination of El Niño, a cooler MDR, and near- or above-average vertical wind shear in the MDR, would likely yield levels of activity toward the lower ends of the predicted ranges.

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.

DISCUSSION

1. Expected 2017 activity

NOAA's 2017 Atlantic Hurricane Season Outlook indicates that an above-normal or near-normal hurricane season is most likely. The outlook indicates a 45% chance for an above-normal season, a 35% chance for a near-normal season, and a 20% chance for a below-normal season. 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 75% chance that the 2017 seasonal ACE range will be 75%-155% of the median. 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 2017 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 11-17 named storms (which includes TS Arlene in April), of which 5-9 are expected to become hurricanes, and 2-4 of those are expected to become major hurricanes. These ranges are centered near or above the 1981-2010 period averages of about 12 named storms, 6 hurricanes and 3 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 2017 Outlook

NOAA's Atlantic hurricane season outlooks are based on predictions of the main climate factors and their associated conditions known to influence seasonal Atlantic hurricane activity. The predictions are based on extensive monitoring, analysis, and research activities, a suite of statistical prediction tools, and dynamical models. The dynamical model predictions come from the NOAA Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) models FLOR-FA and HI-FLOR, the United Kingdom Met Office (UKMET) GloSea5 model, and the European Centre for Medium Range Weather Forecasting (ECMWF) model. ENSO forecasts are also provided from a suite of statistical and other dynamical models contained in the suite of Niño 3.4 SST forecasts, which is compiled by the IRI (International Research Institute for Climate and Society) and the NOAA Climate Prediction Center.

NOAA's 2017 Atlantic hurricane season outlook reflects three main factors during August-October: (1) Either ENSO-neutral or weak El Niño conditions most likely over the tropical Pacific Ocean, (2) Near- or above-average SSTs across much of the Atlantic hurricane MDR, (3) Near-average or weaker-than-average vertical wind shear in the MDR, and (4) Near-average sea level pressures in the MDR.

a. ENSO-neutral or weak El Niño

La Niña dissipated in February, and ENSO-neutral conditions have been present since that time. Recently, above-average SSTs have emerged in the east-central and eastern equatorial Pacific, and near-average SSTs are evident in the central equatorial Pacific. The SST index for the Niño 3.4 region, which spans the east-central equatorial Pacific between 120°W-170°W, is currently near +0.5°C. ENSO-neutral conditions have Niño 3.4 index values between -0.5°C and +0.5°C. El Niño is classified as a sustained Niño 3.4 index value at or above +0.5°C for 5-consecutive months, along with consistent atmospheric impacts. A weak El Niño is defined by a Niño 3.4 index between +0.5° and +1.0°C, and a moderate-strength El Niño is defined by a Niño 3.4 index between +1° and +1.5°C.

El Niño causes increased vertical wind shear in the MDR, along with anomalous sinking motion, and increased atmospheric stability. These conditions are not conducive to hurricane formation and intensification. Conversely, La Niña causes decreased vertical wind shear in the MDR, along with anomalous rising motion and decreased atmospheric stability. These conditions are conducive to a more active Atlantic hurricane season.

Sub-surface ocean temperatures are warmer than average in the central and eastern equatorial Pacific, and near average across the east-central equatorial Pacific. This pattern indicates that the anomalous warmth in the Niño 3.4 region is currently confined to the near-surface, a condition that does not favor a significant short-term evolution toward El Niño.

A time longitude diagram of the equatorial Pacific oceanic heat content anomalies (i.e. average temperature anomalies in the upper 300 m of the ocean) highlights the recent sub-surface temperature evolution and also shows mixed conditions across the central and east-central equatorial Pacific. Following the disappearance of negative heat content anomalies in February, positive anomalies developed in both the central and eastern Pacific. However, the anomalies across the east-central equatorial Pacific have been weak, fluctuating between negative and positive values. Also, the positive anomalies in the eastern Pacific have recently decreased in strength. This variability is linked to a series of equatorial oceanic Kelvin waves, whose downwelling phase (dashed line) produces warming and whose upwelling phase (dotted line) produces cooling. Such intra-seasonal variability can cause large fluctuations in model predictions from one month to the next, and is also making it difficult to predict confidently whether El Niño will develop in time and of sufficient strength to suppress the 2017 Atlantic hurricane season.

Large uncertainties for predictions of El Niño and La Niña are typical at this time of the year, because this is when the model forecast skill is at its lowest. Currently, there is considerable model spread in the SST predictions for the Niño 3.4 region during ASO 2017, generally ranging from ENSO-neutral to a weak El Niño. The average of the dynamical model predictions (thick orange line) indicates a weak El Niño during ASO, perhaps with sufficient strength to have a suppressing influence on the Atlantic hurricane season. The average of the statistical model predictions (thick green line) indicates borderline neutral to weak El Niño conditions during ASO 2017.

There is even considerable spread in the SST forecasts coming from different resolutions of the same model. For example the CFS high-resolution (T-382) model is predicting below-average SSTs in the Niño 3.4 region during ASO, while the lower-resolution CFS T-126 model is predicting a weak El Niño but with no suggestion that El Niño will be strong enough to increase the vertical wind shear in the MDR.

Based on current conditions, the recent oceanic evolution, and the large spread in model forecasts, the latest ENSO outlook issued by the CPC and IRI indicates approximately equal probabilities of ENSO-neutral and El Niño throughout the hurricane season. That outlook also states that "while the Niño-3.4 index may be near or greater than +0.5°C for several months, the warmth may not last long enough to qualify as an El Niño episode and/or may not impact the atmospheric circulation."

b. Near-average or above-average sea surface temperatures in the MDR

Currently, SSTs are above average across the MDR, with the largest departures of between +0.5° and +1°C observed in the Caribbean Sea. For the ASO season, SSTs have been above average in the MDR since 1995. However, there is typically low skill and considerable spread in model predictions of Atlantic SSTs this far ahead of the ASO season. Complicating this situation is the possible continuance of a strong cold bias in forecasts from NOAA's CFS high-resolution model that was evident during past two hurricane seasons. This model is again predicting well below-average SSTs in the MDR, while the lower-resolution CFS runs are predicting overall above-average SSTs in that region.

Uncertainty in the state of the Atlantic Multi-decadal Oscillation (AMO) is another factor complicating the prediction of SST anomalies within the MDR, and their impacts on the Atlantic hurricane season. The AMOis a main climate factor that influences the Atlantic hurricane season, and it sets the backdrop upon which other climate phenomena such as El Niño and La Niña overlay. The AMO results in Atlantic hurricane seasons historically exhibiting 25-40 year periods of generally above-normal activity (called a high-activity era) followed by 25-40 years of generally below-normal activity (called a low-activity era).

At present, there is uncertainty as to whether or not the warm (i.e. positive) phase of the AMO, which has been associated with the high-activity era for Atlantic hurricanes which began in 1995, has ended. There is also uncertainty as to whether a cool (i.e., negative) phase of the AMO and low-activity era has begun similar to that observed during 1971-1994. Two different measures of the strength and phase of the AMO are the Kaplan AMO index and the Klotzbach-Gray AMO index. For Jun.-Nov. (Red line), which is the Atlantic hurricane season, both indices have generally shown strong positive values in association with the recent high-activity era for Atlantic hurricanes. However, in recent years, the Kaplan AMO index has remained positive, while the Klotzbach-Gray index has fluctuated near zero. Similar results apply to the ASO season (Green line). SSTs in the Atlantic MDR have remained well above average since 1995, which is consistent with the warm AMO during that season. The 2017 outlook reflects the likelihood of either the neutral or warm phase of the AMO during the 2017 Atlantic hurricane season.

Conversely, for the cool season January-May (Blue line), both indices have been much cooler during the past few years compared to their Jun.-Nov. and ASO counterparts. Also, the Kaplan AMO index has remained generally positive during the Jan.-May season, whereas the Klotzbach-Gray index has recently turned strongly negative. Thus, the AMO index in the months prior to the Atlantic hurricane season has recently been a poor indicator of the upcoming AMO index during the hurricane season.

c. Near-average or below-average vertical wind shear in the MDR

The strength and spatial distribution of the vertical wind shear is critical for predicting the strength of the hurricane season. Weaker vertical wind shear is conducive to stronger and longer-lasting hurricanes, while strong shear can either prevent a storm from forming or significantly weaken an existing storm. An El Niño of sufficient strength acts to increase the vertical wind shear within the MDR, thereby suppressing the Atlantic hurricane season.

At present, the model forecasts of vertical wind shear vary considerably from one model to the next, are generally dependent upon the model's predicted strength of El Niño. The CFS model is predicting anomalously weak shear in the MDR during ASO 2017. At present, there is no indication that the shear will be excessively weak so as to support an extremely active hurricane season, and also no confident indication that the shear will be exceptionally strong. The MDR will most likely experience near-average or below-average vertical wind shear during ASO 2017.

d. Near-average sea level pressures in the MDR

Relatively small changes in sea level pressures within the MDR can also affect the Atlantic hurricane activity. Higher pressures on the order of 0.5-1.0 mb above average are associated with more sinking and drying of the tropical atmosphere, which is not conducive for hurricane activity. The opposite is true for lower pressures than usual. Currently, pressures across the MDR are near average and forecasts from the various global models do not indicate large anomalies for either higher or lower pressure during ASO 2017.

NOAA FORECASTERS:

Climate Prediction Center

National Hurricane Center

Hurricane Research Division

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.

  • 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.

  • Klotzbach, P.J., and W. M. Gray, 2008: Multi-decadal Variability in North Atlantic Tropical Cyclone Activity, J. Climate, 21, 3929-3935.

  • 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.