The updated 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 no matter what the activity is in a given year. 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 70% of 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, 2) current and predicted atmospheric and oceanic conditions across the tropical Atlantic and Caribbean Sea, and 3) climate models that directly predict seasonal hurricane activity.

Sources of uncertainty in the seasonal outlooks:

1. Is the Atlantic Multi-Decadal Oscillation (AMO) still in its warm phase, and is the Atlantic basin still in a high-activity era?
2. Will the ENSO-neutral conditions stay in place during August-October (ASO), the peak months of the season? 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 often 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, vertical wind shear, moisture, and stability are still showing some spread for 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 updated 2017 Atlantic Hurricane Season Outlook indicates that an above-normal hurricane season is most likely, with the possibility that the season could be extremely active. The outlook indicates a 60% chance of an above-normal season, a 30% chance of a near-normal season, and only a 10% 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.

Above-normal seasons have higher numbers of named storms, hurricanes, and major hurricanes, and these systems tend to last longer than in near- or below-normal seasons. It is possible that the 2017 hurricane season could be the strongest since 2010.

This updated 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:

• 14-19 Named Storms, which includes the six named storms to date
• 5-9 Hurricanes
• 2-5 Major Hurricanes
• Accumulated Cyclone Energy (ACE) range of 100%-170% of the median, which includes the ACE from the six named storms to date.

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.

To date, the 2017 Atlantic hurricane season has produced six named storms, all at tropical storm strength. For the remainder of the season, we expect (with 70% probability for each range) an additional 8-13 named storms, with 5-9 becoming hurricanes and 2-5 of those becoming major hurricanes.

During 1981-2010, Atlantic hurricane seasons averaged about 12 named storms, 6 hurricanes, 3 major hurricanes.

Extremely active seasons (defined by ACE values above 165% of the median) typically have an even greater number of storms threatening the U.S. Gulf Coast and Atlantic Coast, as well as more storms threatening the region around the Caribbean Sea. To date, two tropical storms (TS) made landfall in the U.S. (Cindy in Texas and Emily in Florida), one TS made landfall in Venezuela (Bret), and one TS made landfall on the Yucatan Peninsula (Franklin).

Changes from the pre-season outlook issued on May 25th:

Compared to NOAA's pre-season outlook issued on 25 May, the probability of an above-normal season has increased to 60% (compared to 45% in May), and it is now more likely that the season could be extremely active. Correspondingly, the probabilities of a near-normal season (now a 30% chance compared to 35% in May) or below-normal season (now a 10% chance compared to 20% in May) have decreased. The predicted numbers of named storms, major hurricanes, and seasonal ACE have also increased from the May outlook (which indicated likely seasonal ranges of 11-17 named storms, 2-4 major hurricanes, and an ACE range of 75%-155% of the median).

Reasons why the likelihood of an above-normal season has increased

• 1. The conducive conditions which were predicted in May for the Atlantic hurricane Main Development Region (MDR, which includes the tropical Atlantic and Caribbean Sea) are now present and are expected to persist through the peak months (August-October, ASO) of the hurricane season. These conditions include weaker vertical wind shear, weaker trade winds, more conducive wind patterns coming off of Africa, and a stronger west African monsoon.
  
  
• 2. There is much higher confidence that ENSO-neutral conditions will persist through ASO, and that El Niño will not develop and suppress the season.
  
  
• 3. Warmer sea surface temperatures (SSTs) in the MDR are more likely to persist than key models had previously predicted.
  
  
• 4. All models now predict a more active season than they did in May.
  
  
• 5. Enhanced June-July activity in the tropical Atlantic (tropical storms Bret and Don) reinforces the expectation for an above-normal season. Historically, years with early-season activity in this region have a higher likelihood of being above-normal.
  

[ENSO refers to El Niño/ Southern Oscillation, which has three phases: El Niño, Neutral, and La Niña.]

Preparedness for Tropical Storm and Hurricane Landfalls:

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 the overall activity predicted by this, or any other, seasonal outlook.

DISCUSSION

1. Expected 2017 activity

NOAA's updated 2017 Atlantic Hurricane Season Outlook indicates that an above-normal hurricane season is most likely (a 60% chance), with the possibility that the season could be extremely active. The outlook indicates a 30% chance for a near-normal season, and only a 10% chance for a below-normal season. See NOAA definitions of above-, near-, and below-normal seasons.

The total seasonal activity is measured by NOAA's Accumulated Cyclone Energy (ACE) index, which accounts for the combined intensity and duration of all named storms and hurricanes during the season. This outlook indicates a 70% chance that the 2017 seasonal ACE range will be 100%-170% of the median. This activity is higher than was predicted in May (75%-155% of the median ACE). The updated ACE range indicates the possibility that the 2017 hurricane season could be the most active since 2010. In fact, 2017 could be the first extremely active season (ACE value > 165% of the median) since 2010.

According to NOAA's hurricane season classifications, an ACE value above 120% of the median reflects an above-normal season. ACE values between 71.4% and 120% of the median reflect a near-normal season, and lower values reflect a below-normal season.

The 2017 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 14-19 named storms (which includes the five named storms to date), of which 5-9 are expected to become hurricanes, and 2-5 of those are expected to become major hurricanes. These ranges are centered above the 1981-2010 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 updated 2017 Atlantic hurricane season outlook reflects five main factors which point to a higher likelihood of an above-normal season and also to a higher possibility of an extremely active season. These factors include: (1) a continuation of the conducive atmospheric conditions now present within the MDR (which spans the Caribbean Sea and tropical Atlantic Ocean between 9°N-21.5°N, Goldenberg and Shapiro 1996), (2) a continuation of above-average SSTs in the MDR, along with continued warmer SST anomalies in the MDR compared to the remainder of the global Tropics, (3) an increased likelihood that ENSO-neutral conditions will persist over the tropical Pacific Ocean, and a sharply decreased chance that El Niño will develop and suppress the hurricane season, (4) all models now predict the season to be more active than they did in May, and (5) the occurrence of two named storms in the deep tropical Atlantic during June-July (Bret and Don) reinforces the expectation for an above-normal season.

These expectations are based on extensive monitoring, analysis, and research activities, a suite of statistical prediction tools, and dynamical model forecasts. The dynamical model predictions come from the NOAA Climate Forecast System (CFS), NOAA Geophysical Fluid Dynamics Lab (GFDL) FLOR-FA model, the United Kingdom Met Office (UKMET) GloSea5 model, and the European Centre for Medium Range Weather Forecasting (ECMWF) model. ENSO forecasts also use a combination 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.

a. Conducive atmospheric conditions in the MDR

The inter-related set of conducive atmospheric conditions now present in the MDR are typical of the many above-normal season seen since 1995. The conducive MDR conditions include weaker vertical wind shear, weaker trade winds, a more conducive African Easterly Jet (AEJ) in association with a stronger west African monsoon system, and a stronger upper-level subtropical ridge. This set of conditions allows for stronger African easterly waves, from which tropical storms and hurricanes can more easily develop (Bell and Chelliah, 2006). These storms then have an extended area in which to intensify as they propagate westward over progressively warmer waters, and within an environment of reduced vertical wind shear, increased tropical moisture, and decreased atmospheric stability. The result is higher numbers of tropical storms, hurricanes, and major hurricanes, and these systems tend to last longer than in near-normal or below-normal seasons.

The vertical wind shear is currently weaker than average across the central MDR. Both the CFS high-resolution (T-382) and low-resolution (T-126) models predict weaker shear to persist during ASO. The high-resolution model predicts the weaker shear to remain mostly confined to the central MDR, while the low-resolution model predicts the weaker shear to expand and cover most of the MDR. A larger area of weaker vertical wind shear is conducive to a more active season, and the CFS low-resolution model is indeed predicting a stronger hurricane season than the high-resolution model.

Weaker easterly and northeasterly trade winds are now present, as indicated by westerly wind anomalies across the southern MDR. This wind pattern contributes to reduced vertical wind shear. It is also associated with a more northward penetration of deep tropical moisture and less stable air into the MDR. These conditions contrast sharply with below-normal seasons, which feature a cooler, drier, and more stable air mass within the MDR due to stronger northeasterly trade winds.

A more conducive African easterly Jet is also now present, in response to the upward extension of the reduced easterly winds (i.e., anomalous westerlies) to the 600-700 hPa level along the equatorward flank of the AEJ axis. The resulting increase in cyclonic relative vorticity in this region is more conducive to the intensification of African easterly waves, especially when that same environment features increased moisture and decreased atmospheric stability. This AEJ configuration is consistent with a strong west African monsoon system.

The upper-level (i.e., 200-hPa) subtropical ridge has been stronger than average across the MDR throughout the summer, as indicated by positive streamfunction anomalies extending from the Caribbean Sea to tropical northern Africa. This pattern is associated with weaker upper-level westerly winds (easterly anomalies) across the central MDR, which contributes to the reduced vertical wind shear. Particularly notable is that this anomaly pattern has been embedded within a much larger-scale circulation featuring enhanced ridges extending from the America's to the east of Africa in the subtropics of both hemispheres. This is the most conducive early-summertime upper-level circulation pattern since the above-normal season of 2012.

b. Above-average sea surface temperatures in the MDR

SSTs in the MDR have been well above average this summer, with the largest departures of between +0.5° and +1.0°C recorded across the eastern half of the tropical Atlantic. The average SST anomaly in the MDR was 0.52°C during June-July, and this departure was 0.23°C warmer than that of the remainder of the global tropics. For the ASO season, SSTs have been generally above average in the MDR since 1995.

The increased probability for an above-normal season partly reflects a high confidence that SSTs in the MDR will remain above average. Predictions from the CFS high-resolution and low-resolution models are now notably warmer than those issued in April and May. However, there is still some uncertainty in the expected magnitude and spatial extent of this anomalous warmth. The high-resolution model predicts above-average SSTs in most of the tropical Atlantic during ASO, but also predicts below-average SSTs in the Caribbean Sea. The CFS low-resolution model predicts above-average SSTs of at least 0.5°C across both the tropical Atlantic and Caribbean Sea.

Overall, the current and predicted conditions within the MDR are consistent with the warm phase of the Atlantic Multi-decadal Oscillation (AMO). The AMO is 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. For the Atlantic hurricane basin, the AMO has historically produced 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).

The warm phase of the AMO has generally been present since 1995, and the transition to this warm phase ushered in a high-activity era for Atlantic hurricanes which began that year (Goldenberg et al. 2001). However, there is disagreement between two main AMO indices as to whether or not this latest warm phase of the AMO has ended. The Kaplan AMO index shows that the presence of the warm AMO during the hurricane season (June-November) from 1995 through at least last year, while the Klotzbach-Gray AMO index (Klotzbach and Gray, 2008) shows a more variable AMO over the last few years. Both indices show that the June-November index has been generally warmer than the January-May index over the past few years, suggesting that the early-year AMO index is probably not a good predictor of its strength during June-November.

c. ENSO-neutral likely

ENSO-neutral conditions (no El Niño or La Niña) have been present throughout the summer. The CPC classifies ENSO-neutral conditions as having area-averaged SST anomalies in the Niño 3.4 region between -0.5°C and +0.5°C, as indicated by the Nino 3.4 region index. The Nino 3.4 region is located in the east-central equatorial Pacific between 170°W- 120°W and 5°N-5°S. El Niño (which suppresses the hurricane season; Gray, 1984) is classified as a Niño 3.4 index value at or above +0.5°C for 5-consecutive months, along with consistent atmospheric impacts.

SST's in the central and east-central equatorial Pacific have recently returned to near average, following above-average SSTs of at least 0.5°C in early July. This cooling is seen in the SST indices for both the Niño 4 and Niño 3.4 regions, and follows the cooling seen during late May and June farther east in the Niño 3 and Niño 1+2 regions. The latest weekly SST indices show near-zero values for all four Niño regions.

The average of the dynamical model predictions now indicates a Niño 3.4 index value of roughly 0.25°C (i.e., ENSO-neutral) during ASO, which is a considerable decrease from the value of 0.75°C (i.e., El Niño) predicted earlier in the year. This is one reason why models now predict the season to be more active than they did in May.

Model predictions, along with the demise of the recent Pacific warmth and continued near-average atmospheric conditions over the equatorial Pacific, provide high confidence that ENSO-neutral conditions will persist through ASO. Also, there is a high confidence that El Niño will not develop during the hurricane season. In May, the outlook for ASO reflected approximately equal 47% chances for ENSO-Neutral and El Niño.

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
• Dr. Christopher 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 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.


• 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.


• 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.