The 2017 eastern Pacific hurricane season outlook is an official product of the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC), and is produced in collaboration with scientists from the NOAA National Hurricane Center (NHC). The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140°W north of the equator.
Interpretation of NOAA's eastern Pacific hurricane season outlook
This outlook is general guide to the expected overall activity during the upcoming hurricane season. It is not a seasonal hurricane landfall forecast, and it does not imply levels of activity for any particular location.
Hurricane disasters can occur whether the season is active or relatively quiet. It only takes one hurricane (or even a tropical storm) to cause a disaster. Therefore, 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 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 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 conditions and uncertainties to those expected this year. They do not represent the total possible range of activity seen in past similar years.
This outlook is based on predictions of large-scale climate factors and conditions known to strongly influence seasonal eastern Pacific hurricane activity, along with climate model forecasts. The outlook also takes into account uncertainties inherent in such climate outlooks.
Sources of uncertainty in this seasonal outlook
- This year, it is difficult to assess whether the low-activity era for the eastern Pacific has ended.
- Predicting El Niño and La Niña (also called ENSO) impacts is an ongoing scientific challenge facing climate scientists today. Such forecasts made during the spring generally have limited skill.
- 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.
- Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.
2017 Eastern Pacific Hurricane Outlook Summary
a. Predicted Activity
NOAA's 2017 eastern Pacific Hurricane Season outlook predicts a near-normal or above-normal season, with a 40% chance of an above-normal season, a 40% chance of a near-normal season, and a 20% chance of a below-normal season. See NOAA definitions of above, near-, and below- normal seasons. The eastern Pacific hurricane region covers the eastern North Pacific Ocean east of 140°W north of the equator.
The outlook calls for a 70% probability for each of the following ranges of activity during the 2017 hurricane season, which runs from May 15th through November 30th:
- 14-20 Named Storms
- 6-11 Hurricanes
- 3-7 Major Hurricanes
- Accumulated Cyclone Energy (ACE) range of 75%-145% 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 above the 1981-2010 seasonal averages of 15 named storms, 8 hurricanes, and 4 major hurricanes.
There will be no further updates to this outlook.
b. Reasoning behind the outlook
NOAA's 2017 eastern Pacific hurricane season outlook reflects three main factors during July-September, which is the peak of the hurricane season:
- (1) Either ENSO-neutral or weak El Niño conditions 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) in the main hurricane formation region, and
- (3) Near-average or weaker-than-average vertical wind shear in that same region.
Model forecasts show considerable uncertainty as to whether El Niño, if it forms, will become strong enough to significantly strengthen the hurricane season. The combination of ENSO-neutral, near-average SSTs and near-average vertical wind shear in the eastern Pacific hurricane region would likely yield levels of activity near the lower ends of the predicted ranges. The combination of El Niño, warmer SSTs and weaker vertical wind shear would likely yield levels of activity toward the upper ends of the predicted ranges.
1. Expected 2017 activity
NOAA's 2017 eastern Pacific Hurricane Season outlook indicates a near-normal or above-normal season, with a 40% chance of an above-normal season, a 40% chance of a near-normal season, and a 20% chance of 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 70% chance that the 2017 seasonal ACE range will be 75%-145% of the median. An ACE value of 80%-115% of the median indicates 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 eastern Pacific hurricane season is predicted to produce (with 70% probability for each range) 14-20 named storms, of which 6-11 are expected to become hurricanes, and 3-7 are expected to become major hurricanes. These ranges are centered above the official NHC 1981-2010 seasonal averages of 15 named storms, 8 hurricanes, and 4 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 eastern Pacific hurricane season outlooks are based on predictions of the main climate factors and their associated conditions known to influence the hurricane season. 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 eastern Pacific hurricane season outlook reflects three main factors during the peak months (July-September, JAS) of the hurricane season: (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 hurricane formation region, and (3) Near-average or weaker-than-average vertical wind shear in that same region.
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 and 5°N and 5°S, 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 reduced vertical wind shear in the hurricane formation region, along with anomalous rising motion, and decreased atmospheric stability. These conditions are conducive to hurricane formation and intensification. Conversely, La Niña causes increased vertical wind shear in the hurricane formation region, along with anomalous sinking motion and increased atmospheric stability. These conditions act to suppress the hurricane season.
Sub-surface ocean temperatures are currently 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, which reflect the 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 enhance the 2017 eastern Pacific 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 JAS 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 JAS, perhaps with sufficient amplitude to have an enhancing influence on the eastern Pacific hurricane season. The average of the statistical model predictions (thick green line) indicates borderline neutral to weak El Niño conditions during JAS 2017.
There is even considerable spread in the SST forecasts coming from different resolutions of the same model. For example NOAA's CFS high-resolution (T-382) model is predicting below-average SSTs in the Niño 3.4 region during JAS and August-October, 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 decrease the vertical wind shear in the eastern Pacific hurricane region.
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. Global SST anomalies
The strength of the eastern Pacific hurricane season is also related to the global patterns of SST anomalies that occur on multi-decadal time scales. One such pattern is called the Atlantic Multi-Decadal Oscillation (AMO), and another is called the Pacific Decadal Oscillation (PDO). These climate patterns can last for decades, and are associated with strong multi-decadal fluctuations in hurricane activity. In contrast, ENSO contributes to inter-annual variations in hurricane activity.
The last three eastern Pacific hurricane seasons were above normal, with above-average SSTs across hurricane region. This warmth was associated with the positive phase of the PDO (which features above-average SSTs in the hurricane region and in the far eastern Pacific), along with either a weak warm phase or neutral phase of the AMO. The recent SST anomaly pattern again shows above-average SSTs in the eastern and central Pacific hurricane regions.
The recent increase in hurricane activity is in sharp contrast to the 1995-2013 period, when the eastern Pacific hurricane seasons were generally below normal (called a low-activity era). This reduced activity was associated with a combination of the cold phase of the PDO and warm phase of the AMO. These conditions contributed to anomalously cool SSTs in the eastern Pacific hurricane region. They also contributed to a pronounced east-west dipole pattern in hurricane activity, with a high-activity era for Atlantic hurricanes coinciding with the low-activity era in the eastern Pacific.
For July-September and August-October 2017, NOAA's CFS high-resolution model (Left) predicts a mixed pattern of SST anomalies in the eastern Pacific hurricane region. Conversely, the CFS low-resolution (T-126) model predicts above-average SSTs across the entire eastern and central Pacific hurricane regions. NOAA's 2017 hurricane season outlook reflects a prediction for near- or above-average SSTs in both the Atlantic and Pacific main hurricane development regions. However, the spatial extent and magnitude of the anomalous warmth is uncertain. This favors a near-normal or above-normal season.
Climate Prediction Center
National Hurricane Center