The 2018 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.
Preparedness
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.
2018 Eastern Pacific Hurricane Outlook Summary
a. Predicted Activity
NOAA's 2018 eastern Pacific Hurricane Season outlookindicates an 80% chance of a near-normal or above-normal season. An above-normal season is most likely (45% chance), followed by a 35% 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:
- 14-20 Named Storms
- 7-12 Hurricanes
- 3-7 Major Hurricanes
- Accumulated Cyclone Energy (ACE) range of 80%-160% of the median.
The 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 predicted ranges are centered above the 1981-2010 averages of 15 named storms, 8 hurricanes, and 4 major hurricanes.
The eastern Pacific hurricane season officially runs from May 15th through November 30th.
There will be no further updates to this outlook.
b. Reasoning behind the outlook
This outlook reflects the expectation of ENSO-neutral (no El Niño or La Niña) or weak El Niño conditions, along with above-average sea-surface temperatures (SSTs) across the eastern and central tropical North Pacific. [ENSO refers to El Niño/ Southern Oscillation, which has three phases: El Niño, neutral, and La Niña.].
NOAA's latest ENSO forecast indicates a higher likelihood of ENSO-neutral conditions than El Niño throughout the hurricane season. There is about a 30% chance of El Niño during Jul.-Sep., about a 40% chance of El Niño during Aug.-Oct. and Sep.-Nov, and about a 45% chance that El Niño will develop later in the hurricane season. If El Niño does develop in time, and if the SSTs remain above average in the hurricane region, then the activity could be near the upper ends of the predicted ranges.
DISCUSSION
1. Expected 2018 activity
NOAA's 2018 eastern Pacific Hurricane Season outlook indicates an 80% chance of a near-normal or above-normal season. An above-normal season is most likely (45% chance), followed by a 35% 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 2018 outlook indicates a 70% chance that the ACE range will be 80%-160% 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 2018 eastern Pacific hurricane season is predicted to produce (with 70% probability for each range) 14-20 named storms, of which 7-12 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 hurricane landfalls are ultimately related to the daily weather patterns, storm genesis locations and steering patterns. These patterns 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 2018 Outlook
NOAA's eastern Pacific hurricane season outlook is based on predictions of the main climate factors and their associated conditions known to influence the hurricane season. The outlooks 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, the North American Multi-Model Ensemble (NMME), 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 CPC.
NOAA's 2018 eastern Pacific hurricane season outlook reflects two main factors:
(1) The presence of either ENSO-neutral or El Niño conditions. El Niño generally enhances the eastern (and central Pacific) hurricane seasons, while La Niña usually suppresses them. El Niño enhances hurricane formation and intensification by decreasing the vertical wind shear. The development of El Niño this season could favor hurricane activity near the upper ends of the predicted ranges.
(2) A likely continuation of above-average SSTs across much of the hurricane formation region and subtropical eastern Pacific Ocean. SSTs have generally been well above average in this region since 2015, and models are predicting this warmth to continue through the hurricane season.
a. ENSO-neutral or weak El Niño
La Niña dissipated during April 2018, after developing last fall. Currently, ENSO-neutral conditions are present, with only slightly below-average SSTs remaining in the equatorial Pacific Ocean east of the date line. The corresponding SST index for the Niño 3.4 region is currently -0.1°C. This region spans the east-central equatorial Pacific (between 120°W-170°W and 5°N-5°S). 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 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 +1.0° and +1.5°C.
Sub-surface ocean temperatures are currently warmer than average in the central and eastern equatorial Pacific. However, this anomalous warmth is located well below the ocean surface, and is associated with a slightly deeper-than-average position of the oceanic thermocline. 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) shows that this anomalous sub-surface warmth was associated with a strong downwelling equatorial oceanic Kelvin wave. This wave was initiated in February and propagated eastward during March and April, warming the sub-surface temperatures across the central and east-central equatorial Pacific. This evolution marked the end of La Niña, and the anomalous warmth has since persisted.
The current sub-surface temperature pattern typically precedes El Niño. However, there is considerable uncertainty as to whether El Niño will develop in time, and with sufficient strength, to enhance the eastern Pacific hurricane season. Large uncertainties for predictions of ocean temperatures in the equatorial Pacific are typical at this time of the year, because this is when the model forecast skill is at its lowest.
Model predicted SST anomalies in the Niño 3.4 region for Jul.-Sep. (JAS) 2018 indicate ENSO-neutral. The average of the dynamical model predictions (thick red line) indicates border-line weak El Niño conditions very late in the hurricane season. The average of the statistical model predictions (thick green line) indicates border-line weak El Niño conditions forming sooner, during ASO 2018.
Based on current conditions, the recent oceanic evolution, and the large spread in model forecasts, NOAA's latest ENSO outlook indicates a higher likelihood of ENSO-neutral conditions than El Niño throughout the hurricane season. There is about a 30% chance of El Niño during Jul.-Sep., about a 40% chance of El Niño during Aug.-Oct. and Sep.-Nov, and about a 45% chance that El Niño will develop later in the hurricane season. There is only a small (10%) chance that La Niña will re-develop this season. If El Niño develops in time, it could favor hurricane activity near the upper ends of the predicted ranges.
b. Global and regional SST anomalies
The strength of the eastern Pacific hurricane season is also related to the global patterns of SST anomalies that occur on decadal and 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 are associated with strong decadal and multi-decadal fluctuations in hurricane activity. They produce the background climate patterns upon which the inter-annual ENSO signals overlay.
Three of the last four eastern Pacific hurricane seasons were above normal. This increase in activity compared to the previous 1995-2013 period was accompanied by a sharp increase in SSTs across the hurricane region. The anomalous 2014-17 warmth during the peak months of the season (July-September, JAS) has been far larger than any other period in the historical record dating back to 1950. This warmth was associated with the positive phase of the PDO, and also had a strong projection onto the Pacific Meridional Mode (PMM, Murakami et al., 2016, J. Climate). The PMM features a north-south dipole pattern of SST anomalies across the eastern half of the Pacific Ocean, with warmer (cooler) than average SSTs in the North Pacific (South Pacific). Thus, both the positive PDO and PMM pattern feature above-average SSTs in the eastern and central Pacific hurricane regions.
The most recent SST anomaly pattern again shows above-average SSTs in the eastern and central Pacific hurricane regions, and the March-April 2018 SST anomalies were comparable to those during 2014-17. In addition, the 2018 pattern features below-average SSTs across the eastern half of the tropical South Pacific Ocean. This overall pattern projects very strongly onto the PMM, which Murakami et al. (2016) linked to a more active 2015 eastern Pacific hurricane season.
For July-September 2018, NOAA's CFS model predicts a likely continuation of the anomalous warmth across a large portion of the tropical eastern and central North Pacific. This forecast is consistent with predictions from the NMME model, the GFDL models, and the ECMWF. Consequently, this 2018 hurricane outlook reflects a prediction for above-average SSTs in the eastern Pacific main hurricane development region. The combination of this anomalous warmth and a weak El Niño could put the activity near the higher ends of the predicted ranges.
NOAA FORECASTERS:
Climate Prediction Center
National Hurricane Center
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