The 2018 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 region.
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 the 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:
- 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.
- 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.
- 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.
- Weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.
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2018 Atlantic Hurricane Season Outlook: Summary
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a. Predicted Activity
NOAA's outlook for the 2018 Atlantic Hurricane Season indicates that a near-normal season is most likely (40% chance), followed by a 35% chance of an above-normal season and a 25% 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.
The 2018 outlook calls for a 70% probability for each of the following ranges of activity:
- 10-16 Named Storms
- 5-9 Hurricanes
- 1-4 Major Hurricanes
- Accumulated Cyclone Energy (ACE) range of 65%-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 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.
The Atlantic hurricane season officially runs from June 1st through November 30th. This outlook will be updated in early August to coincide with the onset of the peak months of the hurricane season.
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b. Reasoning behind the outlook
Currently, we have ENSO-neutral (no El Niño or La Niña) conditions in the Pacific Ocean, along with anomalously cool sea-surface temperatures (SSTs) in the Atlantic hurricane Main Development Region (MDR, which includes the tropical Atlantic Ocean and Caribbean Sea). [ENSO refers to El Niño/ Southern Oscillation, which has three phases: El Niño, neutral, and La Niña.].
Looking forward, there is considerable uncertainty in the predictions for both El Niño and Atlantic SSTs during the August-October period. This outlook reflects our expectation of ENSO-neutral conditions or a weak El Niño, along with a return to near-average SSTs in the MDR as the summer progresses.
These conditions are set upon a backdrop of the ongoing high-activity era for Atlantic hurricanes that began in 1995. This increased activity has been associated with the warm phase of the Atlantic Multi-Decadal Oscillation (AMO), which produces a set of atmospheric and oceanic conditions that are conducive to stronger hurricane seasons.
The combination of ENSO-neutral conditions and a warmer MDR could favor hurricane activity near the upper ends of the predicted ranges, while the combination of El Niño and a cooler MDR could favor activity near the lower ends of the predicted ranges.
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Preparedness for Tropical Storm and Hurricane Landfalls:
It only takes one storm hitting an area to cause a disaster, regardless of the overall season strength. 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.
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DISCUSSION
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1. Expected 2018 activity
NOAA's outlook for the 2018 Atlantic Hurricane Season indicates that a near-normal season is most likely (40% chance), followed by a 35% chance of an above-normal season and a 25% 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 seasonal ACE range will be 65%-145% 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 2018 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 10-16 named storms, of which 5-9 are expected to become hurricanes, and 1-4 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 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.
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2. Science behind the 2018 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. These 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) FLOR model, 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 NOAA's IRI (International Research Institute for Climate and Society) and the Climate Prediction Center (CPC).
NOAA's 2018 Atlantic hurricane season outlook reflects three main factors :
1) The presence of either ENSO-neutral or El Niño conditions during the peak months of the hurricane season. El Niño generally suppresses the Atlantic hurricane season, while La Niña usually enhances it. El Niño suppresses Atlantic hurricane formation and intensification by causing increased vertical wind shear, anomalous sinking motion, and increased atmospheric stability in the MDR. The expected lack of a La Niña this season is a limiting factor for the higher end of the predicted hurricane ranges.
2) Near-average SSTs in the MDR . All climate models predict the SSTs in the MDR to be cooler than we saw during the past two above-normal seasons (2016 and 2017). Cooler SSTs in the MDR (especially if cooler than the remainder of the global Tropics) are typically associated with an inter-related set of atmospheric anomalies which favor a less active season. The expected lack of extensive anomalous warmth in the MDR this season is another limiting factor for the higher end of the predicted hurricane ranges.
3) A likely continuation of the high-activity era for Atlantic hurricanes which began in 1995. This period has been associated with the warm phase of the AMO, and with its inter-related set of atmospheric anomalies that favor a more active season (such as weaker vertical wind shear, a stronger West African monsoon system, and more moisture and instability in the MDR).
The combination of a warmer MDR and ENSO-neutral could favor hurricane activity near the upper ends of the predicted ranges, while the combination of El Niño and a cooler MDR could favor activity near the lower ends of the predicted ranges.
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a. ENSO-neutral or weak El Niño
La Niña dissipated during April 2018, after developing last fall and helping to strengthen the 2017 Atlantic hurricane season. Currently, ENSO-neutral conditions are present, with only slightly below-average SSTs remaining in the equatorial Pacific east of the date line. The corresponding SST index for the Niño 3.4 region is currently -0.1°C. The Niño 3.4 region spans the east-central equatorial Pacific between 120°W-170°W and 5°N-5°S. 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 suppress the Atlantic 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 lowest.
Model predicted SST anomalies in the Niño 3.4 region for ASO 2018 generally range from ENSO-neutral to a weak El Niño. The average of the dynamical model predictions (thick red line) indicates neutral conditions during ASO, with a border-line weak El Niño later in the season. The average of the statistical model predictions (thick green line) indicates a border-line weak El Niño 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 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 and with sufficient strength, it could suppress the latter part of the Atlantic hurricane season.
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b. Slightly below average to slightly above average SSTs in the MDR
During March-April, SSTs in the eastern MDR were below average, with the largest departures between -0.5° and -1°C. The most recent 4-week average shows that this cooling has persisted. Conversely, SSTs in the Caribbean Sea were slightly warmer than average during March-April, but have recently become slightly below average. For the MDR as a whole, March-April 2018 SSTs were the coolest since 2014 and 2015. The Atlantic hurricane season was near normal in 2014 and below normal in 2015.
This recent cooling in the MDR is linked to a strong high-pressure system over the central North Atlantic. To the east and south of this anomalous pressure ridge are enhanced northeasterly trade winds, which have acted to cool the MDR. NOAA's CFS model, along with several other dynamical models (GFDL, ECMWF, NMME) are predicting a return to near-average SSTs in the MDR by Aug.-Oct. 2018. However, there is typically low skill and considerable spread in model predictions of Atlantic SSTs this far ahead of the ASO season. During the last few hurricane seasons, SSTs in the MDR have been much warmer than model predictions (Blake et al., 2018). This cool bias greatly limits confidence in the model SST forecasts for this summer and fall.
For the ASO season, SSTs in the MDR have been above average since 1995. It is possible that this region will again be warmer than the models are currently predicting. Warmer SSTs in the MDR (especially if warmer than the remainder of the global Tropics) are typically associated with an inter-related set of atmospheric anomalies which produce more active seasons. This is one reason why we indicate higher probabilities for both a near-normal and above-normal 2018 Atlantic hurricane season than for a below-normal season.
The Atlantic Multi-decadal Oscillation (AMO) is a prominent climate factor which significantly impacts Atlantic hurricane activity for decades at a time. 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). The warm (i.e. positive) phase of the AMO was present during the high-activity eras of 1950-1970 and 1995-present. The cold phase of the AMO was present during the Atlantic low-activity era of 1971-1994.
The warm phase of the AMO is associated with anomalously warm SSTs in the MDR, and with an anomalously warm MDR compared to the remainder of the global Tropics. Both of these conditions have prevailed since 1995.
The time series of one measure of the AMO (the Kaplan AMO index) suggests that the warm (i.e. positive) AMO phase has persisted at least though last year's very active hurricane season. This measure is consistent with the exceptionally warm SSTs in the MDR during ASO 2017 compared to the remainder of the global Tropics.
Another measure of the AMO (the Klotzbach-Gray AMO index) suggests that the AMO may have returned to neutral in the last few years. This latter index is based in part on SST anomalies at the high latitudes of the North Atlantic, and does not incorporate a contribution from SST anomalies in the MDR. Thus, its return to near-zero during the last few seasons is not a reflection of the significant anomalous warmth seen in the MDR during the last few summers.
The AMO sets the backdrop upon which other climate phenomena such as El Niño and La Niña overlay. This 2018 outlook reflects the likelihood that the warm AMO phase has not yet ended, and that the MDR could again become anomalously warm as the summer progresses. However, the outlook also reflects model predictions for near-average SSTs in the MDR. At this time, there is considerable uncertainty in the SST predictions for the MDR.
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NOAA FORECASTERS:
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Climate Prediction Center
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National Hurricane Center
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Hurricane Research Division
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REFERENCES
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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.
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Behringer, D.W., M. Ji, and A. Leetmaa, 1998: An improved coupled model for ENSO prediction and implications for ocean initialization. Part I: The ocean data assimilation system. Mon. Wea. Rev., 126, 1013-1021.
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Blake, E. S., P. Klotzbach, and G. D. Bell, 2018: Climate factors causing the extremely active 2017 Atlantic hurricane season. Presented at AMS 33rd Conference on Hurricanes and Tropical Meteorology, April 2018.
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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.
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Goldenberg, S. B. and L. J. Shapiro, 1996: Physical mechanisms for the association of El Niño and west African rainfall with Atlantic major hurricane activity. J. Climate, 9, 1169-1187.
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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.
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Klotzbach, P.J., and W. M. Gray, 2008: Multi-decadal Variability in North Atlantic Tropical Cyclone Activity, J. Climate, 21, 3929-3935.
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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.
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