The 2021 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-related disasters can occur whether the season is active or relatively quiet. It only takes one hurricane (or tropical storm) to cause a disaster. It is crucial that residents, businesses, and government agencies of coastal and near-coastal regions prepare for every hurricane season regardless of this, or any other, seasonal outlook. 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, and those patterns 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 forecast models that directly predict seasonal hurricane activity.
Sources of uncertainty in the seasonal outlooks:
Predicting El Niño and La Niña events (also called the El Niño-Southern Oscillation, or ENSO) and their impacts on Atlantic region hurricane activity, is an ongoing scientific challenge facing 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 shorter-lived storms or fewer longer-lived storms with greater intensity.
Model predictions of sea surface temperatures (SSTs), vertical wind shear, moisture, atmospheric stability, and other factors known to influence overall seasonal hurricane activity have limited skill this far in advance of the peak months (August-October) of the hurricane season.
Shorter-term weather patterns that are unpredictable on seasonal time scales can sometimes develop and last for weeks or months, possibly affecting seasonal hurricane activity.
a. Predicted Activity
NOAA's outlook for the 2021 Atlantic Hurricane Season indicates that an above-normal season is most likely, with a possibility the season could be extremely active. The outlook calls for a 60% chance of an above-normal season, followed by a 30% chance of a near-normal season, and 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.
The 2021 outlook calls for a 70% probability for each of the following ranges of activity:
- 13-20 Named Storms
- 6-10 Hurricanes
- 3-5 Major Hurricanes
- Accumulated Cyclone Energy (ACE) range of 110%-190% of the median
The 2021 Atlantic hurricane season could extend the record of consecutive seasons with above-normal activity to 6 consecutive seasons.
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 1991-2020 seasonal averages of 14 named storms, 7 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 season.
b. Reasoning behind the outlook
This 2021 seasonal hurricane outlook reflects the expectation of either non-competing or reinforcing climate factors during August-October (ASO), and these factors historically produce Atlantic hurricane seasons with above-normal overall activity. The main climate factors for this outlook are:- The set of conditions that have produced the ongoing high-activity era for Atlantic hurricanes which began in 1995. These conditions include warmer sea surface temperatures (SSTs) and weaker trade winds in the Atlantic hurricane Main Development Region (MDR), along with weaker vertical wind shear and an enhanced West African monsoon. The MDR spans the tropical Atlantic Ocean and Caribbean Sea (Goldenberg and Shapiro 1996).
- The most recent foreast from NOAA's Climate Prediction Center indicates ENSO-neutral conditions are likely, but a redevelopment of La Niña is also possible. The odds are highest for ENSO-neutral (50%), followed by La Niña (42%), and the least favored outcome is El Niño (8%). ENSO-neutral does not inhibit hurricane formation nor suppress the high-activity era conditions. La Niña tends to reinforce those high-activity era conditions and further increases the likelihood of an above-normal season with activity near the upper 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 for the season. 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 2021 activityNOAA's outlook for the 2021 Atlantic Hurricane Season indicates that an above-normal season is most likely (60% chance), with the possibility of it being extremely (aka hyper-) active. The outlook calls for a 60% chance of an above-normal season, followed by a 30% chance of a near-normal season, and a 10% chance of a below-normal season.
The 2021 Atlantic hurricane season is predicted to produce (with 70% probability for each range) 13-20 named storms, of which 6-10 are expected to become hurricanes, and 3-5 of those are expected to become major hurricanes. These ranges are centered above the 1991-2020 period averages of about 14 named storms, 7 hurricanes, and 3 major hurricanes.
The 2021 Atlantic hurricane season could further extend the record streak of consecutive above-normal seasons to 6. Since the current Atlantic high-activity era began in 1995, 18 of 26 (about two-thirds) seasons have been above normal and only 4 (15%) have been below normal based on the 1951-2020 climatology.
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 all named storms and hurricanes during the season. This 2021 outlook indicates a 70% chance that the seasonal ACE range will be 110%-190% of the median. According to NOAA's hurricane season classifications, an ACE value between 75.4% and 130% of the 1951-2020 median reflects a near-normal season. Values above (below) this range reflect an above-normal (below-normal) season. The 2021 predicted ACE range extends well above the thresholds for an above-normal season (130% of median) and into the range for an extremely (or hyper-) active season (≥165% of median).
Predictions of the location, number, timing, and intensity of hurricane landfalls are ultimately related to the daily weather patterns which determine storm genesis locations and steering patterns. These patterns are not predictable weeks or months in advance. As a result, it is not possible to reliably predict the number or intensity of landfalling hurricanes in a seasonal outlook, or whether a given locality will be impacted by a tropical storm or hurricane this season.
2. Science behind the 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, 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 and SPEAR models, the North American Multi-Model Ensemble (NMME), the United Kingdom Met Office (UKMET) GloSea6 model, and the European Centre for Medium Range Weather Forecasting (ECMWF) model. El Niño forecasts are also provided from statistical and other dynamical models contained in the suite of Niño 3.4 SST forecasts, which is compiled by the International Research Institute for Climate and Society (IRI) and the NOAA Climate Prediction Center (CPC).
NOAA's 2021 Atlantic hurricane season outlook reflects the expectation of either non-competing or reinforcing climate factors during August-October, as follows:
The underlying climate factor again this season is the continuation of the high-activity era for Atlantic hurricanes, which began in 1995 in association with a transition to the warm phase of the Atlantic Multidecadal Oscillation (AMO) (Goldenberg et al. 2001, Bell and Chelliah 2006, Klotzbach and Gray 2008). The recently observed and upcoming predicted atmospheric conditions for ASO 2021 reflect the warm AMO phase, and include weaker trade winds and warmer SSTs across much of the MDR, a more conducive African easterly jet, weaker vertical wind shear, an enhanced West African monsoon.
The most recent NOAA ENSO probability forecast indicates a 50% chance that ENSO-neutral conditions will continue through the hurricane season, and a 42% chance that La Niña could redevelop and be in place during the peak months (ASO) of the season. About 25% of past instances where ASO met the oceanic threshold for La Niña had a following year that also met the criteria, so consecutive La Niña episodes are not unusual.
a. ENSO-neutral or La Niña most likely
La Niña represents one phase of the climate phenomenon known as ENSO (i.e. El Niño/ Southern Oscillation). The three phases of ENSO are El Niño, La Niña, and ENSO-Neutral. El Niño tends to suppress Atlantic hurricane activity, while La Niña tends to enhance it (Gray 1984; Goldenberg and Shapiro 1996). These impacts can be strongly modulated by conditions associated with a low- or high-activity era.
ENSO-neutral conditions are present at this time. The latest weekly SSTs are currently 0.4°C below average across the central and eastern equatorial Pacific, and the SST index for the Niño 3.4 region is -0.4°C. The weekly Niño 3.4 index had been below -0.5°C since August 2020. The recent decrease in amplitude of the Niño 3.4 index reflects a weakening of the ongoing La Niña. The wind and OLR patterns over the central Pacific are also reflecting a disruption of the atmospheric response to La Niña.
Looking forward, model-predicted SST anomalies in the Niño 3.4 region generally indicate ENSO-neutral (Niño 3.4 index between ±0.5°C) throughout the hurricane season, though mostly on the cool side. The dynamical model average (red line) and statistical model average (green line) are both on the cool side of zero. ENSO-neutral does not inhibit hurricane formation or suppress the high-activity era conditions.
NOAA's CFS and the NMME are predicting ENSO-neutral. Some of the constituent models in the North American Multi-model Ensemble are predicting the redevelopment of La Niña during ASO 2021. The CFS and NMME both predict slightly below-normal vertical wind shear over the Main Development Region. The shear predicted this year is not as low as what was observed last year, but the odds for either ENSO-neutral or La Niña are both higher than last year, as of the April ENSO forecast. A redevelopment of La Niña would reinforce the high-activity era conditions and further increase the likelihood of an above-normal (or even extremely active) season, with activity more likely to be near the upper ends of our predicted ranges.
Based on current conditions, the recent oceanic evolution, model forecasts, and the low skill inherent in ENSO predictions made at this time of the year, NOAA's ENSO outlook from May of 2021 indicates about a 50% chance of ENSO-neutral during the peak months (ASO) of the hurricane season, followed by a 42% chance of La Niña and only a 8% chance of El Niño.
Therefore, the current hurricane season outlook reflects the expectation that the ENSO will likely either not compete with, or will reinforce, the ongoing set of conditions associated with the current high-activity era for Atlantic hurricanes (discussed below).b. Predicted conditions within the MDR
SSTs are currently near average across the MDR, with an area-averaged anomaly during April of nearly +0.07°C. For the MDR as a whole, both the CFS and NMME models predict slightly above-average SSTs during ASO. Among the NMME models, the predicted values of MDR SST anomalies range from near zero to +1.0°C, with the average being +0.38°C. While models tend to have low skill in predicting the strength of the Atlantic SST anomalies this far in advance, these model predictions are consistent with the ongoing warm phase of the AMO.
Also consistent with the warm AMO phase, two inter-related atmospheric features are now present and are expected to persist through ASO 2021. The first is anomalous westerly winds at 700-hPa across the central and eastern tropical Atlantic. The anomalous winds are not as strong this year as last year. The second is an enhanced West African monsoon system. Intraseasonal oscillations (the Madden-Julian Oscillation) may have played a role in some early enhancement of upper-level divergence and rainfall totals. Should the monsoon stay enhanced, as indicated in many of the model outlooks, these conditions are expected to contribute to an inter-related set of conditions during ASO within the MDR, all of which favor increased Atlantic hurricane activity. These inter-related features include 1) anomalously warm SSTs and decreased vertical wind shear, 2) an African Easterly Jet (AEJ) structure that allows for increased rotation of low-pressure cloud systems (also called African easterly waves) moving westward from Africa, and 3) the combination of increased moisture and decreased atmospheric stability. Because of these conditions, the enhanced African easterly waves can develop more easily into tropical storms and hurricanes, thus increasing the likelihood of above-normal activity this season.
The main uncertainty in this seasonal hurricane outlook is whether ENSO-neutral will persist through ASO, or whether La Niña conditions will redevelop. Most models are predicting ENSO-neutral, while some dynamical models are predicting a transition back to La Niña as the summer progresses. As a result, there are uncertainties in determining the predicted impacts on seasonal activity. If La Niña does redevelop, it would tend to reduce the vertical wind shear across the central and western MDR. This factor, in combination with the predicted continuation of the high-activity era conditions mentioned earlier, would favor seasonal activity near the upper ends of our predicted ranges.
NOAA FORECASTERS
Climate Prediction Center
- Matt Rosencrans, Physical Scientist; Matthew.Rosencrans@noaa.gov
- Dr. Hui Wang, Physical Scientist; Hui.Wang@noaa.gov
National Hurricane Center
- Eric Blake, Senior Hurricane Specialist; Eric.S.Blake@noaa.gov
- Dr. Chris Landsea, Meteorologist; Chris.Landsea@noaa.gov
- Dr. Richard Pasch, Senior 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.
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.
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 activity. J. 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.
