Post Mortem

2023

The 2023 Atlantic hurricane season was characterized as an above-normal season based on the National Oceanic and Atmospheric Administration definition. The season had 20 named storms, of which 7 became hurricanes and 3 became major hurricanes (Category 3+ on the Saffir-Simpson Hurricane Wind Scale). The 20 named storms that formed was tied with 1933 for the 4th most in an individual Atlantic hurricane season on record. The average (1991–2020) Atlantic hurricane season has about 14 named storms, 7 hurricanes and 3 major hurricanes. Accumulated Cyclone Energy (ACE), an integrated index accounting for frequency, intensity and duration of storms, was somewhat above the long-term average.  The 2023 Atlantic hurricane season produced 146 ACE, while the average season produced 123 ACE. 

There was considerable spread in the seasonal hurricane forecasts issued by groups contributing to the Seasonal Hurricane Predictions platform in 2023. This was due to the strong contrasting impacts of a strong El Niño and a record warm tropical Atlantic. The strong El Niño was well anticipated by most climate models, while the record warm tropical Atlantic was not anticipated until closer to the start of the hurricane season. Consequently, publicly-issued seasonal Atlantic hurricane forecasts generally increased their forecasts from March/April to May/June with a slight increase in July/August.  The average of all seasonal outlooks issued at these three lead times was below the observed value for ACE in March/April (114 ACE predicted) and close to the observed value for ACE in May/June (142 ACE predicted) and in July/August (155 ACE predicted).  The record warm tropical Atlantic largely counteracted the hurricane-detrimental impacts of the strong El Niño, resulting in much less vertical wind shear than is typical for El Niño events.

The average values predicted by all groups issuing forecasts in March/April were 15 named storms, 7 hurricane and 2.7 major hurricanes.  These numbers increased in May/June (16 named storms, 8 hurricanes and 3.3 major hurricanes).  The average prediction issued in July/August was for 17 named storms, 8 hurricanes and 3.5 major hurricanes. 

A more in-depth discussion of the 2023 Atlantic hurricane season is available from Colorado State University.

2022

The 2022 Atlantic hurricane season was characterized as a near-normal season based on the National Oceanic and Atmospheric Administration definition. The season had 14 named storms, of which 8 became hurricanes and 2 became major (Category 3+ on the Saffir-Simpson Hurricane Wind Scale) hurricanes. The average (1991–2020) Atlantic hurricane season has about 14 named storms, 7 hurricanes and 3 major hurricanes. Accumulated Cyclone Energy (ACE), an integrated index accounting for frequency, intensity and duration of storms, was slightly below the long-term average.  The 2022 Atlantic hurricane season produced 95 ACE, while the average season produced 123 ACE. 

In general, Atlantic seasonal hurricane forecasts called for an active season at all lead times (from March/April through July/August). Most groups anticipated cool neutral ENSO or weak La Niña conditions and a relatively warm tropical Atlantic. While the season featured La Niña and a warm tropical Atlantic, forecasters generally over-predicted observed activity for most parameters other than hurricane numbers. The average of all seasonal outlooks issued at all three lead times was above the observed value for ACE (154 forecast in March/April, 162 forecast in May/June and 152 forecast in July/August). Overall, the peak of the Atlantic hurricane season (August-October) was characterized by above-normal sea surface temperatures across most of the tropical Atlantic and Caribbean. However, vertical wind shear was elevated in both August and in October, leading to less activity than anticipated.

The average values predicted by all groups issuing forecasts in March/April were 19 named storms, 8 hurricanes, 4 major hurricanes.  These numbers changed little in May/June (18 named storms, 8 hurricanes, 4 major hurricanes).  The average prediction issued in July/August was very similar to May/June, continuing to call for 18 named storms, 8 hurricanes, 4 major hurricanes. 

A more in-depth discussion of the 2022 Atlantic hurricane season is available from Colorado State University.     

2021

The 2021 Atlantic hurricane season was the 6th above-normal Atlantic hurricane season in a row using the National Oceanic and Atmospheric Administration definition of above-normal seasons. The season had 21 named storms – the third most on record. In addition, 2021 had 7 hurricanes and 4 major hurricanes. The average (1991–2020) Atlantic hurricane season has about 14 named storms, 7 hurricanes and 3 major hurricanes. Accumulated Cyclone Energy (ACE), an integrated index accounting for frequency, intensity and duration of storms, was also above average.  The 2021 Atlantic hurricane season produced 146 ACE, while the average season produced 123 ACE.

In general, Atlantic seasonal hurricane forecasts called for an active season at all lead times (from March/April through July/August). Most groups anticipated cool neutral ENSO or weak La Niña conditions and a relatively warm tropical Atlantic. The average of all seasonal outlooks issued at all three lead times was extremely close to the observed value for ACE (152 forecast in March/April, 151 forecast in May/June and 142 forecast in July/August). Overall, the peak of the Atlantic hurricane season (August-October) was characterized by above-normal sea surface temperatures across most of the tropical Atlantic and Caribbean, while vertical wind shear was slightly below-normal in August and September and above-normal in October.

The average values predicted by all groups issuing forecasts in March/April were 17 named storms, 8 hurricanes, 4 major hurricanes and an ACE of 152.  These numbers changed little in May/June (18 named storms, 8 hurricanes, 4 major hurricanes and an ACE of 151).  The average prediction issued in July/August remained relatively unchanged from May/June and called for 18 named storms, 8 hurricanes, 4 major hurricanes and an ACE of 142.  Not all forecast groups issue predictions for all four tropical cyclone intensity metrics.  In terms of Accumulated Cyclone Energy, the most accurate March/April seasonal forecast came from Colorado State University (150 predicted vs. 146 observed), and the most accurate May/June seasonal forecast came from the North American Multi-Model Ensemble which exactly forecast the 146 ACE that was observed. The most accurate July/August seasonal forecast came from WeatherTiger and the National Oceanic and Atmospheric Administration, which predicted 145 ACE. Most groups had overall seasonal hurricane forecasts that were very similar to the observed value.

The 2021 Atlantic hurricane season was also active from a landfalling perspective, with eight named storms and two hurricanes making landfall. The most notable storm of the season was Hurricane Ida, which made landfall in southeastern Louisiana as a Category 4 hurricane. In addition to devastating parts of coastal and inland Louisiana, Ida’s remnants dropped copious amounts of rainfall in the mid-Atlantic and northeast United States, causing significant flooding. Overall ~$79 billion USD in damage has been attributed to Ida.

A more in-depth discussion of the 2021 Atlantic hurricane season is available from Colorado State University.

2020

The extremely active 2020 Atlantic basin hurricane season came to an end on 30 November. The season broke the record for most named storms in a single Atlantic season on record. Year 2020 had 30 named storms, eclipsing the old record of 28 named storms set in 2005. The season also produced the 2nd most hurricanes on record with 13 (trailing only 2005 with 15 hurricanes) and tied for the 2nd most major (Category 3+) hurricanes on record with 6 (trailing only 2005 with 7 major hurricanes). Accumulated Cyclone Energy (ACE), an integrated index accounting for frequency, intensity and duration of storms was also well above-average. The 2020 Atlantic hurricane season produced 180 ACE – the 6th-most ACE for an Atlantic hurricane season in the satellite era (since 1966). Final ACE values may change slightly in post-season reanalysis of tropical cyclone intensity by the National Hurricane Center. The 1981-2010 average values of these quantities are 12 named storms, 6 hurricanes, 3 major hurricanes and an ACE of 106.

In general, Atlantic seasonal hurricane forecasts called for an active season at all lead times (from March/April through July/August). The average of all seasonal forecasts for March/April to May/June changed little, while most seasonal forecasts increased from May/June to July/August, given anomalous warming of the tropical Atlantic and a stronger trend towards La Niña conditions. The average of all seasonal outlooks issued in July/August was extremely close to the observed value for ACE (181 forecast vs. 180 observed). Overall, the peak of the Atlantic hurricane season (August-October) was characterized by above-normal sea surface temperatures and below-normal shear across most of the tropical Atlantic and Caribbean.

The average values predicted by all groups issuing forecasts in March/April were 17 named storms, 8 hurricanes, 4 major hurricanes and an ACE of 148. These numbers changed little in May/June (17 named storms, 8 hurricanes, 4 major hurricanes and an ACE of 152). The average prediction issued in July/August increased from May/June and called for 22 named storms, 10 hurricanes, 5 major hurricanes and an ACE of 181. It should be noted that not all forecast groups issue predictions for all four tropical cyclone intensity metrics. In terms of Accumulated Cyclone Energy, the most accurate March/April seasonal forecast came from Weatherbell (170 predicted vs. 180 observed), while the most accurate May/June and July/August seasonal forecast came from WeatherTiger, which predicted exactly the observed ACE of 180.

In addition to setting the basinwide named storm record, the season was also extremely active from a landfalling perspective, with 12 named storms making landfall in the continental United States (the most on record) and 6 of these at hurricane strength (tied for the most on record). The strongest of these landfalling continental United States hurricanes was Laura, a Category 4 hurricane that devastated southwest Louisiana, causing ~$16 billion dollars in damage and 42 fatalities in the United States. Two Category 4 hurricanes also slammed into Nicaragua just 13 days apart in November – Eta and Iota. Over 250 fatalities and $8 billion dollars in damage have been attributed to these two hurricanes.

A more in-depth discussion of the 2020 Atlantic hurricane season is available from Colorado State University.

Listen to the discussion on the 2020 hurricane season recording - with the participation of L.P. Caron (BSC), P. Klotzbach (CSU) and R. Truchelut (WeatherTiger).

2019

The 2019 Atlantic basin hurricane season came to an end on November 30. The season ended up slightly above normal with a total of 18 named storms, 6 hurricanes, 3 major hurricanes and an Accumulated Cyclone Energy (ACE) of 130. Final ACE values may change slightly in post-season reanalysis of tropical cyclone intensity by the National Hurricane Center. The 1981-2010 average values of these quantities are 12 named storms, 6 hurricanes, 3 major hurricanes and an ACE of 106. While the season was extremely active for the number of named storms, seven of the named storms that formed in 2019 lasted 24 hours or less – the most to last <=24 hours on record.

In general, Atlantic seasonal hurricane forecasts called for a near to slightly above-average Atlantic hurricane season at lead times from March through early August. Seasonal forecasts increased slightly from March/April outlooks to July/August outlooks, given the slightly faster dissipation of El Niño conditions and some anomalous warming of the tropical Atlantic.   Overall, the peak of the Atlantic hurricane season (August-October) was characterized by slightly above-normal sea surface temperatures and below-normal shear across the tropical Atlantic and Caribbean.

The average values predicted by all groups issuing forecasts in March/April were 13 named storms, 6 hurricanes, 3 major hurricanes and an ACE of 101. These numbers increased slightly in May/June to 13 named storms, 7 hurricanes, 3 major hurricanes and an ACE of 113. The average prediction issued in July/August called for 13 named storms, 7 hurricanes, 3 major hurricanes and an ACE of 109. It should be noted that not all forecast groups issue predictions for all four tropical cyclone intensity metrics. In terms of Accumulated Cyclone Energy, the most accurate March/April seasonal forecast came from CFAN (126 predicted vs. 130 observed), the most accurate May/June seasonal forecast came from WeatherTiger (120 predicted vs. 130 observed), and the most accurate July/August seasonal forecast came from NOAA (135 predicted vs. 130 observed).

The 2019 Atlantic hurricane season was quiet in August, very active in September and above-normal in October for named storm activity but below normal for ACE. The most notable storm of the 2019 season was Hurricane Dorian, which devastated the northwestern Bahamas, resulting in over 60 fatalities and costing billions of dollars in damage. Dorian also brought ~1 billion dollars in damage to the southeast United States while making landfall as a Category 1 hurricane near Cape Hatteras. It was also responsible for 9 fatalities in the continental US. The costliest named storm of the 2019 season for the continental US was Tropical Storm Imelda, which while only being a weak tropical storm when it made landfall, deluged southeast Texas with torrential rainfall, causing ~2 billion in damage and resulting in five fatalities.

A more in-depth discussion of the 2019 Atlantic hurricane season is available from Colorado State University.

Listen to the discussion on the 2019 hurricane season recording - with the participation of L.P. Caron (BSC), P. Klotzbach (CSU) and R. Truchelut (WeatherTiger).

2018

The 2018 Atlantic basin hurricane season came to an end on November 30. The season ended up slightly above normal with a total of 15 named storms, 8 hurricanes, 2 major hurricanes and an Accumulated Cyclone Energy (ACE) of 129. Final ACE values may change slightly in post-season reanalysis of tropical cyclone intensity by the National Hurricane Center. The 1981-2010 average values of these quantities are 12 named storms, 6 hurricanes, 3 major hurricanes and an ACE of 106.

In general, Atlantic seasonal hurricane forecasts called for a slightly above-average Atlantic hurricane season in March/April, with agencies generally decreasing their forecasts in May/June and July/August. Unlike most seasons where the skill of the seasonal forecasts increases as the peak of the season approaches, this season’s early forecasts were more skillful than updates issued during July/August. This decrease in predicted storm activity was primarily due to anomalous cooling that took place in the tropical Atlantic. Despite anomalously cool tropical Atlantic conditions during the hurricane season and stronger than normal shear in the Caribbean, the Atlantic hurricane season was slightly above-normal, likely due to favorable conditions in the eastern tropical Atlantic and in the subtropical Atlantic. Warm neutral ENSO conditions prevailed during the season.

The average values predicted by all groups issuing forecasts in March/April were 14 named storms, 7 hurricanes, 3 major hurricanes and an ACE of 109. These numbers decreased in May/June to 12 named storms, 6 hurricanes, 2 major hurricanes and an ACE of 92. The average prediction issued in July/August called for 11 named storms, 5 hurricanes, 2 major hurricanes and an ACE of 71. It should be noted that not all forecast groups issue predictions for all four tropical cyclone intensity metrics. In terms of Accumulated Cyclone Energy, the most accurate March/April seasonal forecast came from Colorado State University (130 predicted vs. 129 observed), the most accurate May/June seasonal forecast came from the North American Multi-Model Ensemble (125 predicted vs. 129 observed), and the most accurate July/August seasonal forecast came from the Antigua Meteorological Service (93 predicted vs. 129 observed).

The 2018 Atlantic hurricane season was extremely quiet in August and quite active in both September and October. The season will be remembered for two hurricanes. Hurricane Florence made landfall near Wrightsville Beach, North Carolina as a Category 1 hurricane and brought record-setting rainfall to the Carolinas as it slowed to a crawl and meandered across the southeastern United States for several days. Hurricane Michael made landfall near Mexico Beach, Florida as a Category 4 hurricane and brought extreme wind and surge damage to portions of the Florida Panhandle, with severe wind impacts also being felt inland in southern Georgia.

A more in-depth discussion of the 2018 Atlantic hurricane season is available from Colorado State University.

2017

The 2017 Atlantic basin hurricane season officially ended on November 30. The season was extremely active with a total of 17 named storms, 10 hurricanes, 6 major hurricanes and an Accumulated Cyclone Energy (ACE) of 226. Final ACE values may change slightly in post-season reanalysis of tropical cyclone intensity by the National Hurricane Center. The 1981-2010 median values of these quantities are 12 named storms, 6.5 hurricanes, 2 major hurricanes and an ACE of 92.

In general, Atlantic seasonal hurricane forecasts called for a slightly below-average Atlantic hurricane season in March/April, with agencies generally increasing their forecasts in May/June and July/August. This increase in predicted storm activity was due to several factors including the lack of development of predicted El Niño conditions as well as anomalous warming in the tropical Atlantic. The average values predicted by all groups issuing forecasts in March/April were 12 named storms, 5 hurricanes, 2 major hurricanes and an ACE of 86. These numbers increased in May/June to 13 named storms, 7 hurricanes, 3 major hurricanes and an ACE of 123. Predictions issued in July/August called for 15 named storms, 7 hurricanes, 3 major hurricanes and an ACE of 123. It should be noted that not all forecast groups issue predictions for all four tropical cyclone intensity metrics. The most successful seasonal forecast for 2017 was issued by the University of Arizona in early June. This forecast called for 11 hurricanes, 6 major hurricanes and an ACE of 181.

The 2017 Atlantic hurricane season was dominated by September, which generated the most ACE by any Atlantic calendar month on record. All other months had near-normal activity. The continental United States was devastated by two Category 4 hurricane landfalls: Harvey and Irma. Irma and Maria brought incredible levels of death and destruction across points of the Caribbean and other parts of the tropical Atlantic.

The 2017 Atlantic hurricane season was generally characterized by neutral El Niño-Southern Oscillation (ENSO) conditions. During the record-breaking September of 2017, vertical wind shear levels were well below normal, providing conditions more conducive for hurricane formation and intensification. Sea surface temperatures in the tropical Atlantic were also much warmer than normal, providing a more conducive thermodynamic environment for hurricanes to thrive.

A thorough discussion of the 2017 Atlantic hurricane season is available from Colorado State University.

2016

The 2016 Atlantic basin hurricane season officially ended on November 30. The season was characterized by overall slightly above-average hurricane activity with a total of 15 named storms, 7 hurricanes, 3 major hurricanes and an Accumulated Cyclone Energy of 135. Final ACE values may change slightly in post-season reanalysis of tropical cyclone intensity by the National Hurricane Center. The ensemble mean forecast called for 15 named storms (-), 8 hurricanes (+1), 3 major hurricanes (-) and an ACE of 108 (-20%). For comparison, the 1981-2010 median values of these quantities are 12 named storms, 6.5 hurricanes, 2 major hurricanes and an ACE of 92.

The aggregate forecast was relatively successful, accurately predicting the total number of storms and major hurricanes, but slightly overestimating the number of hurricanes while at the same time underestimating the ACE. The average forecast outperformed a forecast based on climatology.

Seventeen groups submitted forecasts between early March and early August. It should be noted that not all forecast groups issue predictions for all four tropical cyclone intensity metrics. The number of forecasts that was submitted for each metric is given in the table below.

Fewer forecast groups made a prediction for ACE, but those who did tended to be somewhat more conservative in their overall predictions for 2016. This could explain the difference between higher hurricane forecasts (which almost every group forecasts) and lower ACE forecasts.

The average values predicted by all groups issuing forecasts in March/April were 14 named storms, 7 hurricanes, 3 major hurricanes and an ACE of 107. These numbers increased in May/June to 14 named storms, 8 hurricanes, 3 major hurricanes and an ACE of 117. Predictions issued in July/August were generally lower than in the previous period, with 15 named storms, 7 hurricanes, 3 major hurricanes and an ACE of 97. The number of forecasts for each period was 10 for March-April, 10 for May-June and 6 for July-August. Since the number of forecasts and the composition of the ensemble changes for each 2 month period, it is difficult to analyze how the skill of the average forecast changes as a function of forecast time.

The figure below shows changes in the forecast error of an ensemble constructed using only the individual forecasts which were updated at least once during the months leading up to the hurricane season.

The tendency to increase the predictions for May-June (compared to March-April) and to revise the forecasts downward at the start of the season is clearly visible. And while one would generally expect an increase in skill with forecast time, except for forecasts of named storms, this is not observed in this case. That being said, forecasts of named storms and hurricanes produced in July-August performed better than those produced in March-April and May-June.

Physical mechanism

The 2016 Atlantic hurricane season was generally fairly quiet until late September, when long-lived major hurricane Matthew formed. Matthew was responsible for over 40% of all ACE generated during this season. 2016 was the 3rd year in a row where more ACE was generated in October than in September. On average, about three times as much ACE is generated in the Atlantic in September compared with October.

The 2016 Atlantic hurricane season was generally characterized by cool neutral El Niño-Southern Oscillation (ENSO) conditions. Contrary to the past two years, this season was characterized by below-average vertical wind shear when averaged from August-October across most of the tropical Atlantic. Slightly above-average vertical wind shear prevailed over the Caribbean. While the tropical Atlantic was somewhat warmer-than-normal, the mid-levels of the atmosphere were quite dry this hurricane season. Dry mid-levels enhance downdrafts and suppress the deep convection necessary for supporting hurricanes.

A thorough discussion of the 2016 Atlantic hurricane season is available from Colorado State University.

2015

In general, the 2015 Atlantic basin hurricane season was correctly predicted to be below average by most organizations. Most forecast models called for a strong El Niño to develop during the summer of 2015, and this prediction verified. Eastern and central tropical Pacific sea surface temperatures were at near-record warm levels during August-October, the peak months of the Atlantic hurricane season. Associated with El Niño were very strong upper-level winds that produced extraordinarily strong vertical wind shear in the Caribbean, suppressing storm formation in this area. This strong shear extended far enough east in the tropical Atlantic that any tropical cyclones that did form tended to get sheared apart as they approached the Lesser Antilles.

The primary story of the 2015 Atlantic hurricane season was Hurricane Joaquin. Joaquin generated near half of the Accumulated Cyclone Energy that occurred in 2015. This system formed from a non-tropical area of low pressure and strengthened to a strong Category 4 hurricane. It devastated portions of the central Bahamas, but fortunately, no one on the islands died. Unfortunately, Joaquin was responsible for the sinking of the ship El Faro, killing all 33 people on board. Joaquin’s moisture combined with a mid-latitude system to produce copious amounts of flooding in South Carolina as well.

A much more thorough verification of the 2015 Atlantic hurricane season is available from Colorado State University’s website.