Post Mortem 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.
