Post Mortem 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.