18 cyclones attaining tropical depression status,
17 cyclones attaining tropical storm status,
10 cyclones attaining hurricane status, and
6 cyclones attaining major hurricane status.
Before the beginning of the season, I predicted that there would be
15 cyclones attaining tropical depression status,
15 cyclones attaining tropical storm status,
6 cyclones attaining hurricane status, and
3 cyclones attaining major hurricane status.
The average number of named storms, hurricanes, and major hurricanes for an Atlantic hurricane season (over the 30-year period 1981-2010) are 12.1, 6.4, and 2.7, respectively. The 2017 season was well above average in all categories, especially hurricanes and major hurricanes. In addition, there were several intense and long-lived hurricanes, inflating the ACE (accumulated cyclone energy) index to 223. This value, which takes into account the number, duration, and intensity of tropical cyclones, was the highest since 2005. 2017 was also the first year on record to have three storms exceeding 40 ACE units: Hurricane Jose, with 42, Hurricane Maria, with 45, and Hurricane Irma, with 67.
The ENSO oscillation, a variation in the ocean temperature anomalies of the tropical Pacific, often plays a role in Atlantic hurricane development. At the beginning of the 2017 season, these temperatures were predicted to rise, signaling a weak El Niño event and suppressing hurricane activity. However, this event did not materialize. Though anomalies did rise briefly in the spring, they returned to neutral and even negative by the early fall, when hurricane season peaks. This contributed to the extremely active September. In addition, conditions were more favorable for development in the central Atlantic than they had been for several years, allowing the formation of long-track major hurricanes. Due to these factors, my predictions significantly underestimated the season's extreme activity.
The 2017 Atlantic hurricane season was the costliest ever recorded, with Hurricanes Harvey, Irma, and Maria contributing the lion's share to this total. Among the areas most affected were southeastern Texas (by Harvey), the Leeward Islands (from Irma and Maria), and Puerto Rico and the Virgin Islands (from Maria). Some other notable facts and records for the 2017 season include:
- Tropical Storm Arlene formed on April 20, one of only a small handful of April storms; it also had the lowest pressure ever recorded for an Atlantic tropical cyclone in April
- The short-lived Tropical Storm Bret formed off the coast of South America and made landfall near the northern tip of Venezuela, becoming the southernmost forming June Atlantic cyclone since 1933
- The remnants of Hurricane Franklin regenerated in the eastern Pacific after crossing Mexico and received a new name: Jova
- Hurricane Harvey was the first major hurricane to make landfall in the U.S. since 2005, and the strongest to do so in Texas since 1961; the peak rainfall accumulation of 51.88" in Cedar Bayou, Texas was the largest tropical cyclone rain total ever for the continental U.S.
- Hurricane Irma spent a total of 3 days as a category 5 hurricane, at the time a tie for second most ever recorded in the Atlantic
- When Hurricanes Irma, Jose, and Katia were all at category 2 strength or above on September 8, it marked only the second such occurrence since 1893
- Hurricane Maria reached a minimum pressure of 908 mb, then the tenth lowest ever for an Atlantic hurricane, and the lowest since Dean in 2007
- Becoming a major hurricane near the Azores Islands, Hurricane Ophelia was the easternmost major hurricane ever to form in the Atlantic
- All ten named storms from Hurricane Franklin to Ophelia became hurricanes, the first time ten consecutive names have done so in the Atlantic since 1893
Overall, the 2017 Atlantic hurricane season was exceptionally active and damaging, especially for parts of the Caribbean.
Sources: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf