31 cyclones attaining tropical depression status,
30 cyclones attaining tropical storm status,
14* cyclones attaining hurricane status, and
7* cyclones attaining major hurricane status.
Before the beginning of the season, I predicted that there would be
20 cyclones attaining tropical depression status,
18 cyclones attaining tropical storm status,
9 cyclones attaining hurricane status, and
5 cyclones attaining major hurricane status.
*The operational number of hurricanes was 13 and major hurricanes 6, but Gamma was upgraded to a hurricane and Zeta to a major hurricane in post-season analysis.
The average numbers 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 unprecedented activity of the 2020 season vastly exceeded both these averages and my preditions, above average though they were. With an astonishing 30 named storms, 2020 surpassed 2005's record of 28. Its 14 hurricanes became second-most on record (behind only 2005 with 15), and its 7 major hurricanes tied 2005 for the most ever recorded. 2020 was the second season to use the Greek alphabet to name cyclones after exhausting the 21 names on the ordinary list, and the first to use the Greek letters "Eta", "Theta", and "Iota". The Accumulated Cyclone Energy (ACE) value for the 2020 season (which accounts for duration and intensity of storms as well as number) was around 180, in the "hyperactive" range.
2020 also set a smattering of other activity records. For the third named storm and the fifth onward, each formation was the earliest for that number storm on record. The season had five hurricanes of at least category 4 strength, tying a record. A total of ten storms were named in September, surpassing the previous busiest Septembers of 2002 and 2010, each with eight. This busy month included three storms being (operationally) named the same day: on September 18, Tropical Storm Wilfred, Subtropical Storm Alpha, and Tropical Storm Beta all received names. This was only the second time in the hurricane database that such an event occurred, after August 15, 1893. Note, however, that the post-season analysis on Alpha found that it had in fact formed one day earlier, on September 17. 2020 was the first time three major hurricanes had formed in the month of October. November also had three named storms (tied for most), and two major hurricanes (a first).
Illustrating just how active September was, the above image shows the Atlantic on September 14, when there were five (!) cyclones existing simultaneously. Hurricane Sally is visible, strengthening in the Gulf of Mexico, Hurricane Paulette is off of the east coast of North America, a weakening Tropical Depression Rene is a sheared blob east-southeast of Paulette, Tropical Storm Teddy is strengthening in the tropical Atlantic halfway between the Lesser Antilles and Africa, and a recently formed Tropical Storm Vicky is northeast of Teddy. As if that were not enough, the tropical wave that gave rise to the short-lived Tropical Storm Wilfred can be seen in the bottom-right corner emerging from Africa, as can the disturbance in the Gulf of Mexico southwest of Sally that ulimately became Tropical Storm Beta. That's seven systems in one picture!
There are several factors that contributed to such an active hurricane season. As predicted in pre-season outlooks, a significant La Niña event developed during the summer, shown by the above graphs of sea surface temperature anomalies in different areas of the equatorial Pacific. These relatively cool temperatures are correlated with increased hurricane activity on the other side of the Americas, and this was certainly the case in 2020. A subtler, longer-term cycle also may have contributed to a record season: the Atlantic Multi-Decadal Oscillation (AMO) maintained its positive phase. This index is computed from Atlantic sea-surface temperatures, which were warm even relative to an overall warming planet. This in turn led to a stronger African monsoon and increased rainfall across the tropical Atlantic. As the name suggests, the AMO varies only slowly over dozens of years. Indeed, the five seasons 2016-2020 were all above average.
2020 also saw many, many instances of rapid intensification, as shown in the table above. Ten cyclones satisfied the technical criteria for rapid intensification, namely a 25 kt (30 mph) increase of maximum winds in a span of 24 hours. This tied the record set in 1995. Even more incredible were the top bouts of strengthening on the list: Delta, Eta, and Iota all gained at least 60 knots (70 mph) in 24 hours, which had only been observed a handful of times previously. This points to a final noteworthy aspect of 2020's activity: the absurdly ideal conditions in the western Caribbean sea during October and November. Record low vertical wind shear and high oceanic heat content extending well below the surface led to three of the season's seven major hurricanes developing there in those two months, as well as five of the ten intensfication episodes listed above. Tragically, these favorable conditions led to the season's worst impacts in Nicaragua and Honduras, which experienced a never before seen two consecutive category 4 landfalls in one week (Eta and Iota).
The above graphic (click to enlarge) illuminates yet another metric by which 2020 exceeded all previously known seasons: continental United States landfalls. A record twelve tropical storms and six hurricanes (the latter tying a record) hit the United States, putting the entire Gulf and east caosts under cyclone-related watches and warnings at some point during the year. The epicenter of this barrage was Lousiana, which had five landfalls, three of which were hurricanes and two major hurricanes. The strongest was the devastating Laura, which had the highest winds of a known landfalling Louisiana hurricane since 1856. Even so, the impact of these storms wasn't as great as it could have been, since relatively sparsely populated areas were affected.
Some other notable facts or records from the 2020 Atlantic hurricane season include:
- Tropical Storm Arthur was a pre-season tropical cyclone that formed on May 14, marking the record sixth consecutive season in which a storm formed before the official start of the season on June 1
- When Tropical Storm Bertha formed on May 27, it was the first time since 2012 that there were multiple pre-season storms
- Hurricane Paulette first became a depression on September 6, and transitioned into a remnant low for the last time on September 19; in this interval, a remarkable seven named storms formed in the Atlantic: Rene, Sally, Teddy, Vicky, Beta, Wilfred, and Alpha
- The diameter of tropical storm force winds of Hurricane Teddy just before its extratropical transition was 850 miles, breaking the top 5 for Atlantic hurricanes at the time
- Subtropical Storm Alpha became the easternmost-forming Atlantic named storm on record when it formed near Portugal on September 18; that same day, it became the first named storm to make landfall in that country
- Tropical Storm Theta traversed the farthest east of any November Atlantic tropical cyclone on record, ending up east of 20° W before dissipating.