CSU researchers correctly predicted above-average 2021 Atlantic hurricane season

Hurricane Eta from GOES-16, 11-02-20, CIRA

The 2021 Atlantic hurricane season was above average – in line with forecasts issued by Colorado State University’s Tropical Meteorology Project. Twenty-one named storms formed in 2021 (the 3rd most on record), with seven of these storms becoming hurricanes and four reaching major hurricane strength. The average Atlantic hurricane season has 14 named storms, seven hurricanes and three major hurricanes. Seven named storms and two hurricanes made landfall in the continental United States, with Hurricane Ida striking the central Louisiana coast as a Category 4 hurricane. In addition to devastating winds and storm surge damage near where Ida made landfall, heavy rain from the hurricane’s remnants also brought catastrophic flooding to the mid-Atlantic states.

“The 2021 Atlantic hurricane season was above average, especially when evaluated by named storm frequency. Overall, our seasonal forecasts did an excellent job of predicting an above-average season, with predictions of hurricanes, major hurricanes and Accumulated Cyclone Energy being very close to what actually occurred,” said Phil Klotzbach, lead author of the forecast. Accumulated Cyclone Energy is an integrated metric accounting for intensity and duration of storms. Seasonal Accumulated Cyclone Energy (ACE) was approximately 120% of the 1991-2020 average.

The report summarizes all tropical cyclone activity in the Atlantic basin during the 2021 hurricane season and compares the team’s seasonal and two-week forecasts to what occurred.

When evaluated against ACE, the season was close to what was forecast at all lead times (e.g., early April, early June, early July and early August). El Niño conditions were not expected with any of the outlooks. Most of the season was characterized by cool neutral ENSO conditions, with La Niña conditions developing later in the season. Vertical wind shear in the eastern and central tropical Atlantic was generally weaker than normal during August–September, favoring activity in this region. Surprisingly given the historical relationship between La Niña and vertical wind shear, Caribbean shear was elevated in October-November, leading to a quiet latter part of the Atlantic hurricane season.

The tropical Atlantic and Caribbean were warmer than normal during the hurricane season. Warmer-than-normal sea surface temperatures enhance Atlantic hurricane formation through several mechanisms. Warm sea surface temperatures provide more fuel for developing tropical cyclones. They also tend to be associated with lower pressure and a moister atmosphere, which also enhance hurricane activity.

The Colorado State team was founded by the late William Gray. CSU’s initial forecast for the 2021 season was issued on April 8 and called for an above-average hurricane season. The team predicted 17 named storms, eight hurricanes and four major hurricanes. The CSU team increased its forecast slightly on June 3, calling for 18 named storms, eight hurricanes and four major hurricanes. The July 8 forecast raised the projected numbers and called for 20 named storms, nine hurricanes and four major hurricanes. The numbers were slightly lowered with the Aug. 5 update and called for 18 named storms, eight hurricanes and four major hurricanes.

Observed activity was 21 named storms, seven hurricanes and four major hurricanes. The team predicted above-average ACE of 150 on April 8 and June 3. They increased their ACE forecast slightly to 160 on July 8 and lowered their ACE forecast back to 150 on Aug. 5. Observed ACE through Nov. 30 was 145. The 1991-2020 average Atlantic ACE was 123.

The team bases its annual forecasts on 70 years of historical data and includes factors such as Atlantic sea surface temperatures and sea level pressures, levels of vertical wind shear (the change in wind direction and speed with height), El Niño (an anomalous warming of waters in the central and eastern tropical Pacific) and other factors. While these forecast factors generally work well and explain approximately 50-60% of the year to year hurricane variability in these 70 years of historical data, there remains 40-50% of this variability which is not explained.

Hurricane statistics for 2021 contained in the report include:

  • Twenty-one named storms formed in the Atlantic this season. This is the 3rd most in a single Atlantic season on record, trailing 2020 (30 named storms) and 2005 (28 named storms).
  • Four hurricanes (Grace, Henri, Ida and Larry) formed in the Atlantic between Aug.18-Sept. 2 – the first time on record that more than three hurricanes have formed between these two dates.
  • The Atlantic had no named storm activity between Oct. 3 and Oct. 30 – the first time since 2006 that the Atlantic had no named storm activity between these two dates.
  • Hurricane Elsa was the earliest fifth Atlantic named storm formation on record (named on July 1). Elsa broke the old earliest fifth Atlantic named storm formation record set by Edouard (on July 6, 2020).
  • Hurricane Ida made landfall with maximum sustained winds of 150 mph – tied with the Last Island Hurricane of 1856 and Hurricane Laura (2020) for strongest winds for a Louisiana hurricane on record.
  • Hurricane Sam was a major hurricane for 7.75 days, tied with Hurricane Edouard (1996) for the fourth most consecutive days at major hurricane strength in the satellite era (1966 onwards).

The Tropical Meteorology Project has attributed the upturn in major hurricane activity since 1995 as well as the earlier increase in major hurricane activity from the late 1940s through the mid-1960s to natural multi-decadal variability in the strength of the Atlantic Multidecadal Oscillation (AMO). A concomitant increase in several favorable hurricane-enhancing parameters occur in the tropical Atlantic during the positive phase of this oscillation – while these same parameters tend to suppress hurricanes during the negative phase of this oscillation.

The Tropical Meteorology Project has been issuing forecasts for the past 38 years. A brief qualitative outlook for the 2022 hurricane season will be issued on Thursday, Dec. 9, with a first full forecast to be issued in early April 2022.