CSU researchers correctly predicted extremely active 2020 Atlantic hurricane season

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

The 2020 Atlantic hurricane season was extremely active. This heightened level of hurricane activity was relatively well anticipated by Colorado State University’s Tropical Meteorology Project. The season broke the single-season Atlantic named storm record with 30 named storms and also featured 13 hurricanes and six major hurricanes – both the second most on record. Twelve named storms, of which six were at hurricane strength, made landfall in the continental United States with the strongest of these hurricanes being Category 4 Hurricane Laura, which made landfall in southwest Louisiana.

“The 2020 Atlantic hurricane season was extremely active, especially when evaluated by named storm frequency and the number of tropical cyclones hitting the United States. Overall, our seasonal forecasts somewhat underestimated the number of named storms, hurricanes and major hurricanes but were quite accurate for Accumulated Cyclone Energy (ACE),” said Phil Klotzbach, lead author of the forecast and a research scientist in the CSU Department of Atmospheric Science. Accumulated Cyclone Energy is an integrated metric accounting for intensity and duration of storms. Seasonal Accumulated Cyclone Energy (ACE) was approximately 170 percent of the 1981-2010 average. The latter part of the season was extremely active, with four of the six major hurricanes that formed in 2020 occurring in October-November. No season on record prior to 2020 had more than two Atlantic major hurricane formations in October-November.

The report summarizes all tropical cyclone activity in the Atlantic basin during the 2020 hurricane season and compares the team’s seasonal and two-week forecasts to what occurred.

When evaluated against ACE, the season was slightly more active than anticipated with the early April, June and July forecasts. El Niño conditions were not expected with any of the outlooks; however, La Niña was not anticipated to be as strong as it ended up being during the season. Consequently, vertical wind shear across the tropical Atlantic and Caribbean was even weaker than was thought with the early outlooks. However, by early August, the Tropical Meteorology Project forecast an extremely active season, due in part to weak vertical wind shear during July in the tropical Atlantic and Caribbean. Weak vertical wind shear was likely the primary reason why October-November 2020 was so active in the western Caribbean and Gulf of Mexico.

The tropical Atlantic and Caribbean were also much warmer than normal during the hurricane season. Anomalously warm 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 moister mid-level air, which also enhance hurricane activity.

The Colorado State team was founded by the late William Gray. CSU’s initial forecast for the 2020 season was issued on April 2 and called for an above-average hurricane season. The team called for 16 named storms, eight hurricanes and four major hurricanes. The CSU team slightly increased its forecast on June 4 and called for 19 named storms, nine hurricanes and four major hurricanes. The July 7 forecast increased the projected numbers slightly and called for 20 named storms, nine hurricanes and four major hurricanes. The numbers were raised again with the August 5 update and predicted 24 named storms, 12 hurricanes and five major hurricanes. Observed activity was 30 named storms, 13 hurricanes and six major hurricanes. The team predicted a well above-average ACE of 150 on April 2, increased its ACE forecast to 160 on June 4 and July 7 and raised its forecast ACE to an extremely high 200 on Aug. 5. Observed ACE through November 30 was 180. The 1981-2010 average Atlantic ACE was 106.

The team bases its annual forecasts on 60 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 percent of the year to year hurricane variability in these 60 years of historical data, there remains 40-50 percent of this variability which is not explained.

Hurricane statistics for 2020 contained in the report include:

  • 30 named storms formed in the Atlantic this season. This is the most on record, eclipsing the old record of 28 named storms set in 2005.
  • The 2020 Atlantic hurricane season had the earliest forming ‘C’ named storm and the earliest named storm formation from the ‘E’ storm onwards.
  • 10 named storms formed in September. This is the most September named storm formations on record, breaking the old record of 8 named storms set in 2002, 2007 and 2010.
  • 12 Atlantic named storms made landfall in the continental US in 2020 – the most on record. The prior record was 9 named storm landfalls in the continental US set in 1916.
  • Hurricane Laura made landfall with maximum sustained winds of 130 kt in Cameron Parish, Louisiana. Laura is tied with the Last Island Hurricane of 1856 for the strongest winds for a Louisiana hurricane landfall on record.
  • Hurricane Iota made landfall in Nicaragua with maximum sustained winds of 135 kt – the strongest November landfalling hurricane in Nicaragua on record.

The TMP 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 37 years. A brief qualitative outlook for the 2021 hurricane season will be issued on Thursday, December 10, with a first full forecast issued in early April 2021.