Inside the eye of Hurricane Epsilon viewed from a Hurricane Hunter aircraft near the top of the cyclone. The radar overlay (lower left) shows the aircraft entering the eye and the surrounding clouds. NOAA Image.

After the Record Breaking Hurricane Season 2020, What Will Happen in 2021?

Last January, no one expected the 2020 Atlantic Hurricane Season to close as the most active season on record. We had 31 tropical cyclones. All but one became tropical storms. Thirteen of the 30 tropical storms intensified into hurricanes, and six into major hurricanes. Twelve hurricanes or tropical storms made landfall in the contiguous United States. The previous record was nine. Twenty-seven storms broke previous records for the earliest formation beginning with Tropical Storm Cristobal.

The formation of a La Niña in the southeastern Pacific Ocean combined with an above average Atlantic Multi-Decadal Oscillation (AMO) fueled the record number of storms and the rate at which they formed.

The first annual meteorological discussion of the upcoming season takes place in mid-December just two weeks after the previous hurricane season ends. Colorado State University’s Tropical Meteorology Project issues the first possibilities for the coming season. Another authoritative body is Tropical Storm Risk, a consortium of tropical weather specialists.

Damage from Hurricane Laura

Devastating Damage Following Hurricane Laura in Southwest Louisiana. Four more tropical cyclones would hit Louisiana before the season ended. Storm Surge damage exceeding 17 feet was documented along the coast.

El Niño–Southern Oscillation

One the leading indicators of Atlantic Hurricane Activity comes from the Pacific Ocean. The El Niño–Southern Oscillation or ENSO is the fluctuation of wind and water temperature over the tropical eastern Pacific Ocean which affects climate patterns in the tropics and subtropics.

El Niño is a sea temperature warming phase while La Niña is a cooling phase. High atmospheric pressure accompanies El Niño while low air surface pressure follows a La Niña. These periods of warming and cooling generally last about two months and occur approximately every two years with varying intensity. Science has yet to explain all the mechanisms that cause a La Niña or El Niño and forecasts for either condition remain uncertain beyond two or three months. That was the case in December 2019 through late winter 2020 when forecasters issued the first predictions for the 2020 Hurricane Season. In December 2019, a La Niña was somewhat possible but far from certain.

Although ENSO has a definite impact on the Atlantic Hurricane Season, it is not the only factor to consider. Rather, it shifts the chances for a specific weather pattern.

Twin globes showing the La Nina and El Nino. Image via Climate.gov

The upper image shows the cooler water temperatures—La Niña—near the equator in the eastern Pacific ocean. A La Niña promotes the formation of storms in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.

The lower image shows warmer water temperatures—El Niño—which generally suppresses tropical cyclone formation in the Atlantic Basin. 

A neutral condition of average water temperatures is the norm.  

CSU and TSR Forecasts and Discussions

The Tropical Meteorology Project at Colorado State University and Tropical Storm Risk will issue their first forecasts for the 2021 Atlantic Hurricane Season on April 8 and April 9, 2021. They had their first discussion on the upcoming season on December 9 and 10, 2020.

CSU and TSR rely on two primary factors for the December outlook. The strength of the Atlantic Multi-Decadal Oscillation (AMO) and the phase of ENSO.

Historical Data provides a basis for forecasting the AMO component of the discussion. Three quiet hurricane seasons (low AMO) from 2013 to 2015 were followed by a slightly above-average 2016 season (increasing AMO.) The 2017 hurricane season had almost record-breaking levels of activity (high AMO) with above average activity in 2018 and 2019. We had an extremely active 2020 season. This leads to a high confidence that the AMO in 2021 will remain in a positive phase in spite of cooler than normal sea temperatures in the North Atlantic.

Forecasting the phase of the ENSO this far in advance is difficult and computer models produce wide-ranging forecasts for summer and fall of 2021. Most models indicate that La Niña will transition to a neutral phase, but statistical analysis points toward a continuing La Niña at a weaker magnitude than current observations.

Tropical Meteorology Project

The Colorado State University Tropical Meteorology Project anticipates five possible outcomes for the 2021 Atlantic Hurricane Season and assigns a percent chance to each possible outcome.

AMO El Niño ACE Chance
Very Strong No El Niño 170 25%
Above Average No El Niño 130 35%
Above Average El Niño Develops 80 20%
Below Average No El Niño 80 10%
Below Average El Niño Develops 50 10%

 

Forecasters use the Accumulated Cyclone Energy (ACE) as a basis for long-range forecasts.

ACE Named Storms Hurricanes Major Hurricanes
170 14-17 9-11 4-5
130 12-15 6-8 2-3
80 8-11 3-5 1-2
50 5-7 2-3 0-1

 

Averages from 1981 to 2010

  • 1 Named Storms
  • 4 Hurricanes
  • 7 Major Hurricanes
  • ACE ~ 106

Based on CSU’s discussion and probable outcomes, a 60 percent chance exists for an above average season with at least 12-15 named storms, 6-8 hurricanes, and 2-3 major hurricanes.

Tropical Storm Risk

TSR also expects a weak La Niña phase going into the summer and fall of 2021. The July to September trade winds affect vertical wind shear and the ability of cyclones to spin up in the main hurricane track through the Caribbean Sea and Tropical North Atlantic. Tropical Storm Risk expects weaker than normal trade winds which will enhance the ability of tropical storms to develop.

Tropical Storm Risk also warns that precision six months in advance of the season start is low, typically below 30 percent overall except for intense (major) hurricanes.

A 50 percent possibility exists for an above average (>126) ACE, 31 percent for near-normal (74-126) ACE, and 19 percent for a below-average (<74) ACE.

This aligns closely with the Tropical Meteorology Project discussion.

ACE Named Storms Hurricanes Major Hurricanes
> 126 16 7 3
74-126 9.5 4.5 1.5
> 74 6 2.5 1
Hurricane Preparedness. Prepare Now for the 2021 Atlantic Hurricane Season

2021 Hurricane Season Forecast

Based on long-range assessments from the Tropical Meteorology Project and from Tropical Storm Risk, the 2021 Atlantic Hurricane Season will probably be an active season, but not as intense as the 2020 Hurricane Season.

Overall, we can probably expect 12 to 16 Named Storms of which 6 to 8 will become hurricanes and 2 to 3 will further strengthen to category 3 or higher.

Tropical cyclones can form at any time of year.

  • 2016: Hurricane Alex in January
  • 2017: Tropical Storm Arlene in April
  • 2018: Tropical Storm Alberto in May
  • 2019: Tropical Storm Andrea in May
  • 2020: Tropical Storm Arthur in May
  • 2020: Tropical Storm Bertha in May

Each of the years from 2016 to 2020 was an above active season. If the trend continues, a tropical cyclone could form before the start of the 2021 Hurricane Season.

Are you ready?