The 2021 Atlantic hurricane season was the third-most active Atlantic hurricane season on record in terms of the number of tropical storms, although many of them were weak and short-lived. With 21 named storms forming, it became the second season in a row and third overall in which the designated 21-name list of storm names was exhausted. Seven of those storms strengthened into hurricanes, four of which reached major hurricane intensity, which is slightly above-average. The season officially began on June 1 and ended on November 30. These dates historically describe the period in each year when most Atlantic tropical cyclones form. However, subtropical or tropical cyclogenesis is possible at any time of the year, as demonstrated by the development of Tropical Storm Ana on May 22, making this the seventh consecutive year in which a storm developed outside of the official season.
Three named storms formed in June, tying the record for the most to develop in that month. Among them was Tropical Storm Claudette, which brought flooding to portions of the Deep South. Then, on July 1, Elsa developed and became the earliest-forming fifth named storm on record, surpassing Tropical Storm Edouard in 2020. The storm later caused significant impacts from Barbados to much of the East Coast of the United States, with about $1.2 billion in damage in the latter region. In August, Tropical Storm Fred flooded parts of the Caribbean and Southeastern United States, resulting in roughly $1.3 billion in damage. Hurricane Grace intensified to a Category 3 major hurricane before making landfall in the Mexican state of Veracruz, causing 17 deaths and about $513 million in damage in the Greater Antilles and Mexico. On August 22, Henri struck Rhode Island and brought flooding and high winds to the Northeastern United States, with damage estimated at $700 million.
Hurricane Ida became the deadliest and most destructive tropical cyclone of the season after striking southeastern Louisiana at Category 4 strength in late August, 16 years to the day after Hurricane Katrina decimated that same region. After devastating Louisiana and moving farther inland, Ida caused catastrophic flooding and spawned several destructive tornadoes across the Northeastern United States. Damage estimates from the storm exceeded $75 billion, contributing to over 93% of the total damage done in the season. Additionally, Ida killed 107 people, directly or indirectly, throughout the impacted regions. In September, Hurricane Larry peaked as a powerful Category 3 hurricane over the open Atlantic before making landfall in the Canadian province of Newfoundland and Labrador as a Category 1 hurricane. Later in the month, Hurricane Nicholas moved erratically both on- and offshore the coasts of Texas and Louisiana. Freshwater flooding, coastal flooding, and winds generated by Nicholas left about $1.1 billion in damage. Hurricane Sam became the most intense system of the season, peaking as a strong Category 4 hurricane in late September. Tropical cyclones during this season collectively caused 194 deaths and nearly $80.78 billion in damage, making it one of the costliest Atlantic hurricane seasons on record.
Nearly all forecasting agencies predicted above-average activity during the season, due to expectations of abnormally warm sea surface temperatures, the unlikelihood of an El Niño, and the possibility of a La Niña. Although these forecasted conditions transpired during the season, the agencies slightly underestimated the number of named storms, but nearly all were fairly accurate with the number of hurricanes and major hurricanes. This season, the National Hurricane Center (NHC) began issuing regular Tropical Weather Outlooks on May 15, two weeks earlier than it had done in the past. The change was implemented given that named systems had formed in the Atlantic Ocean prior to the start of the season in each of the preceding six cycles. Prior to the start of the season, NOAA deployed five modified hurricane-class saildrones at key locations around the basin, and in September, one of the vessels was in position to obtain video and data from inside Hurricane Sam. It was the first-ever research vessel to venture inside the middle of a major hurricane.
Seasonal forecasts
In advance of, and during, each hurricane season, several forecasts of hurricane activity are issued by national meteorological services, scientific agencies, and noted hurricane experts. These include forecasters from the United States National Oceanic and Atmospheric Administration (NOAA)'s Climate Prediction Center, Tropical Storm Risk (TSR), the United Kingdom's Met Office (UKMO), and Philip J. Klotzbach, William M. Gray and their associates at Colorado State University (CSU). The forecasts discuss weekly and monthly changes in significant factors that help determine the number of tropical storms, hurricanes, and major hurricanes within a particular year. According to NOAA and CSU, the average Atlantic hurricane season between 1991 and 2020 contained roughly 14 tropical storms, seven hurricanes, three major hurricanes, and an accumulated cyclone energy (ACE) index of 72–111 units. Broadly speaking, ACE is a measure of the power of a tropical or subtropical storm multiplied by the length of time it existed. It is only calculated for full advisories on specific tropical and subtropical systems reaching or exceeding wind speeds of 39 mph (63 km/h). NOAA typically categorizes a season as above-average, average, or below-average based on the cumulative ACE index, but the number of tropical storms, hurricanes, and major hurricanes within a hurricane season is sometimes also considered.
Pre-season forecasts
On December 9, 2020, TSR issued an extended range forecast for the 2021 hurricane season, predicting slightly above-average activity with 16 named storms, 7 hurricanes, 3 major hurricanes, and an ACE index of about 127 units. TSR cited the expected development of a weak La Niña during the third quarter of 2021 as the main factor behind their forecast. CSU released their first predictions on April 8, 2021, predicting an above-average season with 17 named storms, 8 hurricanes, 4 major hurricanes, and an ACE index of 150 units, citing the unlikelihood of an El Niño and much warmer than average sea surface temperatures in the subtropical Atlantic. On the same day, Pennsylvania State University predicted 9–15 named storms, anticipating slightly above-average sea surface temperatures and a neutral El Niño–Southern Oscillation. TSR updated their forecast on April 13, with 17 named storms, 8 hurricanes, and 3 major hurricanes, with an ACE index of 134 units. On the same day, the University of Arizona (UA) issued its seasonal prediction of above-average hurricane activities, with 18 named storms, 8 hurricanes, 4 major hurricanes, and an ACE index of 137 units. North Carolina State University (NCSU) made its prediction for the season on April 14, calling for an above-average season with 15 to 18 named storms, 7 to 9 hurricanes, and 2 to 3 major hurricanes. The Weather Company (TWC) issued their first predictions on April 15, forecasting 18 named storms, 8 hurricanes, and 3 major hurricanes, citing warm sea surface temperatures, a La Niña, and other active seasons with similar atmospheric conditions.
On May 13, TWC updated their forecast for the season, calling for an active season, with 19 named storms, 8 hurricanes, and 4 major hurricanes. On May 20, NOAA's Climate Prediction Center issued their forecasts for the season, predicting a 60% chance of above-average activity and 30% chance for below-average activity, with 13–20 named storms, 6–10 hurricanes, and 3–5 major hurricanes. The same day, UKMO issued their own forecast for the 2021 season, predicting an average one with 14 named storms, 7 hurricanes, and 3 major hurricanes, with a 70% chance that each of these statistics will fall between 9 and 19, 4 and 10, and 1 and 5, respectively. TSR's third seasonal forecast, issued on May 27, predicted 18 named storms, 9 hurricanes, and 4 major hurricanes.
Mid-season forecasts
The second prediction by CSU, issued on June 3, increased the number of named storms to 18, while leaving the number of hurricanes and major hurricanes unchanged at 8 and 4, respectively. On June 16, UA updated their forecast for the season, with 19 named storms, 6 hurricanes, 4 major hurricanes, and an ACE index of 183 units. On July 6, TSR released their third forecast for the season, slightly increasing their numbers to 20 named storms, 9 hurricanes, and 4 major hurricanes. This prediction was largely based on their expectation for a weak La Niña to develop by the third quarter of the year. On July 8, CSU updated their prediction to 20 named storms, 9 hurricanes, and 4 major hurricanes. UKMO's updated forecast on August 2 called for 15 named storms, 6 hurricanes, and 3 major hurricanes. The second and final forecast by NOAA, issued on August 4, predicted 15 to 21 named storms, 7 to 10 hurricanes, and 3 to 5 major hurricanes. On August 5, CSU's final seasonal predictions decreased the number of named storms to 18 and hurricanes to 8, while the number of major hurricanes remained unchanged. TSR issued their final forecast for the season on the same day, lowering their numbers to 18 named storms, 7 hurricanes, and 3 major hurricanes.
Seasonal summary
The 2021 Atlantic hurricane season officially ran from June 1 to November 30. A total of 21 tropical depressions formed, all of which reached at least tropical or subtropical intensity. This made it the third-most active Atlantic hurricane season on record, behind only 2005 and 2020. Like both those seasons, the 2021 season exhausted its primary list of storm names. Nine of the systems lasted for two days or less, tied with 2007 for the most since the NHC began monitoring subtropical systems in 1968. Although the season was highly active in terms of the number of named storms, seven of those tropical or subtropical systems intensified into a hurricane and four of those became a major hurricane, which is near-average and just slightly above-average, respectively. Nonetheless, 2021 marked the record sixth consecutive above-average season in the Atlantic. The ongoing warm Atlantic multidecadal oscillation, which began in 1995, contributed to the season's high level of activity, as it led to above-average sea surface temperatures in the Atlantic basin. Other factors included the presence of a La Niña and abnormally heavy West African Monsoon precipitation.
Collectively, the tropical and subtropical systems of the 2021 Atlantic hurricane season caused 194 deaths and about $80.727 billion in damage, making it the fourth costliest season on record. Eight named storms struck the United States, which is the third most ever, behind only the previous season and 1916. In conjunction with 2020, 19 systems of at least tropical storm intensity made landfall in the country during the two seasons, surpassing the record of 15 during the 2004 and 2005 seasons combined. As a result, some regions significantly impacted during the 2020 season were once again hit hard in 2021, especially eastern Louisiana and portions of the Northeastern United States. Rhode Island was struck by two tropical systems, Elsa and Henri, an unusual occurrence especially given that the state had recorded no landfalls since Bob in 1991. Four tropical cyclones or their remnants – Elsa, Fred, Ida, and Nicholas – each caused at least $1 billion in damage in the United States.
There were three named storms in June, tying 1886, 1909, 1936, 1959, 1966, 1968, 2023, and 2024 for most named storms in June. Elsa formed on June 30 and became a tropical storm on the following day, making it the earliest fifth-named storm on record, surpassing the previous record by five days, set by Tropical Storm Edouard in 2020. After Elsa dissipated, activity came to a monthlong halt due to unfavorable conditions across the basin. Tropical cyclogenesis also paused for much of the month of October, primarily due to the presence of drier air. For the first time since 2006 and only the second time during the hyperactive era which began in 1995, no named storms developed between October 6 and October 30.
The ACE index for the 2021 Atlantic hurricane season was approximately 145.6 units. The totals represent the sum of the squares for every (sub)tropical storm's intensity of over 39 mph (63 km/h), divided by 10,000. Therefore, the ACE index value does not include tropical depressions. Each season dating back to 2016 recorded ACE index values exceeding 129, which senior research associate Brian McNoldy of the University of Miami described as "unprecedented even for four years, let alone six!"
Throughout the season, NOAA Hurricane Hunters logged 462.2 flight hours, conducting 58 eyewall passages and deploying 1,310 dropsondes in the process. NOAA also deployed 66 underwater gliders, which made 78,328 observations on oceanic salinity and temperatures. Additionally, NOAA used five unmanned saildrones to increase documentation on atmospheric and oceanic conditions across the Atlantic basin. One of the five saildrones became the first research vessel to ever enter a major hurricane when it reached Hurricane Sam on September 30. It recorded sustained winds of 125 mph (201 km/h) and waves up to 50 ft (15 m) in height while also capturing video footage from inside the storm.
Systems
Tropical Storm Ana
An upper-level trough drifted across the western Atlantic on May 19, inducing the formation of a low-pressure area along a stalled front the next day. This system became co-located with an upper-level low, reducing wind shear and causing atmospheric instability, which allowed convection to develop despite cold sea surface temperatures. After the disturbance shed its frontal characteristics, it became Subtropical Storm Ana at 06:00 UTC on May 22 about 200 mi (320 km) northeast of Bermuda. This made 2021 the seventh consecutive year in which a tropical or subtropical cyclone formed before the season's official start. It formed in a location where no tropical storms within the month of May had been documented since before 1950. Ana then made counter-clockwise loop due to weak steering currents. After the wind field became more symmetrical and convection organized further by early on May 23, Ana transitioned into a tropical storm as it accelerated northeastward amid southwesterly mid-level flow. Ana peaked with maximum sustained winds of 45 mph (72 km/h) before it encountered increasing wind shear and began losing convection. By 18:00 UTC on May 23, the system transitioned into an extratropical cyclone about 605 mi (975 km) northeast of Bermuda. The extratropical cyclone merged with a trough early on May 24 and was then absorbed by a frontal system later that day.
On May 20, the Bermuda Weather Service issued a tropical storm watch for the precursor to Ana, before canceling it two days later. The storm's precursor produced mostly light winds on Bermuda, although the Pearl Island station observed a sustained wind speed of 44 mph (71 km/h) and a gust of 49 mph (79 km/h).