The 2021 Atlantic hurricane season draws to an official close on November 30, after generating an extraordinary 21 named storms (third highest on record), seven hurricanes, four major hurricanes, and an accumulated cyclone energy (ACE) of 145. Those numbers compare with the 1991-2020 averages for an entire season of 14.4 named storms, 7.2 hurricanes, 3.2 major hurricanes, and an ACE index of 123. As documented by Brian McNoldy, Senior Research Associate at University of Miami’s Rosenstiel School of Marine and Atmospheric Science, 2021 marked the sixth consecutive year with an ACE index above 129: “this has never happened before, not during the satellite era, not since records begin in 1851. This sustained level of tropical cyclone activity in the Atlantic is unprecedented even for four years, let alone six!”

One hurricane will surely get its name retired – Ida, with $64.5 billion in damage and 96 deaths, ranking as the fifth most costly weather disaster in world history, according to NOAA. Two other hurricanes generated more than $1 billion in damage: Hurricane Elsa, which affected the eastern U.S. July 7-9, killing one person and causing $1.2 billion in damage, and Tropical Storm Fred, with $1.3 billion in damage during its trek up the U.S. East Coast August 10-17.

Digging beyond these numbers, we’ve come up with a list of the top-10 most unusual things about the Atlantic hurricane season of 2021:

Figure 1. Preliminary track map for the 2021 Atlantic hurricane season. (Image credit: National Hurricane Center)

1. Second consecutive year to run through the entire alphabet

Perhaps the most remarkable trait of the 2021 Atlantic hurricane season is that for the second year in row, the entire alphabetical list of names for storms was exhausted. The 21 named storms of 2021 were the third highest on record, behind only 2020 (30) and 2005 (28). However, nine of this year’s storms were “shorties” – named storms that lasted two days or less. This ties a record set in 2007 for the most “shorties” since 1968, when the National Hurricane Center began tracking subtropical storms and counting them in seasonal totals of activity. (Note that these subtropical storms were numbered rather than named until 2004’s Subtropical Storm Nicole.)

As explained in a post here earlier this year, improved technology has allowed the identification of weak, short-lived tropical cyclones that would have escaped detection in previous years. No link between warming of the oceans from increased greenhouse gases and the observed increase in the number of Atlantic named storms has been firmly established. According to this year’s Sixth Assessment Report from the Intergovernmental Panel on Climate Change, the global frequency of tropical cyclones will likely hold steady or decrease as global warming continues. Among those tropical cyclones, though, the proportion that reach Category 4 or 5 will very likely increase.

Figure 2. Number of named Atlantic storms lasting two days or less, 1968-2021. The trend (blue line) is decidedly upwards.

2. A record 19 U.S. landfalls in two years

Eight named storms made landfall in the U.S. in 2021 (though one could argue that the first named storm to hit the U.S., Claudette in southeastern Louisiana on June 19, did not count as a landfall, since it wasn’t named until it was centered over land). The eight U.S. landfalls in 2021 is the third highest on record, behind 2020 (11) and 1916 (nine).

The two-year period 2020-2021 had a truly astonishing 19 landfalls in the contiguous U.S., six times the average for a two-year period, and beating the previous two-year landfall record of 15, set in 2004-2005. Third place is held by 1915-1916, with 13 landfalls. From 1950 through 2020, the U.S. averaged three landfalling tropical storms (with one a hurricane) per year.

Figure 3. Visible satellite image (with lightning) of Tropical Storm Ida at 9:41 a.m. EDT August 27, 2021. At the time, Ida had top winds of 65 mph and was about to make landfall in Cuba, on its way to a devastating category 4 landfall in southeastern Louisiana. (Image credit: NOAA/RAMMB/Colorado State University)

3. For the second year in a row, Louisiana suffers a record-strength hurricane landfall

Category 4 Hurricane Ida struck a catastrophic blow on Louisiana on August 29, making landfall at the key oil industry hub of Port Fourchon with 150 mph winds and a central pressure of 930 mb. That day was also the 16th anniversary of Hurricane Katrina’s devastating landfall in Louisiana and Mississippi in 2005. Katrina, the most expensive weather disaster in world history, at $176 billion, caused between 1,085 and 1,387 deaths.

Only four hurricanes (all Cat 5s) have made landfall in the contiguous U.S. with stronger winds than Ida, and Ida is tied with Laura, in 2020, and with the 1856 Last Island Hurricane as the strongest ever to hit Louisiana (by sustained winds). As measured by central pressure at landfall, Ida ranks as the ninth-strongest to hit the contiguous U.S., and second-strongest to hit Louisiana. Only Hurricane Katrina of 2005, with a 920 mb pressure at landfall near Buras, had a lower pressure.

4. In its first forecast for Ida, NHC predicted a near-major hurricane in 72 hours

When Tropical Depression Nine formed in the Western Caribbean on August 26, the National Hurricane Center made the most aggressive forecast of rapid intensification it had ever made for a tropical depression, calling for the system to intensify into a 110-mph hurricane in 72 hours, just 5 mph short of major hurricane strength, when Ida would be nearing the coast of southeastern Louisiana. According to Sam Lillo, this forecast tied the record as being the highest three-day intensity forecast ever made for a tropical depression, set in 2020 for Hurricane Iota.

Ida did indeed rapidly intensify, making landfall as a powerful 150-mph category 4 hurricane in southeastern Louisiana three days later, very close to the location predicted in the first advisory. Though the amount of rapid intensification was under-predicted, the initial intensity forecast for Ida was unquestionably a major success, as it gave an unusually long period of time to prepare for the arrival of a destructive hurricane.

5. Ida’s rapid intensification

Hurricane Ida put on a furious display of rapid intensification as it steamed northwards towards Louisiana, with the pressure dropping from 985mb to 929mb (a fall of 56 mb) in just 24 hours. As documented by Sam Lillo (see Tweet above), only nine hurricanes on record in the Atlantic have done this, with Ida being the furthest north and the closest to a U.S. landfall. In one 12-hour period, Ida’s pressure fell by a remarkable 40 mb – the third fastest pressure fall observed in a Gulf of Mexico hurricane since 1979. Sea surface temperature of 30-31 degrees Celsius (86-88°F) and the warm Loop Current eddy that Ida passed over were both key factors in its intensification.

Ida’s winds increased by 65 mph in the 26 hours leading up until landfall. Interpolation of Ida’s winds yields a 24-hour intensification before landfall of 55 mph, making Ida the fourth-fastest intensifying hurricane on record before making a contiguous U.S. landfall. Historical records show that since 1950, nine storms have intensified by at least 40 mph in the 24 hours before landfall. It is sobering to note that four of those storms, below in bold face, occurred in just the past five years; All four were category 4 or category 5 hurricanes:

Humberto, 2007 (65 mph increase);
King 1950 (60 mph increase);
Eloise 1975 (60 mph increase);
Ida, 2021 (55 mph increase);
Danny 1997 (50 mph increase);
Laura 2020 (45 mph increase);
Michael 2018 (45 mph increase);
Harvey 2017 (40 mph increase); and
Cindy 2005 (40 mph increase).

Five out of seven of this year’s Atlantic hurricanes rapidly intensified by at least 35 mph in 24 hours – the official National Hurricane Center definition of rapid intensification – with only Hurricane Henri and Hurricane Nicholas missing the mark:

Sam: 45 mph ending at 0Z Sep. 26;
Larry: 35 mph ending at 12Z Sep. 4;
Ida: 65 mph ending at 12Z Aug. 29;
Grace: 60 mph ending at 6Z Aug. 21; and
Elsa: 35 mph ending at 18Z Jul. 2.

Another weird aspect of Ida is that much of its damage, and the bulk of its fatalities, occurred long after the storm’s demise as a hurricane. As Ida moved eastward from the Appalachians as a remnant center of low pressure, it triggered tornadoes and torrential rains across the mid-Atlantic and Northeast, causing widespread and devastating floods, including across the New York City area.

6. A hurricane of non-tropical origin threatens New England

A powerful low-pressure system that passed though Michigan on August 11 brought a line of severe thunderstorms that knocked out power to more than 800,000 customers (myself included) in Michigan – one of the state’s largest power outages on record. Little did I suspect that the storm would go on to spawn a hurricane that would threaten New England! But that’s exactly what happened when the storm system, which caused $1.1 billion in damage in the Midwest and mid-Atlantic, moved over the warm ocean waters off the mid-Atlantic coast.

The waters north of Bermuda, where the storm began to fester, were 28-29 degrees Celsius (82-84°F), about 0.5-1.0 degree Celsius above average for the time of year. On August 16, Tropical Depression Eight was born, becoming Tropical Storm Henri that evening. The system gradually strengthened to a category 1 hurricane with 75 mph winds four days later, prompting hurricane warnings for the coasts of New York, Connecticut, Rhode Island, and Massachusetts. Fortunately, Henri moved unusually slowly for a New England hurricane, and as it approached the coast, much cooler waters there were able to weaken Henri to a 60-mph tropical storm by the time of its landfall in Rhode Island. Henri’s slow forward speed of 11 mph at landfall resulted in torrential rains and a significant storm surge, and the storm was responsible for $650 million in damage, according to Aon.

Henri’s unusual origin is likely to be repeated more frequently in coming years, as sea surface temperatures continue to increase as a result of global warming, allowing hurricanes to form in regions where waters used to be too cold. A study published this month found that global warming could be expected to cause a substantial increase in the number of tropical storms forming off the East Coast, and that these storms would likely move more slowly at landfall, increasing their damage potential by subjecting the coast to a longer period of impacts. The greatest threat for slower-moving storms was found for the Virginia and North Carolina coasts.

On July 9, Tropical Storm Elsa made landfall in Long Island, New York, and then in Westerly, Rhode Island, as a tropical storm with 50 mph winds (though a post-season analysis may reveal that Elsa had become an extratropical storm by that time). Elsa was the first named storm to hit Rhode Island since the 1991 landfall of Hurricane Bob. Remarkably, Rhode Island suffered a second landfall this season, when 60-mph Tropical Storm Henri struck the state on August 22, very close to where Elsa had made landfall.

The only other named storm to make landfall on the U.S. East Coast in 2021 was Tropical Storm Danny, which hit South Carolina on June 29 with 40 mph winds.

Figure 4. Radar image of Tropical Storm Claudette at 7:53 a.m. EDT June 19, four hours after Claudette was named. Claudette was named while centered inland, 45 miles southwest of New Orleans. At the time of this image, Pensacola, Florida, recorded a wind gust of 81 mph. (Image credit: Mark Nissenbaum)

8. Tropical Storm Claudette forms over land

In a rare occurrence, Tropical Storm Claudette was named when the storm’s center was inland. The first advisory for Claudette was issued at 5 a.m. EDT June 19, when the storm, with winds of 45 mph, was centered about 45 miles southwest of New Orleans, Louisiana. The formation of Claudette over land may have been a result of the brown ocean effect, the result of very wet soils over southern Louisiana.

Claudette brought torrential rain, flooding, and tornadoes to portions of the central Gulf Coast, with 24-hour rainfall amounts of over 10 inches in portions of Louisiana and Mississippi. Claudette killed 14 people in Alabama, and caused $350 million in damage in the southeastern U.S.

At least two previous Atlantic storms were named while over land: Tropical Storm Julia in September 2016 near Jacksonville, Florida (named after making landfall as a tropical depression), and Tropical Storm Beryl in August 1988, which was declared a tropical depression and then a tropical storm while over southeastern Louisiana.

9. Hyperactive through late September, then nothing until October 30

After blitzing through the first 20 names in the Atlantic list of storms by September 29, when Tropical Storm Victor formed, the 2021 season went eerily cold during October; After October 5, when Hurricane Sam became post-tropical, the Atlantic experienced no named storm activity until Subtropical Storm Wanda formed on October 30. Since the current active Atlantic hurricane period began in 1995, only one other year has had no named storm activity between October 6 and 30: the El Niño year of 2006. It is very unusual to see such a quiet October during a year with La Niña conditions present, as was the case in October 2021. The lack of October 2021 activity was primarily the result of dry air over the typical spawning grounds for October storms.

Figure 5. Satellite images of the nor’easter that killed two and caused over $200 million in damage in the Northeast U.S. in late October (left, at 17:45Z October 27) and the named storm it evolved into, Tropical Storm Wanda (right, at 15:30Z November 3).