Thursday, December 5, 2024

The weirdly hyperactive 2024 Atlantic hurricane season ends » Yale Climate Connections

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With the calendar turned to December, we now close the book on the unusually deadly and destructive Atlantic hurricane season of 2024. There were 18 named storms, 11 hurricanes, and five major hurricanes. An average season has 14 named storms, seven hurricanes, and three major hurricanes. The season’s accumulated cyclone energy (ACE) reached 162 (33% above average), which officially qualifies 2024 as a hyperactive season, according to the definition used by the Colorado State University seasonal forecast group – and that’s in spite of a month-long pause in activity at the climatological peak of hurricane season.

Tropical cyclone tracks for the 2024 Atlantic hurricane season
Figure 1. Observed tropical cyclone tracks for the 2024 Atlantic hurricane season. (Image credit: NHC)

Record-warm waters and the lowest wind shear on record over the tropical Atlantic helped fuel two Cat 5s more than three months apart – Beryl and Milton – making 2024 the first season since 2019 with two category 5 storms. Beryl made a catastrophic hit on Carriacou Island, Grenada, on July 1 as a Cat 4 with 150 mph winds, making it the strongest landfalling Atlantic hurricane of 2024. Milton was the season’s strongest storm, peaking with 180 mph winds and a central pressure of 897 mb on Oct. 7 in the Gulf of Mexico, making it the fifth-strongest Atlantic hurricane on record (by pressure) and sixth-strongest by winds:

1. 190 mph (Allen 1980)
2. 185 mph (Dorian 2019, Labor Day 1935, Gilbert 1988, Wilma 2005)
3. 180 mph (Milton 2024, Mitch 1998, Rita 2005, Irma 2017)
4. 175 mph (nine storms, including Maria 2017, Katrina 2005, Andrew 1992, Camille 1969)

The U.S. endures an epic pummeling by hurricanes in 2024

The season’s deadliest and most destructive hurricane was Hurricane Helene, which made landfall in Florida’s Big Bend region on Sep. 26 as a Category 4 storm with 140 mph winds. As documented by Michael Lowry, at least 243 people lost their lives in Helene across seven states, making it the deadliest hurricane to hit the mainland U.S. since Hurricane Katrina killed an estimated 1,392 people in 2005. Flood damage from the hurricane was catastrophic in western North Carolina, and the final damage tally is likely to make Helene one of the top-10 most expensive hurricanes on record.

Helene’s landfall gives the U.S. a record eight Cat 4 or Cat 5 Atlantic hurricane landfalls in the past eight years (2017-2024), seven of them being continental U.S. landfalls. That’s as many Cat 4 and 5 landfalls as occurred in the prior 57 years. The only comparable beating the U.S. has taken from Category 4 and 5 landfalling hurricanes occurred in the six years from 1945 to 1950, when five Category 4 hurricanes hit South Florida. Furthermore, the U.S. has now suffered a major hurricane strike in five consecutive years. Only one other time since accurate hurricane landfall records began in 1900 has the nation seen a streak that long: way back in 1915-1919.

Hot on Helene’s tail came Hurricane Milton, which made landfall on Oct. 9 in Sarasota, Florida, as a Cat 3 with 120 mph winds. Milton killed 25 people and inflicted tens of billions in damage. But as bad as Milton was, Florida got a major break when the eye of the storm made landfall just 20 miles south of the entrance to Tampa Bay, sparing Florida’s most vulnerable city from a massive 10-foot storm surge that would have cost tens of billions of dollars.

Read: Four ways climate change likely made Hurricane Helene worse
Read: Without climate change, Hurricane Milton would have hit as a Cat 2, not a Cat 3

Warmer oceans=stronger hurricanes. A study by Climate Central released today found that all 11 of 2024’s Atlantic hurricane were intensified by human-caused ocean warming, with a doubling of destructive power a common result. My post:yaleclimateconnections.org/2024/11/huma…

— Dr. Jeff Masters (@drjeffmasters.bsky.social) 2024-11-20T17:17:41.248Z

Three other hurricanes hit the U.S. in 2024, all along the Gulf Coast:

Category 1 Hurricane Beryl made landfall in Texas on July 8. According to NOAA, Debby killed 45 people in the U.S. and did $7.2 billion in damage. After Beryl struck, some 400 Texans were hospitalized for carbon monoxide poisoning after turning to generators to keep their power and A/C going in July’s brutal heat.

Category 1 Hurricane Debby made landfall on August 5 in Florida’s Big Bend region with 80 mph sustained winds, then made a second landfall near Bulls Bay, South Carolina, as a tropical storm on August 8. According to NOAA, Debby killed 10 people and did $2.5 billion in damage.

Category 2 Hurricane Francine made landfall on Sep. 11 in central Louisiana with 100 mph winds. According to Gallagher Re, losses are at US$1.5 billion as of October 2024, but no deaths occurred.

Beryl and Milton had unusually intense tornado outbreaks associated with them, spawning a total of four EF3 tornadoes. Remarkably, only five EF3 tornadoes had previously been recorded in tropical cyclones going back to 1995. Beryl spawned a total of 67 tornadoes, mainly on July 8 across eastern Texas, making it the fifth most prolific tornado-producing tropical cyclone on record. Milton spawned 46 tornadoes in Florida on Oct. 9, making it the prolific tornado-producing tropical cyclone in state history.

A list of the deadliest U.S. hurricanes since 1963. Three of the top five deadliest hurricanes have occurred since 2017.A list of the deadliest U.S. hurricanes since 1963. Three of the top five deadliest hurricanes have occurred since 2017.
Figure 2. Deadliest U.S. hurricanes since 1963. Helene ranks as the 4th deadliest U.S. hurricane in at least 60 years. (In 2023, NHC revised its official death toll for Katrina from 1,833 to 1,392, based on two studies published by the American Meteorological Society that drew on more than 1,000 medical logs on storm victims in Louisiana and Mississippi.)

A weird progression to the season

As the 2024 Atlantic hurricane season began, hurricane experts viewed the upcoming season with dread: record-warm ocean temperatures, combined with the likely onset of a La Niña event in the Eastern Pacific were sure to cause a serious onslaught of hurricanes. La Niña conditions often lead to above-average hurricane seasons because of a decrease in wind shear, causing less disruption of the structure of a tropical cyclone. And warm sea surface temperatures tend to lead to above-average hurricane activity.

Atlantic sea surface temperatures Jun-Nov 2024Atlantic sea surface temperatures Jun-Nov 2024
Figure 3. Sea surface temperatures across much of the tropical Atlantic were record-high during the 2024 hurricane season. (Image credit: Michael Lowry)

According to The Colorado State forecast team’s summary of the 2024 Atlantic hurricane season, vertical wind shear during August–October of 2024 in the hurricane main development region (MDR) was the lowest on record, and surface temperatures were at near-record warm levels during the peak of the 2024 hurricane season. These conditions helped fuel the hyperactive season that occurred. But the season did not progress like a typical one.

Departure of vertical wind shear from average in Aug-Oct, 1950-2024, in the Atlantic main development region for hurricanesDeparture of vertical wind shear from average in Aug-Oct, 1950-2024, in the Atlantic main development region for hurricanes
Figure 4. Departure of vertical wind shear from average in August to October, 1950-2024, in the Atlantic main development region (MDR) for hurricanes, defined to be 10–20°N, 85– 50°W. Wind shear was record-low in 2024. (Image credit: Colorado State University)

The season got off to a record start, with Hurricane Beryl becoming a Category 5 hurricane on July 2, making it the earliest forming Atlantic Cat 5 on record by over two weeks (previous record: Hurricane Emily, July 17, 2005). But a highly abnormal lull in activity occurred between August 20 – September 23, which is typically the peak of the Atlantic hurricane season. The ACE index was 8 during this period, the lowest since 1994, and no named Atlantic storms formed from August 13 – September 8. The last time that this occurred was in 1968.

The lull was likely due to a combination of factors, including an unusual northward-displaced African monsoon over the Sahara Desert (causing a northward shift in easterly wave activity with increased dry air moving into the MDR), unusually warm upper-level temperatures (which increased atmospheric stability), too much wind shear in the eastern/central Atlantic from strong winds out of the east, and a phase of the Madden-Julian Oscillation (MJO) unfavorable for hurricanes during the early to middle part of September (causing stable, sinking air over the tropical Atlantic).

The season then resumed in ferocious fashion, with 11 named storms, seven hurricanes, and four major hurricanes after Sep. 24, setting numerous records for late-season activity (e.g., the seven hurricanes that formed after September 25 were the most on record for the tail end of a season; see the records list from Colorado State University below).

Progression of the 2024 Atlantic hurricane season using the seven-day accumulated cyclone energy (ACE). Progression of the 2024 Atlantic hurricane season using the seven-day accumulated cyclone energy (ACE).
Figure 5. Progression of the 2024 Atlantic hurricane season using the seven-day accumulated cyclone energy (ACE). Huge peaks in ACE came early and late in the year, with a highly abnormal pause in activity during the usual August and September peak portion. (Image credit: Michael Lowry)

While some of the ominous seasonal outlooks for the Atlantic did end up on the high side, the forecasts as a group performed well in calling for a markedly busier season than usual, as evident on a verification website hosted by the Barcelona Supercomputing Center (see Fig. 6 below for the ACE results).

 Seasonal outlooks for accumulated cyclone energy (ACE) issued at various points by various groups before the peak of the 2024 Atlantic season had arrived.  Seasonal outlooks for accumulated cyclone energy (ACE) issued at various points by various groups before the peak of the 2024 Atlantic season had arrived.
Figure 6. Seasonal outlooks for accumulated cyclone energy (ACE) issued at various points by various groups before the peak of the 2024 Atlantic season had arrived. The orange dot at left is the mean value of the various outlooks, while the red dot is the actual observed ACE though Dec. 2. The horizontal dotted lines in red and orange show the typical range of seasonal ACE. (Image credit: Barcelona Supercomputing Center/AXA/Colorado State University)

A remarkably high death toll from the 2024 Atlantic hurricane season

The 2024 Atlantic hurricane season had a remarkably high death toll. Unofficially, 11 storms caused a total loss of life of 390:

Hurricane Helene: 243 (NC/SC/GA/FL/TN/VA/IN)
Hurricane Beryl: 71 (44 in TX, 2 in LA, 2 in VT, 23 in the Caribbean)
Hurricane Milton: 35 (32 in Florida, 3 in Mexico)
Hurricane Debby: 10 (9 in Florida, 1 in Canada)
Tropical Storm Chris: 6 (Mexico)
Tropical Storm Alberto: 5 (4 in Mexico, 1 in the U.S.)
Hurricane Rafael: 5 (Panama)
Hurricane Ernesto: 3 (rip current drownings in NC/SC, U.S.)
Tropical Storm Sara: 3 (2 in the Dominican Republic, 1 in Honduras)
Tropical Storm Nadine: 3 (floods in Mexico)
Hurricane Kirk: 1 (tornado in France)

Notable records from the 2024 Atlantic hurricane season

The Colorado State forecast team put together a selection of some of the notable statistics from the 2024 season:

  • 11 hurricanes formed in the Atlantic. 2024 tied with 1995 for 5th place for most hurricanes produced in the satellite era (1966-onward).
  • Five major hurricanes formed in the Atlantic. 2024 tied with 1995, 1999, 2008 and 2010 for 6th place for major hurricanes produced in the satellite era (1966-onward).
  • Five hurricanes made landfall in the continental US (Beryl, Debby, Francine, Helene, and Milton). 2024 tied with 1893, 2004, and 2005 for the 2nd-most continental US hurricane landfalls in a season. 1886, 198,5 and 2020 had 6 continental US hurricane landfalls.
  • 162 ACE units were generated during 2024, making the season hyperactive by NOAA’s definition. 2024 is the 11th hyperactive season in the satellite era (1966-onward).

Intra-seasonal statistics

  • 36 ACE units were generated by 8 July – the most on record by that date. The prior record through 8 July was 32 ACE set in 1933.
  • Three hurricanes formed in the Atlantic by 14 August. Four other years in the satellite era (1966-onward) have had 3+ hurricane formations by 14 August: 1966, 1968, 1995, 2005.
  • No named storms formed in the Atlantic between 13 August – 8 September. The last time that this occurred was in 1968.
  • Seven ACE units were generated in the Atlantic between 20 August – 23 September. 2024 produced the least ACE between 20 August – 23 September since 1994.
  • 11 named storms formed in the Atlantic from 24 September onward. 2024 is tied with 2005 for the record for most named storm formations from 24 September onward.
  • Seven hurricanes formed in the Atlantic from 25 September onward, the most on record for that period.
  • Four major hurricanes formed in the Atlantic since 26 September – the 2nd most on record from 26 September onward, behind only 2020, which produced 5 major hurricanes from 26 September onward.
  • 100 ACE units were generated in the Atlantic since 24 September – the 2nd most on record from 24 September onward. 2024 trails 1878, which produced 109 ACE units from 24 September onward.
  • Hurricanes Kirk and Leslie both set the easternmost record for hurricane formations in the tropics (<=23.5°N) from October onward.
  • Kirk, Leslie and Milton were hurricanes simultaneously – the first time on record that the Atlantic has had 3 hurricanes simultaneously from October onward.

Individual storm/landfall statistics

  • Hurricane Beryl became a Category 5 hurricane on 2 July – the earliest-forming Atlantic Category 5 hurricane on record. The prior record was Emily (2005) on 17 July.
  • Hurricane Beryl intensified by 55 kt in 24 hrs from 29 June (1800 UTC) to 30 June (1800 UTC) – the most rapid 24-hr intensification by an Atlantic hurricane prior to 1 July on record. The old record was Hurricane Alice (1954), with 45 kt in 24 hours.
  • Hurricane Beryl had maximum sustained winds of 145 kt – the strongest Atlantic hurricane prior to August on record. The prior record was Emily (2005; 140 kt).
  • Hurricane Beryl made landfall on Carriacou Island, Grenada with maximum sustained winds of 130 kt – the strongest Grenada hurricane landfall on record. Hurricane Ivan (2004) passed about 10 miles south of the main island of Grenada with maximum winds estimated at 110 kt.
  • Hurricane Helene (Category 4; 120 kt) was the strongest hurricane to make landfall in the Big Bend on record. The prior record was the Cedar Keys Hurricane of 1896 (Category 3; 110 kt).
  • Hurricane Kirk reached hurricane strength at ~40°W on 1 October – the farthest east that an Atlantic hurricane had formed in the tropical Atlantic (<=23.5°N) from October-onward on record. Old record was ~59°W set by Jose (1999) and then tied by Tammy (2023).
  • Hurricane Kirk reached Category 4 intensity at ~47°W, breaking the old record for farthest east a Category 4-5 Atlantic hurricane had occurred in October – November. The old record was Sam (2021) at ~60°W.
  • Hurricane Leslie reached hurricane strength at ~34°W on 4 October – the farthest east that an Atlantic hurricane had formed in the tropical Atlantic (<=23.5°N) from October onward, breaking the old record set by Kirk just three days prior.
  • Hurricane Milton had maximum sustained winds of 155 kt – the strongest for an Atlantic hurricane since Dorian (2019) and the strongest for a Gulf of Mexico hurricane since Rita (2005).
  • Hurricane Milton had a lifetime minimum central pressure of 897 mb – the lowest for an Atlantic hurricane since Wilma (2005).
  • Hurricane Milton intensified by 80 kt in 24 hrs from 6 October (1800 UTC) to 7 October (1800 UTC) – the most rapid 24-hr intensification by an Atlantic hurricane since Felix in 2007 (85 kt in 24 hrs).
  • Hurricane Rafael made landfall in Cuba as a major hurricane with maximum sustained winds of 100 kt – the first major hurricane landfall in Cuba in November since Michelle (2001).
  • Hurricane Rafael was only the 2nd major hurricane on record in the Gulf of Mexico in November. The other Gulf major hurricane in November was Kate (1985).

Below-average activity in the Eastern Pacific

The 2024 Eastern Pacific hurricane season was below average in activity, with 13 named storms, five hurricanes, three major hurricanes, and an Accumulated Cyclone Energy (ACE) of 82. In comparison, the long-term averages for the period 1991-2020 were 16.6 named storms, 8.8 hurricanes, 4.6 major hurricanes, and an ACE of 133.

The only significant damage from this year’s Eastern Pacific season was from compact, slow-moving Hurricane John, which took 29 lives and caused about $1 billion in damage. John made two landfalls as it churned westward along the coast of southern Mexico, dumping torrential rains that totaled as much as 1442 millimeters (56.8 inches).

Bob Henson contributed to this post.

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