Monday officially marks the final day of Atlantic hurricane season, and it has been one for the record books.
2020 has been undoubtedly a crazy year – with the Covid-19 pandemic, murder hornets and some of the largest wildfires in recorded history. It seems understandable that hurricane season would also be memorable.
This season was forecast to be a busy season. Two of the most well-known and respected entities that forecast their predictions for the upcoming hurricane season are Colorado State University (CSU) and the National Oceanic and Atmospheric Administration (NOAA).
CSU’s Tropical Meteorology Project team predicted an above-average Atlantic hurricane season on April 2. The team forecast 16 named storms, including eight hurricanes.
“We were forecasting a well above-average season in April, June and July, and increased that forecast to an extremely active season in August,” said Phil Klotzbach, a research scientist at the university’s Department of Atmospheric Science.
“Active seasons can play out very differently,” Klotzbach said. “For example, both 2004 and 2005 had comparable levels of ACE (227 ACE in 2004 and 245 ACE in 2005), but 2005 had 28 named storms while 2004 had only 15 named storms.”
ACE stands for “accumulated cyclone energy.” It is the metric used by meteorologists to account for both a storm’s strength and how long the storm lasts. Typically, the more ACE there is in a single hurricane season, the more active the season is.
Seven weeks after CSU put out its initial forecast, NOAA forecast a 60% chance for an above-normal season, predicting a 70% chance of having 13 to 19 named storms, of which six to 10 could develop into hurricanes, including three to six major hurricanes.
“Obviously, given our forecast named storm numbers, we were quite surprised to see 30 named storms, but as you can see from other indices that we forecast, most of them were close to in line with our predictions,” Klotzbach said.
Researchers at CSU use forecast models mainly based on ACE to make their hurricane season predictions.
The season began early when Tropical Storm Arthur formed on May 14, more than two weeks before Atlantic hurricane season officially began. The season runs from June 1 through November 30.
“The 2020 Atlantic hurricane season ramped up quickly and broke records across the board,” said Neil Jacobs, acting NOAA administrator, in a media release.
Every named storm so far this season except three (Arthur, Bertha, and Dolly) set a record for the earliest named storm ever recorded.
For example, Cristobal was the earliest third named storm on record when it formed on June 2, beating the previous record – Colin in 2016 – by three days. By the time Wilfred formed, the earliest 21st named storm, these systems were beating the previous records by nearly three weeks.
When Hurricane Delta was churning in the Atlantic, it broke numerous records, only to see many of them broken a few weeks later when Hurricane Iota moved through the western Caribbean.
During the peak of the season, there were five tropical cyclones in the Atlantic at the same time – Paulette, Rene, Sally, Teddy and Vicky – for only the second time in history.
The only other time there were five active tropical cyclones – hurricane, tropical storm and/or tropical depression – in the Atlantic was in 1971.
This year, six storms reached major hurricane status – Laura, Teddy, Delta, Epsilon, Eta and Iota. This ties for the second highest number of major hurricanes in a single season. A major hurricane is a Category 3 or larger storm with winds of at least 111 mph (178 kph).
There were four major hurricanes that formed in October and November only. Before this year, no year ever had more than two major hurricane formations in those two months.
The season’s strongest storm was Hurricane Iota, which peaked at 160 mph. It was the second major hurricane to form in the month of November, which has never happened in recorded history – Eta was the first.
Iota made landfall in Nicaragua as a Category 4 hurricane with sustained winds of 155 mph, just 2 mph shy of the Category 5 threshold. It was the strongest November hurricane on record to hit Nicaragua, breaking the record set by Eta two weeks before.
A record 12 named storms made landfall across seven states this year: Bertha, Cristobal, Fay, Hanna, Isaias, Laura, Marco, Sally, Beta, Delta, Eta and Zeta.
People along nearly every mile of coastline from Texas to Maine were affected by at least one named storm this season.
“Every mile of the US Gulf and Atlantic coast has been under a tropical storm or hurricane watch or warning, except for one single county with coastline: Wakulla County, Florida,” said Jake Carstens, a meteorology graduate research assistant at Florida State University.
In 2020, every month of hurricane season saw a storm make landfall in the US. May, considered pre-hurricane season, also experienced a storm landfall, meaning there were seven straight months of direct landfalls.
Despite most of the storms hitting the Gulf Coast, the Northeast was affected by three named storms – Fay, Isaias and Zeta. Tropical Storm Fay was the only storm to make landfall in the Northeast, hitting New Jersey on July 10. Hurricane Isaias, which made landfall in North Carolina in August, triggered a huge swath of power outages along the East Coast.
Remarkably, Florida made it almost to the very end of hurricane season before a storm made landfall. Eta became the first November landfall for Florida since Mitch in 1998. And since Eta made two landfalls in Florida, it added to the many miles of coastline under tropical alerts this season.
But of all the areas affected by tropical cyclones this year, Louisiana was the most frequent target. It had a record-breaking five storms make landfall: Cristobal, Laura, Marco, Delta and Zeta.
Hurricane Laura made landfall as a strong Category 4 storm near Cameron, Louisiana, on August 27. Six weeks later, Hurricane Delta struck the same area, battering homes and businesses that were still being repaired from Laura.
Zeta was the fastest of these storms, making landfall at 24 mph. The slowest was Hurricane Sally, which was moving at 3 mph at landfall. Even though the storms were Category 2, the varying landfall speeds changed how the storms affected the local communities.
An average human walks at 3 to 4 mph, which means a person could have walked faster than Sally. But Sally’s super slow movement allowed the storm to dump a tremendous amount of rain over a prolonged period of time in the same locations. An average September sees 4-5 inches of rainfall along the Florida-Alabama-Mississippi panhandle, but Sally dropped that in just a couple of hours. By the time the storm left the region, at least three months of rain had accumulated in some spots.
Zeta’s fast speed allowed the tropical storm-force winds to travel very far inland, not just along the coast, and those winds felled trees and power lines from Louisiana to Virginia. More than 2 million people lost power from Zeta.
But that speed also meant that rainfall totals were not as high as they were with Sally. Widespread totals were within the 2-4 inch range, with one small area of 6 inches near the Mississippi-Alabama border.
Isaias, the name people struggled to pronounce, affected almost everyone along the Eastern Seaboard. More than 100 million people were under either a hurricane watch or warning or tropical storm watch or warning stretching from Florida to Maine.
Damaging winds triggered power outages for more than 3 million customers. Tornadoes were also a big factor with Isaias – at one point more than 30 million people were under tornado watches. The storm produced more than 50 tornado reports in two days, a high number given that tropical systems in the Gulf are more likely to produce tornadoes than their Atlantic counterparts.
Texas had two landfalls, Hanna in July and Beta in September. Alabama was hit by Hurricane Sally in September. South Carolina was hit by the preseason storm Bertha in May.
The Greek alphabet
For the second time in recorded history, the National Hurricane Center used every name on the pre-determined list of names for tropical systems in the Atlantic basin, prompting the use of the Greek alphabet to name storms for the remainder of the season.
And there were a record number of Greek alphabet letters used for storm names – nine: Alpha, Beta, Gamma, Delta, Epsilon, Zeta, Eta, Theta and Iota. Four of the 12 US named storms were from the Greek alphabet. Two of the top five worst storms to ever hit Nicaragua in recorded history were from the Greek alphabet.
The latter portion of the season was remarkably the more intense portion of the season.
Of the first 21 named storms, on the regular hurricane season list, only two were major hurricanes – Laura and Teddy. However, of the nine names used in the Greek alphabet, four were major hurricanes – Delta, Epsilon, Eta and Iota.
The Greek alphabet storms alone produced enough ACE equal to that of a normal hurricane season. If you tally the amount of ACE produced in November, October, and half of September (since Tropical Storm Alpha was named on September 16), the total comes to 106. An average season during those same months would produce an ACE of 46 and a normal entire season produces a total ACE of 104.
The addition of the Greek alphabet named storms pushed the total ACE for the 2020 season to 180, however this was not enough to break the all-time record of ACE, which was 258 set in 1933. However it is important to note that since 1933 was before the satellite era, when data was not always as reliable, some entities cite the record ACE as 245 set in 2005.
When Iota reached Category 5 strength on November 16 with sustained winds of 160 mph, it became the first Greek alphabet storm to ever reach Category 5 intensity.
“So many storms stand out, but I think Iota really does put an exclamation point on what has been a crazy season,” Klotzbach said. “Iota was the latest forming Category 5 hurricane in the Atlantic on record.”
Hurricane hunters and the data
There are two distinctive types of hurricane hunters, NOAA and the US Air Force Reserve. Both organizations fly missions into tropical systems to record invaluable data to be used by forecasters at the National Hurricane Center (NHC).
Maj. Jeremy DeHart is a meteorologist and aerial reconnaissance weather officer with the Air Force Reserve’s 53rd Weather Reconnaissance Squadron. The squadron took to the skies eight times to fly in 11 different storms, including one two-week marathon session flying back-to-back storms.
“Another four deployments were to evacuate the aircraft due to the threat of direct impacts at our home base in Mississippi, which takes an extra toll on our personnel. So we are proud of the work we’ve put in, but like everyone else, we’re ready get some rest and put a lid on the 2020 season,” DeHart said.
NOAA’s hurricane hunters flew 86 missions with nearly 700 flight hours logged.
“There were definitely a lot to choose from, but the biggest surprise for me was the entire Marco and Laura tasking,” DeHart said. “Simultaneous tropical cyclones in the Gulf of Mexico, which were flown from four different locations … that was just not something I ever thought I’d see.”
While hurricane hunter aircraft capture atmospheric data above the storms, hurricane ocean gliders capture data below the storm in the ocean. Incorporating both atmospheric data and ocean temperature and salinity data gathered by the gliders help forecasters provide more accurate intensity forecasts.
The remotely piloted gliders use sensors to measure salt content, temperature and other parameters as they travel in the ocean down to a half mile below the surface. Even under hurricane conditions, they can transmit ocean data via satellite for immediate use in hurricane forecast models.
The science behind the record-breaking season
So what made this season different from other seasons and, more specifically, why did it produce so many storms? The answer is a combination of things: La Niña, lack of vertical wind shear and warmer ocean temperatures.
“Going into this hurricane season, we knew sea surface temperatures were running above average across much of the Atlantic, and that really continued to be the case through the entire season,” CNN meteorologist Taylor Ward said. “In many regions the water temperatures were 2 to 4 degrees Celsius above average, especially in critical areas like the Gulf of Mexico, Caribbean, and the main development region of the tropical Atlantic.”
This provided extreme amounts of fuel for storms that developed in the low wind shear conditions provided by La Niña.
A La Niña watch was issued in August, one month before peak hurricane season, which is very important, because with El Niño/La Niña, timing is everything.
There had been 11 named storms in the Atlantic when the La Niña watch was issued. An average season normally doesn’t produce 11 named storms until November 23. The season was well ahead of schedule before La Niña officially arrived on September 10, which is also the statistical peak of hurricane season.
La Niña is essentially the “cool phase” of the El Niño-Southern Oscillation (ENSO) climate pattern, which is a naturally occurring phenomenon that involves fluctuating ocean temperatures in the Pacific. El Niño is the warm phase of the cycle. Both La Niña and El Niño occur every three to five years on average, according to NOAA.
La Niña weakens high atmospheric winds. In the case of tropical systems, it allows warm air pockets to grow vertically and develop into hurricanes.
“In addition to the La Nina conditions that you mentioned, the tropical Atlantic was much warmer than normal,” Klotzbach explains. “Also, vertical wind shear in July was extremely low across the tropical Atlantic and Caribbean, which is one of the reasons why we increased our forecast from July to August.”
Vertical wind shear is the change of wind speed and direction with height. For a hurricane to form, it needs little to no wind shear and a very moist atmosphere.
This lack of vertical wind shear is part of the reason why the early portion of this year’s hurricane season was so active. Tropical Storm Arthur formed before the season officially began and the season’s rapid pace continued when a record nine named storms developed between May and July.
Wind shear can often make or break a system. When a storm moves into a high shear environment, it often spells doom or at the very least weakens it or prevents it from intensifying further. Having extremely low areas of wind shear allowed several storms to intensify easily and quickly.
“August-October-averaged zonal vertical wind shear was extremely low across the tropical Atlantic and Caribbean,” Klotzbach said.
Climate change may have been a contributing factor to the record hurricane season. While a warming planet does not directly impact the total number of storms, there is mounting evidence that climate change does affect the intensity, forward speed, and the amount of rain produced by tropical cyclones.
Rapid intensification is one way scientists believe the climate crisis is affecting hurricanes, with warmer waters helping storms strengthen more quickly. Rapid intensification is where a named storm strengthens at least 35 mph in 24 hours or less.
This season 10 storms rapidly intensified, six of them were Greek alphabet storms. This set yet another season record for most rapidly intensifying storms in a single season since 1979 (tied with 1995).
“Although most hurricanes tend to weaken as they approach land, some rapidly increase in strength just prior to landfall – a phenomenon that is both dangerous and hard to forecast,” according to NASA. “As the climate continues to warm, the number of storms that fall into the latter category is likely to increase, presenting a stark reality for communities in their paths.”
Eight storms rapidly intensified 24 to 48 hours before landfall. Those storms were Hanna, Laura, Sally, Delta, Gamma, Zeta, Eta and Iota. Rapid intensification is especially dangerous when it happens in the 24 to 48 hours before a storm reaches land. People living along the coastline go to bed anticipating a Category 1 but wake up to a Category 3 major hurricane far stronger than anticipated.
Besides rapid intensification, several studies have suggested that since the 1950s, the forward speed of all tropical storms and hurricanes has slowed as much as 10-15%. Something that was far too evident with Hurricane Sally this year.
“Hurricane Sally stood out to me this year. The storm’s sluggish pace, at times moving as slow as 2 mph just miles off the Gulf Coast, was as memorable as any storm this season,” CNN meteorologist Pedram Javaheri said. “Sally’s complete lack for a defined steering characteristic as it meandered along the northern Gulf coast, soaking the region for days before it ever made landfall resonated with me.”
Recent findings suggest that the warmer temperatures across the Arctic, an area that is warming at a disproportionately higher rate, are playing a role in weakening atmospheric circulations. These weaker circulations can reduce forward speed of tropical systems, hence allowing for wetter and more destructive storms.
Hurricanes are getting wetter as a warmer atmosphere leads to more rainfall and research shows that the heaviest rainfall in storms has increased around 7% compared to last century.
“Simply put, warmer air holds more water vapor,” said Jim Kossin, an atmospheric research scientist at NOAA’s National Centers for Environmental Information.
Scientists are confident the climate crisis is making storm rainfall worse by increasing the rate at which it falls and the amount of rain storms produce. Gulf of Mexico water temperatures are largely above average, and the Northern Hemisphere just experienced its hottest summer on record. That allowed storms like Sally to hold more moisture and produce more rain.
If that is not enough climate change evidence, a new study published earlier this month found that hurricanes are maintaining their strength after landfall for much longer, exposing areas far inland to damaging winds that they have rarely experienced before.
And while researchers say we can’t draw climate change conclusions based on any one storm, they say the overall trend line is clear: Hurricanes are packing a punch farther inland than in recent decades.
Monday is the last official day of the 2020 Atlantic hurricane season, but it’s 2020, so Mother Nature may not have gotten that message. There is currently one system brewing in the Atlantic which has the potential to become a named storm in the next couple days. The next name on the list is the Greek letter Kappa. December storms have happened before. The last year a named storm developed in December was Olga in 2007.
CNN’s Pedram Javaheri, Brandon Miller and Taylor Ward contributed to this story