A season of “fish storms”?
The 2023 North Atlantic hurricane seasongot off to an unusually early start, with an unnamed subtropical storm off the north-eastern coast of the US in January – well before the hurricane season’s official start date of June 1.
By November 23, the 2023 season had witnessed 20 named storms: six more than the average pre-season forecasts announced*. This makes it the fourth highest total of named storms in a year since 1950, according to the National Oceanic and Atmospheric Administration (NOAA) . Of these, seven intensified into hurricanes, with three hurricanes escalating to major hurricane status: Lee (Category 5), Franklin (Category 4), and Idalia (a Category 4 hurricane and Category 3 upon landfall).
Despite it being an active season, only eight storms have made landfall so far in 2023, and only tropical storms Harold and Ophelia and Hurricane Idalia made landfall in the US. This has led to most storms being referred to as “fish storms” – storms that pose virtually no risk to land but can be a threat to boats and ships and produce dangerous currents along the coast .
Hurricane Franklin was the first major hurricane of the 2023 Atlantic hurricane season. It made landfall as a tropical storm on the southern coast of the Dominican Republic and triggered heavy rainfall and destructive winds. After passing the Dominican Republic, it further intensified into a Category 4 hurricane on the high seas.
In late August, Hurricane Idalia made landfall in Florida’s Big Bend region (where the narrow Florida Panhandle transitions to the wider Florida Peninsula) as a Category 3 storm, becoming the first major hurricane to make landfall in that area since record-keeping began in 1842.
At peak season in early September, the massive and expansive Category 1 Hurricane Lee threatened to impact the eastern coast of the US and Canada. A long-lived storm, Lee underwent a remarkable transformation, intensifying from a Category 1 to a Category 5 storm in just 24 hours. However, it quickly weakened and transitioned to an extratropical storm before making landfalls in Nova Scotia, Canada, as a Category 1 storm, then moving out into the far northern Atlantic. Lee’s extratropical phase brought rain and gale-force winds to parts of the UK and Ireland. Meanwhile, swells generated by the storm triggered dangerous surf and rip currents along the entire Atlantic coast of the US. Aon estimates economic losses due to Hurricane Lee to be around US$50mn .
How did the 2023 season compare with the forecasts?
Contrary to pre-season predictions of a near-average season, the 2023 Atlantic hurricane season has turned out to be above average, with Hurricane Idalia making it to the list of costliest Atlantic hurricanes in recorded history.
El Niño, a natural climate pattern associated with warmer-than-average sea surface temperatures (SSTs) in the central and eastern tropical Pacific Ocean, typically casts a dampening effect on Atlantic hurricane activity. This is because El Niño can increase wind shear, a disruptive force that can hinder hurricane formation and intensification. However, the 2023 Atlantic hurricane season has defied this El Niño-induced suppression, producing an unusually high number of named storms. This can be attributed to exceptionally warm SST anomalies recorded in the north Atlantic Ocean. These record-breaking SSTs were caused by a combination of short-term anomalous circulation in the atmosphere and longer-term changes in the ocean .
The complex interplay of these contrasting climate signals made it difficult for forecasters to accurately predict the activity of the 2023 hurricane season. Forecasting institutions were faced with the challenge of balancing the typically hurricane-suppressing effects of El Niño with the potent hurricane-fueling conditions created by the warm Atlantic waters.
Midway through the season, forecasting bodies like the NOAA National Hurricane Center and other institutions revised their initial predictions, indicating that the season would be more impactful than previously anticipated.
Deep dive: Hurricane Idalia
Hurricane Idalia roars into the Gulf of Mexico; Source: NASA Earth Observatory
On August 30, 2023, Hurricane Idalia made landfall along the Florida coast, unleashing its fury upon the Big Bend region. The system briefly attained a Category 4 status on its approach to Florida, but its intensity waned, resulting in a reduction of maximum sustained wind speeds to 201km/h (125 miles per hour) before it made a powerful and destructive landfall as a Category 3 hurricane near Keaton Beach, Florida. Despite a gradual weakening after landfall, Idalia’s initial intensity and rapid forward speed propelled it across northern Florida and into southern Georgia within a mere nine hours, maintaining hurricane strength throughout its path.
Idalia unleashed catastrophic storm surges with inundation levels in coastal areas ranging from 2 to 3.7 meters (7 to 12 feet). These surges were among the highest recorded since the 1993 Storm of the Century , leaving a trail of destruction along the coast.
Heavy rainfall also accompanied Idalia, leading to flash flooding in some areas. The storm’s impact extended beyond Florida, with heavy rainfall and strong winds affecting Georgia. As Idalia weakened further, it continued its path into southern South Carolina, where it still posed a significant threat as a tropical storm.
Moody’s RMS estimates insured losses from Hurricane Idalia to range between US$3bn and US$5bn, with a best estimate of US$3.5bn. Additionally, Moody’s RMS anticipates the National Flood Insurance Program (NFIP) could incur losses of around US$500mn . It expects the US private market insured losses to be driven by wind, while storm surge and flood could contribute to around 40% of total private market losses, and around 30% of the total event losses (incl. NFIP). It also estimates most private market insured losses (around 70%) and NFIP losses (around 90-95%) from Idalia to be in Florida.
Even though losses from Idalia surpassed the billion-dollar mark, two factors may have helped to mitigate its impact. Firstly, the landfall area has a significantly lower population and exposure density compared to much of Florida. Secondly, it has a relatively small wind field, which helped to reduce the spatial extent of wind-induced damages. However, some of this mitigation was counteracted by the greater vulnerability of the properties in the region, which were mostly built during the 1980s and 1990s, before modern building codes were implemented .
Where are we now?
The Atlantic hurricane season has officially ended (on November 30), having had notable impacts on several regions. The impact of Hurricane Idalia, particularly its storm surge and floods (both pluvial and fluvial), underscores the urgent need to accurately model and incorporate secondary perils into probabilistic catastrophe models. The increasing relevance of these perils due to climate change necessitates further research and development in secondary peril modeling to fully assess and mitigate disaster risks. While advancements in these models have been significant, adequately capturing the complexity and potential impact of secondary perils remains a challenge.
Comparison of the 2023 North Atlantic hurricane season storms to the pre-season forecast averages, mid-season averages of AccuWeather, Colorado State University (CSU), Tropical Storm Risk (TSR), National Oceanic and Atmospheric Administration (NOAA) and North Carolina State University (NCSU) and 2022 season actuals.
Source data: National Hurricane Center. Graphics by Allianz Commercial