As the human and economic costs mount in the aftermath of two deadly hurricanes, Helene and Milton, we look at why they were so damaging and whether there’s anything we can learn from them.
At 11.25pm on 26 September, Hurricane Helene made landfall in Florida’s Big Bend region before heading inland to cause unprecedented damage across the southeast of the US. By the time the hurricane had dissipated, fatalities were at least 250, millions had been left without clean water or power, and a state of emergency had been declared in North Carolina.
Less than two weeks later, Hurricane Milton started forming in the south-west Gulf of Mexico. At the time of writing the death toll was 33 and preliminary estimates of the cost of damage were around $50bn.
While Helene was the most devastating in terms of loss of life, both events have brought into sharp focus how climate change is impacting risk in the US – and how far communities still have to go towards fully understanding, and adapting to, that risk.
Why was Helene so severe?
One reason both events caused so much damage was their sheer magnitude. Helene was in the 90th percentile of hurricanes in terms of its size, and its path of destruction extended over 500 miles, affecting areas that were far from the coast. Another was the incredible speed of their intensification. Milton saw its sustained wind speeds go from 80 mph to 175 mph and its pressure drop from 988hPa to 911hPa in just 12 hours.
Helene was also particularly damaging because it produced extreme levels of all three hazards associated with tropical cyclones – wind, storm surge and rain. Dr Natalie Lord, Principal Climate Scientist at Fathom, explains: “Some storms produce one or two of these hazards that are particularly severe, but Helene had exceptionally strong winds, huge amounts of rain, and a large storm surge.”
How Hurricane Helene developed
Helene started forming on 17 September as a monsoon-type low-pressure system in the Caribbean Sea. It intensified rapidly as it moved upwards to the Gulf of Mexico, fuelled by abnormally high ocean surface temperatures – above 28°C at the time Helene was forming, which is nearly 1 °C warmer than the 2003–14 mean.
By the time Helene made landfall as a category 4 hurricane in the Big Bend region of north-west Florida, its sustained wind speeds were more than 140 mph and the air pressure at its center was 938hPa. Both these factors were major influencers in the height of the storm surge.
Helene had also drawn up large amounts of moisture from the warm ocean and, as it moved inland, it dropped approximately 151 trillion liters of rainfall, half of which fell in Florida, Tennessee, Georgia and the Carolinas. There was particularly severe flooding inland in the southern and central Appalachians, where an earlier rainfall system unrelated to tropical cyclones had been pushed ahead of the storm, saturating the ground, swelling the rivers and compounding the storm’s impact.
Milton’s unusual trajectory
Milton’s story was somewhat different. It formed in the southwest Gulf of Mexico – unusual for October storms, which typically start life in the Caribbean Sea. The conditions were conducive to rapid intensification – record-breaking ocean temperatures and low wind shear. It quickly gained strength and size, but this time took a perpendicular trajectory, heading east-northeast, again unusual for the time of year.
As it headed directly towards Tampa, the potential damage was predicted to be huge. In the end, the storm surge was not as high as had been feared (see below), so although the hurricane caused massive destruction, the worst-case scenario was avoided.
The complexity of storm surges
Storm surge height is dependent on wind speed and air pressure, but also the length and depth of the continental shelf. The bathymetry of Florida’s west coast makes it susceptible to storm surges, as the continental shelf is both wide and shallow and gently slopes up to the coastline.
Helene’s massive storm surge tore apart several hundred structures on the coast, but because it made landfall in a relatively unpopulated area, the areas that observed the highest surges avoided the worst damage.
Hurricane Milton, on the other hand, was forecast to hit Tampa directly. Fortunately, the hurricane made landfall to the south of the city, so catastrophic damages were avoided.
Because winds spin counterclockwise around the eye of the storm in the Northern Hemisphere, surges form to the right of the eye of the hurricane (relative to its direction of travel), as strong onshore winds push water towards land. As Tampa is to the north of the landfall location, it experienced negative surges, with the winds pushing water offshore and away from the coast.
The figure below shows the water levels during Milton from a tide gauge near Tampa. It also shows the large positive surge during the earlier Hurricane Helene (on the left).
What part did climate change play?
Many extreme events are expected to become more severe with climate change, with studies suggesting that tropical cyclones are now likely to be more severe, with more rapid intensification and higher probabilities of more extreme rainfall and wind speeds, as summarized in this research paper.
Both Milton and Helene were characterized by extremely rapid intensification, fuelled in part by the record-breaking temperatures of the sea’s surface. (This review paper, with contributions from Fathom’s Dr Natalie Lord, explores the effects of climate change on tropical cyclones, as well as other climate processes which can impact them, such as the El Niño Southern Oscillation (ENSO).)
While it is difficult to determine the precise role climate change played in these two events, this early attribution study estimates that climate change caused over 50% more rainfall during Hurricane Helene in some parts of Georgia and the Carolinas. The observed rainfall was estimated to be up to 20 times more likely in these areas because of global warming.
Inland flood: a rising risk
Whereas the damaging effects of Milton were mainly felt in Florida, the worst impacts of Helene were over the southern Appalachians, particularly in western North Carolina, hundreds of miles inland and far from areas you would expect to be hit by a tropical cyclone.
While people along the coast were ordered to evacuate as Helene approached, some communities further north only received warnings after floods had already caused significant damage.
Why were inland communities taken by surprise?
Half of the fatalities related to Helene were in North Carolina, with the city of Asheville, in the foothills of the Blue Ridge Mountains at 2,000 ft elevation, particularly hit hard by flooding.
FEMA is responsible for mapping flood risk for communities in the US, with its maps covering 100-year and, in some areas, 500-year flood. The image below shows FEMA’s simulation of the 100-year flood zone, alongside a drone image of the actual observed flood extent in Asheville.
According to research by Fathom’s Chief Scientific Officer, Dr Oliver Wing, the county in which Asheville sits, Buncombe, ranks 76 of over 3,000 in terms of modeled average annual loss within US counties ($66.5 million in 2020) as this interactive map shows.
The upshot is that the high-risk indicators are there. As Fathom’s Dr Oliver Wing points out, “We should not be surprised when extreme events occur. The annual chance of a 100-year flood occurring on any given river in the UK, for example, is 78%; very different to the 1% chance people typically associate with 100-year events. In the US, with its geographic size and diversity of flood drivers, that probability is going to be even closer to 100%. I am not sure this is well appreciated – there is a distinct lack of statistical literacy in the public discourse.”
What do Helene and Milton mean for the insurance industry?
Counting the financial impacts of events is a long and complicated process, but according to some estimates, combined private insured losses from Helene and Milton could reach $55bn.
While the damage from Milton was largely wind-related, Helene’s was mainly caused by storm surge and inland flooding, which is less likely to be covered by insurance (most US homeowners’ insurance policies include wind-damage cover but flood has to be bought separately).
That’s why estimates for insured losses (covering wind, storm surge and inland flooding damage) from Hurricane Helene are lower, at around $6.4bn. Economic losses, meanwhile, were estimated to be in triple digits, with some estimates reaching more than $100bn.
The growing insurance gap
This stark discrepancy between insured and economic losses highlights the growing insurance gap in the US. In some areas that were hit the hardest by Helene, it’s estimated that less than 1% of homes had flood insurance. According to Swiss Re, in Buncombe County, North Carolina, there were 140,000 housing units before the storm but only 941 flood insurance policies were active in August.
One positive outcome from tragic events like Helene and Milton could be wider provision and a higher uptake of insurance – as well as interest in mitigation initiatives such as those piloted in New York City by Swiss Re and partners.
Want to know more about Fathom’s state-of-the-art flood data for the US? Explore our US Flood Map.
The need to mitigate, adapt and expect the unexpected
If Helene and Milton have shown us anything, it is that extremely rare events can happen in quick succession, that they can impact areas that are not necessarily prepared for them, and that we might need to rethink our understanding of risk and probability.
Climate change is making hurricanes wetter and more severe while development, population growth and urbanization are putting more people in harm’s way. This means everyone, from insurers to governments, has an urgent need to help communities adapt to this evolving risk. While forecasting natural catastrophes is anything but certain, we can be ~100% certain that one will happen again.