What is a storm surge?
As a hurricane approaches a coast, the churning winds force ocean water up on to land; atmospheric pressure from the storm also helps squeeze the water ashore. The shallower the continental shelf, the higher the threat of a dangerous surge. The water may take a couple of days to fully subside.
Water is heavy – about 1,700lb, or 770kg, to a cubic yard (0.76 cubic meters) – and it can move fast in a surge, sweeping people to their deaths, throwing about boats and vehicles, and pulverizing structures. Six inches of fast-moving water is enough to knock over an adult, the National Hurricane Center (NHC) says. Surges become even more dangerous as they coincide with high tide.
Hurricane Katrina in 2005 caused storm surges of more than 25ft on the Mississippi coastline. The NHC says many of the 1,500 people killed lost their lives, directly or indirectly, due to the storm surges.
What will Hurricane Milton’s impact be?
Storm surge is often the greatest threat to life and property from a hurricane, according to the National Weather Service, which added that in the past, large death tolls from major hurricanes have often resulted from the rise of the ocean.
The National Weather Service said on Wednesday it was expecting life-threatening storm surges, with some areas along the west coast predicted to receive a storm surge of as much as 10-15ft above ground level on Wednesday and Thursday.
The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline, the National Weather Service said.
“If you are in the Storm Surge Warning area, this is an extremely life-threatening situation and you should evacuate if ordered to do so by local officials,” the National Weather Service said on Wednesday.
On Wednesday morning, Ron DeSantis, the governor of Florida, said it was not clear exactly where the eye of the storm would come ashore but the impact would be “broader than that”, adding that “absolutely every place on the west coast of Florida could get major storm surge”.
Officials in Sarasota county, Florida, have predicted storm surge there from Milton will be about double the amount from Hurricane Helene. The National Weather Service has warned that areas of Sarasota, Venice and Englewood have the potential for 11-15ft of storm surge above ground.
In Pinellas county, officials have told residents that the expected 10-15ft of storm surge there is not only “unimaginable” for their coastal community, but is also “not survivable”.
Jane Castor, Tampa’s mayor, has told residents that if they are in a single-story home “that is hit by a 15ft storm surge, which means that water comes in immediately, there’s nowhere to go”, adding: “So if you’re in it, basically that’s the coffin that you’re in.”
According to the National Oceanic and Atmospheric Administration (Noaa), hurricane storm surges can cause extensive damage, and can “severely erode beaches and coastal highways” as well as take out boats and buildings.
“As the waters move inland, rivers and lakes may be affected, and add to the rising flood levels,” the administration said.
In 2022, Hurricane Ian produced catastrophic storm surge along the south-west and north-east coasts of Florida. In some areas, the storm surge was up to 18ft.
How is the climate crisis a factor?
As greenhouse gases help trap heat in the atmosphere, they are also helping to supercharge the oceans with record-breaking temperatures. The heat in the Gulf of Mexico, where many of these storms develop, has been abnormally high and this extra heat acts as a sort of jet fuel for hurricanes, quickly turning them into major storms.
Researchers have found that since the 1970s, the number of storms escalating into category 4 or 5 hurricanes, with winds of at least 131mph (210 km/h), has roughly doubled in the North Atlantic. “If you look back in time, historically, storms intensified at a slower rate than they do now,” said Phil Klotzbach, a researcher at Colorado State University who specializes in hurricane forecasting.
What will the long-term costs be?
The costs of extreme weather linked to the climate crisis keep on rising. In the US last year, there were a record number of 28 disasters that each cost at least $1bn.
In the aftermath of Hurricane Helene, one estimate, by the private forecaster AccuWeather, said the damages could be between $225bn and $250bn.
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