Hot Atlantic sets the stage for extreme hurricane season

This year’s Atlantic hurricane season will bring between 17 and 25 named tropical storms, including as many as seven major hurricanes, according to the US National Oceanic and Atmospheric Administration (NOAA). That is the highest number of storms the agency has ever projected in its seasonal forecast.

The stage is set for an “extraordinary” season, said Rick Spinrad at NOAA, in a briefing on 23 May.

The annual forecast considers tropical storms that reach sustained wind speeds above 63 kilometres per hour (39 miles per hour). This year, of the named storms, NOAA predicts that eight to 13 will be hurricanes with wind speeds at least 119 kilometres per hour, and four to seven will be major hurricanes with wind speeds at least 179 kilometres per hour.

The combined strength of these storms across the entire season, known as accumulated cyclone energy, ranks the second highest ever forecast by the agency.

The high number of predicted storms is due to high surface temperatures in the tropical Atlantic Ocean and a shift to the cooler La Niña climate pattern in the Pacific Ocean. The above-average temperatures in the Atlantic can strengthen storms and cause them to intensify more rapidly, and La Niña reduces patterns of wind shear that typically weaken hurricanes. An above-normal African monsoon season that can seed storms also contributes to the high projection.

“Everything has to come together to get a forecast like this,” said Ken Graham at the US National Weather Service.

The agency’s forecasts broadly align with earlier projections from other groups, including the UK Met Office, which projected as many as 28 named storms, far above the 14 or so seen on average over the past three decades. Another forecast from Michael Mann and his colleagues at the University of Pennsylvania projected an even more extreme 27 to 39 named storms. The 2020 hurricane season, which was the most active on record, had 30 named storms.

“It’s a ‘double whammy’ of factors that are responsible for our prediction of a record active season, and both factors are favoured by human-caused climate change,” says Mann. Warming driven by greenhouse gas emissions has played a substantial role in the warm Atlantic waters, and climate models suggest human-caused warming could lead to a greater tendency for La Niña conditions, he says.

The forecasts contrast with the 2023 season, which saw a more moderate 20 named storms. Atlantic Ocean temperatures were also high last year, but the effect of this was reduced by wind shear driven by the El Niño climate pattern in the Pacific Ocean. And most of those hurricanes blew out over the ocean rather than making landfall on the coast. Still, last year’s storms caused about $4 billion in damage to the US, says Spinrad.

The direction in which wind will steer 2024’s storms is now “the million dollar question”, says Phil Klotzbach at Colorado State University. Steering patterns are difficult to forecast far ahead of time because they depend on shorter-term weather. But conditions look favourable for storms to form in the Caribbean, where they can impact islands and quickly hit the US coast, he says.

While the forecast is alarming, officials say people can take steps to minimise danger and damages from the expected storms. “It’s the highest number, but it’s about being ready,” says Graham.