Weatherwatch: slower tropical storms are raising flood threat


Falls in the average tracking speeds of hurricanes and typhoons, attributed to global warming, put more lives at risk

A flooded street in Houston, Texas.
People in Houston being evacuated from floodwaters brought on by Hurricane Harvey when it stalled over Texas in August 2017. Photograph: David J Phillip/AP

Northern hemisphere weather is stealing the limelight right now. Hurricane Florence slammed into the eastern US on Friday as the Philippines and Hong Kong braced for Typhoon Mangkhut. Hurricanes and typhoons are no surprise at this time of year, but what is noticeable is that these tropical storms are not barrelling along as fast as they used to.

Research published in Nature earlier this year showed that the average speed at which tropical storms track has slowed down by 10% since 1949. Over land, speeds have decreased even further; around 30% for western North Pacific storms and 20% for North Atlantic ones. Changes in atmospheric circulation, caused by global warming, are thought to be behind the decrease in pace.

Hurricanes that proceed at a trot instead of a canter are not good news. “The slower the storm moves, the more rain falls on the area under it. Inland fresh-water flooding increases and this is the No 1 factor, by far, that causes loss of life associated with hurricanes,” says James Kossin, an atmospheric scientist at the University of Wisconsin. Hurricane Harvey demonstrated this last year when it stalled over Texas, and right now Hurricane Florence appears to be lingering too.