How hurricanes like Milton spawn tornadoes

October 9

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Commuters on Interstate 75 driving away from Florida’s west coast ahead of the arrival of Hurricane Milton

Marco Bello / Reuters

Hurricane Milton has spawned at least two large tornadoes ahead of making landfall on Florida’s west coast later tonight, and a tornado watch has been issued for much of the southern part of the state.

Such tornadoes are not uncommon – they are observed in more than 80 per cent of hurricanes that made landfall in the Gulf Coast – but it is unusual for them to be so clearly visible ahead of the hurricane, says William Gallus at Iowa State University. “There have been a surprisingly large number already and they look like Great Plains tornadoes,” he says. “They’re wide.”

There are two main ingredients needed for hurricanes to spawn tornadoes, which add to the destructive potential of a storm. The first is instability created by heat and humidity in the atmosphere. The second is differences in wind speed and direction at different altitudes, known as wind shear.

Hurricanes moving over water normally have relatively low wind shear because there isn’t much friction between the storm and the sea surface. “It is like this giant spinning cylinder, so the winds aren’t very different on the ground than they are up high,” says Gallus.

That changes as the storm makes landfall, and friction with the ground slows winds at lower altitudes, which also drives them towards the centre of the storm. When the air is hot and humid enough, these intense winds can form tornadoes.

In this case, bands of wind ahead of the main body of Milton have reached the coast, creating wind shear and spawning tornadoes, says Gallus.

Along with other hurricane hazards like storm surge and heavy precipitation, such tornadoes can cause substantial destruction, their path sometimes visible in the pattern of debris they leave behind. By one estimate, about 3 per cent of tropical-storm related deaths in the US were caused by the tornadoes they spawned; an earlier estimate put the number at 10 per cent of fatalities.

Such tornadoes may also become more frequent as climate change raises temperatures in the lower atmosphere, adding to the unstable conditions under which tornadoes form. In a recent study, Gallus and his colleagues simulated how four different hurricanes – Ivan, Katrina, Rita and Harvey – may have behaved with warming expected by mid-century under a very high emissions scenario. They found the number of tornadoes spawned by each storm in their simulation rose substantially, ranging from a 56 per cent increase for Harvey to a 299 per cent increase for Katrina.

“Even if you only end up with half as much it would be a very noticeable increase in the number of tornadoes,” says Gallus.

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