How hurricanes like Milton spawn tornadoes

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

Such tornadoes are not uncommon — they are seen in more than 80 percent of hurricanes that make landfall on the Gulf Coast — but it is unusual for them to be so clearly visible before the hurricane, says William Gallus of Iowa State University. “There are already a surprising number of them and they look like tornadoes from the Great Plains,” he says. “They are wide.”

Two main ingredients are needed for hurricanes to produce tornadoes, which add to a storm’s destructive potential. The first is the instability caused by heat and humidity in the atmosphere. The second is differences in wind speed and direction at different altitudes, known as wind shear.

Hurricanes that move over water normally have relatively low wind shear because there is not much friction between the storm and the sea surface. “It looks like this giant spinning cylinder, so the wind is not much different on the ground than it is at high altitude,” says Gallus.

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

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

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

Such tornadoes may also become more common as climate change increases temperatures in the lower atmosphere, contributing 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 – might have behaved with expected warming by mid-century in a very high emissions scenario. They found that the number of tornadoes spawned by each storm in their simulation increased significantly, ranging from a 56 percent increase for Harvey to a 299 percent increase for Katrina.

“Even if you only get half that, it would be a very noticeable increase in tornadoes,” Gallus said.