How hurricanes like Milton create tornadoes

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

William Gallus of Iowa State University said such tornadoes are not uncommon and have been observed in more than 80% of hurricanes that make landfall in the Gulf, but it’s unusual for them to be so clearly visible in front of a hurricane. . “We’ve already seen an alarming number of tornadoes, and they look like the Great Plains tornadoes,” he says. “It’s spacious.”

Two main elements are needed for a hurricane to generate a tornado, which increases the destructive power of the storm. The first is instability caused by heat and moisture in the atmosphere. The second is the difference in wind speed and direction at different altitudes, known as wind shear.

Hurricanes that move over water typically have relatively low wind shear because there is not much friction between the storm and the ocean surface. “It’s like a giant rotating cylinder, so the wind doesn’t change much between the ground and the sky,” Gallus said.

Conditions change when a storm makes landfall, with friction with the ground slowing the winds at lower altitudes and pushing them toward the center of the storm. When the air becomes hot and humid enough, these strong winds can form tornadoes.

In this case, a band of wind in front of Milton’s main body reaches the coast, causing wind shear and spawning a tornado, Gallus said.

Like other hurricane hazards such as storm surge and heavy rain, these tornadoes can cause significant destruction, and the pattern of debris they leave behind can sometimes indicate their path. By some estimates, approximately 3 percent of tropical cyclone-related deaths in the United States are caused by tornadoes spawned by tropical cyclones. Previous estimates put the number at 10% of the deaths.

Such tornadoes could also become more frequent as climate change warms the lower atmosphere, making conditions more unstable for them to form. In a recent study, Gallus and his colleagues investigated how four different hurricanes, Ivan, Katrina, Rita, and Harvey, would be affected by the expected warming by mid-century under very high emissions scenarios. We simulated how it would behave. They found a significant increase in the number of tornadoes produced by each storm in their simulations. The range was from a 56% increase in Harvey to a 299% increase in Katrina.

“Even if only half of them occur, the number of tornadoes will increase very significantly,” Gallus said.