Hurricane Milton made landfall in Florida along with 19 previous violent tornadoes believed to have been formed by wind shear, thermal instability and atmospheric moisture.
Storm Milton landed in Florida with winds of 193 km/h, equivalent to level 3 on the 5-level storm scale in the US at about 8:30 p.m. on October 9 (7:30 a.m. on October 10, Hanoi time).
However, heavy rain, strong winds and violent tornadoes appeared in Florida before the storm hit shore. Florida Governor Ron DeSantis said at least 19 tornadoes appeared in Florida and 116 tornado warnings were issued across the state. St. Louis County Sheriff Lucie, Keith Pearson, said several people were killed at the Spanish Lakes Country Club after tornadoes devastated the area, but did not provide details.
Tornadoes accompanying hurricanes are not rare, they appear in more than 80% of storms that land in the Gulf of Mexico coastal area. But it’s unusual for tornadoes to be seen so clearly ahead of the storm, according to Iowa State University meteorology professor William Gallus. “The number of tornadoes is surprising and they look similar to tornadoes in the North American Great Plains. They are very large,” he said.
There are two main “ingredients” needed for storms to create tornadoes, increasing their destructive power. 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, called wind shear.
Storms moving over water often have relatively small wind shear because there is not much friction between the storm and the sea surface. “It’s like a giant rotating cylinder, so the wind at the surface isn’t much different than up high,” Gallus said.
But that changes when the storm hits land. Friction with the ground slows winds at low altitudes, while also pushing them toward the center of the storm. When the air is hot and humid enough, these strong winds can form tornadoes. In the case of Hurricane Milton, as the storm approached shore, wind bands ahead of the main body of the storm reached the coast, creating wind shear and causing tornadoes.
Tornado in Fort Myers city before Hurricane Milton made landfall on October 9. Video: WYZE
“You suddenly create a situation where the wind speed and direction change relative to the winds aloft,” said Jana Houser, an assistant professor of meteorology at Ohio University, explaining the landfall scenario. This is wind shear and can cause the air to spin.
Initially, this creates a rotating horizontal cylinder of air, parallel to the ground. Like any thunderstorm, thunderstorms in hurricanes create strong updrafts of air. These air currents can straighten the air cylinder, creating a tornado.
If the rising airflow is very strong, the rotating air column will be compressed more tightly, with a smaller diameter. When this happens, the tornado can become stronger, much like a figure skater retracting his arms to increase his spin speed.
Along with other storm dangers such as storm surge and heavy rain, tornadoes can cause significant damage. According to an estimate in the magazine Bulletin of the American Meteorological Societyabout 3% of hurricane-related deaths in the US are caused by accompanying tornadoes. An older estimate put the figure at 10%.
Such tornadoes may become more frequent as climate change increases temperatures in the lower atmosphere, increasing the unstable conditions that cause tornadoes to form. In a recent study, Gallus and his colleagues simulated the behavior of four hurricanes Ivan, Katrina, Rita and Harvey in the middle of this century in a situation where the Earth warms due to large emissions. They found that the number of tornadoes associated with each storm in the simulation increased significantly, from 56% with Hurricane Harvey to 299% with Hurricane Katrina.