Why tornadoes form within hurricanes — and how climate change could make it more common

The extreme weather associated with Hurricane Milton has included tornadoes that have killed at least five in Florida’s St. Lucie County, a phenomenon known to occur as such storms move inland.

Tornadoes are more commonly associated with areas less vulnerable to coastal storms, like the southern and midwestern U.S. But they can form once a hurricane transitions from sea to land and its winds encounter greater friction.

Between 1994 and 2014, two out of three tornadoes that hit the U.S. between August and September had a link to tropical cyclones, according to The Weather Channel.

While hurricanes are over the ocean, “the water is kind of acting like a frictionless surface [and] winds at the ground don’t get a whole lot slower as they’re encountering the water,” said Jana Houser, an associate professor at Ohio State University who specializes in radar analysis of tornadoes.

“Water does not have much friction,” noted William Gallus, a professor of meteorology at Iowa State University, who compared a hurricane over the ocean to a can of soup being spun, spinning as a single unit in one direction.

But once a storm approaches land, Houser said, the winds closer to the ground begin to slow as they encounter resistance from the ground, while those at higher altitudes accelerate.

This alone doesn’t form a tornado because the winds are still spinning horizontally, Houser said, but when a strong thunderstorm is added to the mix, it reorients the winds from “spinning like a bike tire to spinning like a top.”

A tornado results when two key ingredients combine, Gallus said: wind shear, or a quick change in wind speed and direction, and instability as a result of the system becoming warm and humid closer to the ground but colder higher up. When that happens, he said, “blobs of hot air can rise on their own” and form tornadoes like the ones seen with Milton.

Tornadoes most frequently form within the thunderstorms embedded in the hurricane’s outer rain bands, far from the storm's eye, due in part to the increased instability there, according to the National Weather Service.

The Milton tornadoes, Gallus said, were somewhat atypical by virtue of their clear visibility. In most cases, tornadoes that form in such storms are hidden by large areas of rain and cloud cover. Milton's high level of instability however, meant the outer bands where the tornadoes formed moved far from the rest of the storm.

Climate change always looms large in the background of extreme weather, but Houser said it does not necessarily have a direct causal link to the tornadoes. Rather, she said, the well-documented association between warmer sea surface temperatures and heightened hurricane intensity make the conditions that lead to the tornadoes more likely. 

“There is a relationship between the intensity of the hurricane and the ability of the environment to facilitate tornado production,” she said. 

In Milton, Gallus said, “some of the tornadoes acted a lot like tornadoes in Tornado Alley in the Great Plains.”

“You can never take one weather event and say anything pro or con” in terms of a direct causal relationship to climate change, Gallus added, “but it’s better to say it is consistent with what our study found, that the number of tornadoes associated with hurricanes in the future will probably increase.”

Milton, Houser noted, was particularly ripe for tornado development because as the storm moved toward Florida, an area of wind higher up in the atmosphere — around the same point as airplane flight paths — was already seeing high winds.

Those winds “acted to weaken the center part of the hurricane but then these outer bands ended up being more enhanced,” she said.

While the visibility of the Milton tornadoes was unusual, along with how many of them occurred across a small area, other hurricanes, including ones that have moved over Texas or further up the East Coast, have been associated with as many as hundreds of tornadoes, Gallus said.

The tornadoes created by hurricanes tend to be weaker: According to the Weather Channel analysis, none of the 558 hurricane-linked tornadoes in 2004 and 2005 was higher than an F3 on the five-point Fujita scale used to measure tornado intensity.

Gallus pointed to earlier research finding that up to 10 percent of fatalities in hurricanes are tornado-related.

However, cases like Milton's, in which such tornadoes cause fatalities, may be dangerous simply because the tornadoes catch people who don’t associate them with tropical storms off-guard, Houser said — even though in this instance, tornado activity was forecast.

“When your attention is focused so much on the hurricane the idea of tornadoes isn’t really something you’re processing,” she said.