A tornado that tore through the Houston area on Tuesday was the kind of early-season storm that scientists say has been occurring with increasing regularity — a sign that patterns of severe weather are shifting.
The tornado struck Pasadena, southeast of Houston, seriously damaging homes and other buildings and knocking out power to thousands in the city and its surrounding areas. More than a dozen other tornado sightings were reported as the storm moved across the Gulf Coast, although they have yet to be confirmed by the National Weather Service.
The spate of reported tornadoes adds to changes that experts have been observing in recent years — specifically, that where and when tornadoes occur has begun to shift.
Historically, tornadoes were most likely to strike within a column of the central U.S. that was nicknamed "Tornado Alley." The area includes parts of Texas, Oklahoma, Kansas and Nebraska. In recent years, however, researchers noticed that fewer tornadoes were touching down in the Great Plains and that more were hitting the Southeast.
Tornado Alley's changing borders can have deadly consequences, said Victor Gensini, an associate professor in the department of earth, atmosphere and environment at Northern Illinois University, who published key research on the topic in 2018 and has studied the shift extensively.
"The No. 1 thing is that we have greater population density in the Mid-South," Gensini said. "There are basically more targets to hit on the dartboard."
The differing landscapes are also a factor in a storm's destructiveness. In areas with tightly packed cities, lots of trees and less open space, for example, a tornado can cause more catastrophic damage.
There are also more vulnerable communities across the Southeast, particularly among people who live in mobile homes, Gensini said.
"Half of all tornado fatalities happen in mobile homes," he said. "If you're living in a mobile home during a tornado warning, it's already too late. That's a major vulnerability."
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Tornadoes can develop at any time with the right atmospheric ingredients, but they are typically concentrated within a season that lasts from March through June. Increasingly, however, such storms are spinning up earlier in the year, in months not usually associated with strong tornado activity.
The first three weeks of 2023 have already had some of the highest numbers of reported tornadoes for this time of year, Gensini said.
"We're well above average in terms of the number of tornadoes that we would expect for the month of January," he said.
A major reason for the stormy start to the year is a naturally occurring climate pattern known as La Niña, in which cooler-than-average water in the Pacific Ocean influences weather systems around the world.
Both La Niña and its warmer counterpart, El Niño, affect the positioning of the jet stream, which is powered by temperature differences between the cooler polar region to the north and warmer air masses to the south. Scientists think changes in the jet stream may help explain the uptick in storms — with increasing atmospheric instability possibly affecting areas under the flow of fast-moving air — but it's still an area of active research.
"The jury is still kind of out on that, but that's kind of a preliminary hypothesis that our group is trying to start testing," Gensini said.
Global warming is also likely to be a factor, experts said, although detecting the specific effects of climate change on tornadoes has been more challenging than with other extreme weather events, such as hurricanes.
Part of the difficulty arises because studies have suggested competing effects, said Christopher Weiss, a professor of atmospheric science at Texas Tech University.
As the world warms, atmospheric instability will increase, creating conditions more favorable for storms to develop. Yet, at the same time, strong wind shear, where winds increase and rapidly change direction at different atmospheric heights, is forecast to decrease as a result of climate change. Weaker wind shear could reduce the amount of warm, rising air in storms and therefore suppress the formation of tornadoes. Scientists are still trying to understand what these seemingly conflicting outcomes could mean for tornadoes, but Weiss said studies have suggested that a warmer climate will produce more storms.
"Even if wind shear remains fairly steady, we're increasing instability on top of that, so we can expect to see more in the way of tornado occurrence," he said.
Next month, Weiss and his colleagues will embark on a three-month research project in the Southeast focused on studying storms in the region and how to enhance predictions of when they form. The research, part of the National Oceanic and Atmospheric Administration's VORTEX program, will also examine societal aspects, including available shelters and how to communicate threats to communities at risk, Weiss said.
"It's a really holistic approach to the tornado threat in that section of the country, so hopefully we can make some impact," he said.
The project may be well-timed, with the year already off to an active start.
La Niña conditions will most likely continue to fuel atmospheric instability, which could mean more tornadoes ahead, Gensini said.
"It's pretty rare to see an active January and February that all of a sudden goes dull in April and May," he said. "When you look at tornado statistics, if it starts early in January and February, it'll typically just keep going."