By SETH BORENSTEIN
AP Science Writer
WASHINGTON (AP) — Over the past few decades tornadoes have been shifting — becoming less common in Oklahoma, Texas and Kansas but spinning up more in Wisconsin, other states along the Mississippi River and even farther east, a new study shows. Scientists aren’t quite certain why.
The increase in tornado numbers is happening most markedly not only in Wisconsin but also Mississippi, Arkansas, Tennessee, Louisiana, Alabama, Kentucky, Missouri, Illinois, Indiana, Iowa and parts of Ohio and Michigan, according to a study in the journal Climate and Atmospheric Science on Wednesday. There has been a slight decrease in the Great Plains, the biggest drop being seen in central and eastern Texas. Even with the decline, Texas still gets the most tornadoes of any state.
The shift could be deadly because the area with increasing tornado activity is home to more people than the places where twisters are becoming less common, said the lead study author, Victor Gensini, a professor of atmospheric sciences at Northern Illinois University.
Also in the places were tornado numbers are increasing, more people live in vulnerable mobile homes and the destructive storms are more likely to happen at night, he said.
Even though Texas, Kansas and Oklahoma get many more tornadoes, the four states where tornadoes are deadliest are Alabama, Missouri, Tennessee and Arkansas, according to the National Oceanic and Atmospheric Administration.
“More folks are generally at risk because of that eastward shift,” Gensini said.
Because tornadoes sometimes go undercounted, scientists don’t like to study trends by using counts of tornadoes. Gensini and the tornado scientist Harold Brooks of the National Severe Storms Lab instead looked at “significant tornado parameters,” a measurement of tornado-fostering conditions. Their method looked at differences between wind speed and direction at different altitudes, how unstable the air is and humidity. The more those three ingredients are present, the greater the likelihood that tornadoes will form.
The increases in this measurement mirrored slightly smaller increases found in the number of twisters.
The study looked at changes since 1979. Everywhere east of the Mississippi, except the west coast of Florida, is seeing some increase in tornado activity. The biggest increase occurred in states bordering the Mississippi River.
Why is this happening?
“We don’t know,” Gensini said. “This is super consistent with climate change.”
As the Great Plains dry out, there’s less moisture, which helps give rise to the type of storms that spawn tornadoes, Gensini said.
Tornadoes form along the country’s “dry line,” an imaginary boundary between dry places to the west and wetter places to the east.
That dry line is moving east.
“This is what you would expect in a climate-change scenario, we just have no way of confirming it at the moment,” Gensini said.
Gensini said unless there are specific detailed studies, he and others cannot say the changes are being caused by global warming, just that recent developments have been expected.