Given that “tornado alley” doesn’t have a strict geographical definition, the authors looked at Nebraska, Kansas, Oklahoma, and northern Texas. In these locations, the peak of tornado season was around May 26 in the 1950s. By the early 2000s, it had shifted to May 19.
That seven day shift was found by looking at tornadoes of all intensity. However, by removing F0 tornadoes, the least damaging tornadoes on the Fujita scale, that shift increases to 14 days earlier.
While it’s clear that tornadoes are occurring earlier now than they were in the mid-20th century, it’s not readily apparent what is causing the shift. Given that good tornado records only date back to 1950, it is inherently difficult to do an in-depth analysis of changing trends. There are also regional influences at play — topography and land use.
However, Greg Carbin, the warning coordination meteorologist at the Storm Prediction Center, notes that it’s possible a warming climate is behind some of the changes, due to the influence of seasonal temperature changes on the jet stream. “If winters are not as cold, or if spring times are warmer, the location of the jet stream is most likely displaced north of where it has been in the past,” said Carbin, who was not involved in either study.
In addition to potential changes in the jet stream, researchers on the study found a link between El Niño events and tornado activity in Oklahoma. “The relationship we do see in Oklahoma is a light but significant connection to El Niño,” said Paul Stoy, a professor in the Department of Land Resources and Environmental Sciences at Montana State University, and co-author of the study. “This makes one suspect that if global climate change is changing these larger circulations, then there is a connection between a global [variability] and tornado activity.”
Not only is the tornado season shifting, but year-to-year swings in tornado activity are growing. A study released this week concludes that the volatility of tornado reports has increased since 2000 — changes which cannot simply be explained by reporting practices.
Michael Tippett, a researcher of applied physics and mathematics at Columbia University, and author of the study, defines tornado volatility as the change in tornado reports from year to year. Recent years have proven how extreme these swings can be. In 2011, a notoriously active year for severe weather, nearly 1,900 tornadoes were reported across the U.S., and 1,700 of those were confirmed. Then in 2012, 937 tornadoes were confirmed.
Throughout the 20th century, changes in tornado volatility could be easily explained by changes in reporting practices. As population grew, especially in rural areas, tornado reports increased. The use of Doppler radar and the popularity of storm chasing also played an obvious role.
However, the volatility after the year 2000 can not be explained so easily, and those changes were dramatic. Tippett found that between the periods 1980-2004 and 2005-2013, the volatility of tornadoes doubles.
Digging deeper, Tippett found that the volatility in the physical nature of the atmosphere coincides with a large portion of the tornado volatility in recent years. Unsurprisingly, fluctuations in storm relative helicity, which is the vertical spin in the atmosphere that is necessary for tornadoes to form, as well as general thunderstorm precipitation, appear to be well-linked to tornado volatility changes.
While Tippett stops short of connecting these results to climate-related changes in the atmosphere, recent studies have drawn a link between global warming and increase in environments favorable for severe weather. In addition, a study published in early August found that days with multiple tornadoes have increased over the last 60 years.
Thus, while climate change is expected to have wide-reaching impacts on extreme weather, how exactly tornado activity is changing as a result is still a subject of investigation.