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Dustin Shaw raises debris as he searches what is left of his sister's house in the Parkwood Meadows neighborhood after a tornado in Vilonia, Arkansas. (Danny Johnston / AP)
The frequency of tornadoes has increased in the eastern third of the United States and especially in the south-central, according to a new study in the journal Climate and Atmospheric Science.
While tornadoes have increased in the east, tornado activity has declined significantly in much of the southern plains of Texas and Oklahoma, as well as in the Colorado High Plains. These focal areas are part of a broad downward trend across the plains – historically known as the tornado alley.
According to the study, there are problematic changes, given the trend of moving an increased tornado activity in a particularly vulnerable region. The frequency of tornadoes has increased most dramatically in a region including Mississippi, Alabama, Arkansas, Missouri, Illinois, Indiana, Tennessee and Kentucky.
This was already on average the most deadly terrain for tornadoes in the country, considering multiple factors, such as a higher population density than the more western areas. There are also more trees and less visibility, weaker dwellings and a tendency to develop fast storms that cause nocturnal tornadoes, which are the most deadly.
Change in frequency of tornadoes from 1979 to 2017, by calculating potential for significant tornadoes. The red increases, the blue decreases. (Vittorio Gensini and Harold Brooks in Nature)
The authors also highlight the increased variability in the number of tornadoes from one year to the next.
The 2010s have demonstrated: 2010 was active, then 2011 was the year of the "super epidemic" among many tornadoes. Since then, the majority of tornado years have been relatively calm. This is the theme of 2018, as it threatens to become the first in modern history without recorded tornado rated EF4 or higher (on the 0-5 scale).
The study was written by Vittorio Gensini and Harold Brooks, authoritative personalities on severe weather and tornadoes. Gensini is a professor of atmospheric science at Northern Illinois University, and Brooks is a scientist at the Severe Storms National Laboratory of the Federal Government in Oklahoma.
For several years, ideas about the changing frequency of tornadoes – a nudge east of the hottest traditional sites on the Plains – have circulated, but few reliable data have supported them.
"[W]We believe that these trends in tornado environments are significant and have not been documented with this level of detail by previous research, "write Gensini and Brooks in the study.
The authors used a metric known as the Important Tornado Parameter (STP) to approximate the activity of tornadoes during the years of their study (1979-2017). This is a complex index of Twister ingredients that measures the potential power of a tornado environment.
"[We found] a strong downward trend in the annual accumulation of PTS in the central and southern Great Plains, "says the study. "Meanwhile, a strong upward trend is found in parts of the southeast, west-central and northeast."
Changes in the environment of tornadoes from 1979 to 2017, by seasonal period of three months. DJF = December to February, and so on. The yellows and oranges increase, the blues diminish. (Vittorio Gensini and Harold Brooks in Nature)
Gensini and Brooks compare the results obtained with STP to the actual tornado reports to validate their results. There are many similarities plus some conflicts. STP has an advantage over actual reports, as it is unaffected by the increase in the number of meteorologists and better detection capabilities that have resulted in an increase in the number of low tornadoes observed in recent decades.
The study found that in the majority of sites, the agreement between STP environments and reports was solid. The south-central, in particular, recorded a significant increase in STP environments and tornado confirmations. They have less confidence in certain areas, like eastern Kansas, for example. It's a bit of a battlefield between growing activity in the east and shrinking west. In this area, reports have increased, but the climate favorable to twisters has deteriorated in recent years.
With climate change underway in the background, the question of the relationship between changes in the activity of tornadoes is obvious.
The answer to the question on climate change remains to be determined, says the study. An inherent difficulty in assessing the possible links between climate change and tornadoes is that we are just beginning to understand unstable tornado behavior and patterns.
Although there are fundamental truths, such as the frequency of tornadoes tends to cluster in the spring (although this might change), it is difficult to detect signals in the data. Swarms of tornadoes tend to come in gusts. A few big days or a big week can help make a month or even a year an active month. These active stretches are often followed by seasonal and multi-year breaks.
Keeping these facts in mind, previous studies have shown that the frequency of severe weather events in parts of the Midwest and Southeast is expected to increase by the end of the 21st century as the climate warms. This is generally the same area where Gensini and Brooks have identified the recent tornado increase.
Gensini told the Associated Press that "we do not know" the cause of the observed changes, "
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