FILE - This April 30, 2017, file photo, provided by the Warren County Emergency Management Agency shows what looks like a tornado that approached Vicksburg, Miss. A new study finds that tornado activity... (Associated Press) 
 
 WASHINGTON (AP) — Over the past few decades tornadoes have been shifting — decreasing in Oklahoma, Texas and Kansas but spinning up more in states along the Mississippi River and farther east, a new study shows. Scientists aren't quite certain why. 
 
 Tornado activity is increasing most in Mississippi, Arkansas, Tennessee, Louisiana, Alabama, Kentucky, Missouri, Illinois, Indiana, Wisconsin, Iowa and parts of Ohio and Michigan, according to a study in Wednesday's journal Climate and Atmospheric Science. There has been a slight decrease in the Great Plains, with the biggest drop 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 bigger and home to more people, said study lead author Victor Gensini, a professor of atmospheric sciences at Northern Illinois University. Also more people live in vulnerable mobile homes and tornadoes are more likely to happen at night in those places, he said. 
 
 Even though Texas, Kansas and Oklahoma get many more tornadoes, the four deadliest states for tornadoes 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, especially in the past and in less populous areas, scientists don't like to study trends by using counts of tornadoes. Gensini and tornado scientist Harold Brooks of the National Severe Storms Lab looked at "significant tornado parameters," a measurement of the key ingredients of tornado conditions. It looks at differences between wind speed and direction at different altitudes, how unstable the air is and humidity. The more of those three ingredients, the more likely tornadoes will form. 
 
 The increases in this measurement mirrored slightly smaller increases found in 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. 
 
 Overall there is a slight increase in tornado activity, but it's not too much and not nearly like what's happening in the east, Gensini said. 
 
 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 to have the type of storms that spawn tornadoes, Gensini said. Tornadoes form along the "dry line" where there are more thunderstorms because there's dry air to the west and moist air from the Gulf of Mexico 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 this is caused by global warming, just that it looks like what is expected. 
 
 Pennsylvania State University meteorology professor Paul Markowski, who wasn't part of the research, praised the study as careful and well done. 
 
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 Follow Seth Borenstein on Twitter: @borenbears . 
 
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 The Associated Press Health & Science Department receives support from the Howard Hughes Medical Institute's Department of Science Education. The AP is solely responsible for all content. ||||| Despite the aforementioned environmental caveats, annual accumulated diurnal max STP is a statistically significant covariate to annual United States tornado reports (Fig. 1). This degree of explanatory power (44% of the variance in annual tornado counts is explained by accumulated diurnal maximum STP) suggests that STP may be used as a climatological proxy for annual tornado report counts. At the monthly scale, accumulated STP exhibits a seasonal trend in explanatory power. Monthly standardized anomalies of accumulated diurnal maximum STP for January, February, March, May, and December all explain roughly half of the variance associated with the standardized anomaly of monthly United States tornado counts (Fig. 2). August displays a minimum in variance explanation (9%), and it is speculated this is a function of the mesoscale nature of late boreal summer tornado environments and larger contributions to the monthly count climatologies from tropical storms and hurricanes. Standardized anomaly regression slopes reveal that monthly accumulated STP is a conservative estimator of monthly tornado counts, and this bias is greatest in the boreal summer (Fig. 2). The null hypothesis of, “Monthly accumulated STP does not explain statistically significant amounts of variance in monthly tornado counts” was rejected at the 0.01 significance level for all months except August. Thus, the STP covariate can be applied at the monthly scale, but caution should be used due to the seasonal changes in the coefficient of determination and level of statistical significance. 
 
 Fig. 1 Scatter plot of United States annual tornado counts vs. United States annual accumulated significant tornado parameter (1979–2017) from NARR Full size image 
 
 Fig. 2 1979–2017 Unites States monthly tornado reports and accumulated diurnal maximum STP (expressed here as a standardized anomaly by month) from NARR. Regression r2 and p values are listed next to their respective months Full size image 
 
 Examining annual tornado report counts and accumulated annual STP through time confirm this explanatory relationship and give further confidence in the use of STP as a climatological tornado report proxy (Fig. 3). Both tornado counts and STP indicate a slight positive trend through time over the analysis period, which starts at the beginning of the modern Geostationary Operational Environmental Satellite (GOES) era. Consistent tornado reporting data exists back to the early 1950s, but there are currently no datasets that allow for the space and time scales necessary to adequately retroactively compute STP back to 1950, and hence, we are limited here to the analysis period 1979–2017. Most of the increase in tornado reporting is observed from the late 1980s to the early 2000s, with a steady or even declining trend thereafter which is consistent with previous research.3 
 
 Fig. 3 Interannual variability of United States tornado reports (blue; axis left) and the United States annual sum of daily max significant tornado parameter (red; axis right) from NARR Full size image 
 
 Of greatest interest here is the potential for shifts in the spatial location of tornado frequency. Research has identified evidence of a “Dixie Alley,” which represents an eastward extension of the traditional "Tornado Alley" in the central Great Plains.26 However, this could be due to the techniques used to smooth reports in space and time.27 An eastward shift in tornado frequency was also suggested, but results were limited to two arbitrary regions, coarse spatial environmental covariates, and lacked a robust environmental tornado report proxy such as STP.12,13 Using Theil-Sen slope analysis—a robust linear trend estimator which is insensitive to outliers—and Kendall’s τ test for statistical significance, we show a robust downward trend in the annual accumulation of STP across the central and southern Great Plains. Meanwhile, a robust upward trend is found in portions of the Southeast, Midwest, and Northeast (Fig. 4). No significant increase (decrease) in tornado environments is observed west (east) of the 95th meridian. We believe these trends in tornado environments are significant and have not been documented with this level of detail by previous research. 
 
 Fig. 4 Theil-Sen slope analysis of 1979–2017 annual grid-point sum of daily max STP from NARR. p values are hatched at values ≤ 0.05 significance using Kendall’s τ statistic. Slope units are sum of daily max STP per year Full size image 
 
 Given these environmental tendencies, one would also suspect to find such evidence of trends in tornado reports given the explanatory power of the STP covariate. Indeed, significant trends in the spatial frequency of annual tornado reports are found over the analysis period (Fig. 5). Both tornado reports and environments indicate significant decreasing trends in frequency over portions of Texas, Oklahoma, and northeast Colorado. Agreement in the sign of the Theil-Sen slope is also noted for significant increasing trends in portions of Mississippi, Alabama, Arkansas, Missouri, Illinois, Indiana, Tennessee, and Kentucky. We are most confident about the trends in these areas, where both tornado reports and environments share the same significant sign of Theil-Sen’s slope. Confidence in the trends of tornado frequency are reduced in areas where the sign of Theil-Sen’s slope is opposite or do not share statistical significance. For example, significant increases in tornado reporting trends are found across Kansas, but environments indicate a neutral or downward trend. Additionally, tornado environments indicate a robust increasing trend in portions of Arkansas and Louisiana that are not supported by tornado reporting trends. We cannot offer a physical reason for these discrepancies at the current time, but they may be a function of non-continuous regional/seasonal efficiencies of the STP covariate (e.g., biases shown in Fig. 2) and biases in the tornado report database. We speculate the non-continuous regional/seasonal efficiencies of STP are a function of convective initiation and forcing for ascent. In addition, STP was designed with the intention of evaluating environments favorable for right-moving supercells in the Great Plains of the U.S. and not necessarily non-mesocyclone tornadoes.15,28 However, results associated with reports herein are generally consistent with an increasing trend found in the Southeast United States due to a greater propensity for tornado outbreaks east of the Mississippi River.29,30 
 
 Fig. 5 Theil-Sen slope analysis of 1979–2017 annual gridded tornado reports. p values are hatched at values ≤ 0.05 significance using Kendall’s τ statistic. Slope units are reports per year * 10−1 Full size image 
 
 Further examination reveals that spatial trends in tornado environment frequency are modulated by seasons (Fig. 6). Positive trends in tornado environment frequency are shown to progress northward through the annual cycle. Most of the decreasing trends across Texas are due to the months of March, April, and May (Fig. 6b), and most of the Great Plains has shown a decreasing trend in tornado environments during June, July, and August (Fig. 6c). It should be noted that our results do not indicate that maximum United States tornado environment and report frequency is no longer in the Great Plains. Rather, we demonstrate there has been a robust downward trend of tornado environments and reports in portions of the Great Plains concurrent with a simultaneous robust increasing trend in areas surrounding the Mississippi River Valley and the Midwest.
– Over the past few decades, tornadoes have been shifting—decreasing in Oklahoma, Texas, and Kansas, but spinning up more in states along the Mississippi River and farther east, a new study shows. Scientists aren't quite certain why, the AP notes. The study, which looked at changes since 1979, found everywhere east of the Mississippi, except the west coast of Florida, is seeing some increase in tornado activity. Tornado activity is increasing most in Mississippi, Arkansas, Tennessee, Louisiana, Alabama, Kentucky, Missouri, Illinois, Indiana, Wisconsin, Iowa, and parts of Ohio and Michigan, per a study in the Climate and Atmospheric Science journal. The four deadliest states for tornadoes are Alabama, Missouri, Tennessee, and Arkansas, per the NOAA. There's been a slight dip in the Great Plains, with the biggest drop in eastern and Central Texas. Even with the decline, Texas still gets the most tornadoes. The shift may be deadly, as the area with increasing tornado activity is bigger and home to more people, per study lead author Victor Gensini. He adds that in those places, more people live in vulnerable mobile homes, and tornadoes happen more at night. Gensini and tornado scientist Harold Brooks examined "significant tornado parameters," a measurement of the key ingredients of tornado conditions that looks at differences between wind speed and direction at different altitudes, how unstable the air is, and humidity. The more of those three ingredients, the more likely tornadoes will form. Overall, there's a slight increase in tornado activity, but it's not nearly like what's happening in the east, Gensini says. Why is this happening? "We don't know," he says, though "this is super-consistent with climate change." (How RV parks are affected.)