Annual and seasonal tornado trends in the contiguous United States and its regions

Abstract

The numbers of E(F)1+ tornadoes per year generally declined in all regions of the United States, except the Southeast, where they increased in frequency. The South Great Plains had the greatest median tornado frequency early in the study period whereas the Southeast had the greatest in the middle and late periods. The proportion of tornadoes occurring in the contiguous United States in summer are decreasing whereas the proportion occurring in fall are increasing. Annual tornado frequency in the (a) West, (b) North Great Plains, (c) South Great Plains, (d) Midwest, (e) Southeast, and (f) Northeast regions. The dashed line is the Theil–Sen slope estimate and the solid line is the local polynomial regression curve with a smoothing span of 0.66.

Using the standard deviational ellipse to document changes to the spatial dispersion of seasonal tornado activity in the United States

“…11,13,16,17 These changes have led to an increase in the mean and variability of the number of tornadoes per tornado outbreak and day. 14,16 Recent empirical studies also illustrate that the spatial distribution of tornadoes has shifted over time by analyzing tornado density maps, [17][18][19] regional and gridded trends, 17,[20][21][22] and trends in the center of tornado activity. 17,[23][24][25] The observed maximum of gridded tornado and tornado day counts, for example, has shifted from the Great Plains to the Southeast between the periods 1954-1983 and 1984-2013.…”

“…19 Regional and gridded annual tornado counts have similarly trended downward in portions of the Great Plains and upward throughout most of the Southeast. 17,[20][21][22] The decrease in the Great Plains is observed in all seasons, whereas the increase in the Southeast is driven mostly by recent increases in tornado activity in winter and fall. 17,22 These regional trends have led to eastward shifts in the center of annual tornado activity and in spring, summer, and fall.…”

Quantifying relationships between environmental factors and power dissipation on the most prolific days in the largest tornado “outbreaks”

“…Outbreaks are largely confined to the Southeast during the late fall and winter months (Dean, 2010). The percentage of all U.S. tornadoes occurring in clusters is on the rise Fuhrmann et al, 2014;Tippett et al, 2014;Elsner et al, 2015;Tippett et al, 2016;Moore, 2017;Moore, 2018;Moore and DeBoer, 2019). Tippett et al (2016) attributed the upward trend in tornado clusters to significant increases in storm relative helicity.…”

Evaluating tornado casualty rates in the United States

“…Recent studies find upward trends in the interannual variability and clustering of tornadoes over the past few decades (Brooks et al, 2014;Elsner et al, 2015;Tippett et al, 2015). They also find a shift in the areal extent of tornado occurrence (Agee et al, 2016;Moore, 2017), and increasing tornado power (Elsner et al, 2018b). Other research argues that an increase in total population and dispersal of the built environment will lead to an increase in the amount of tornado destruction (Strader et al, 2017).…”