Just How Violent are the Tornadoes this Year?

The tornado death toll this year has been unprecedented, the greatest since 1953. So the public and the media have been asking, are the tornadoes simply more violent this year? This analysis should shed some light on the issue.

Update (6/1/11): One of the tornadoes from May 24th has been upgraded to an EF5, and this post has been edited accordingly. Further updates are possible after evaluation of today’s MA tornadoes.

For this (E)F3 and greater tornadoes will be considered “violent”. The traditional (E)F4/5 criteria have great variability due to the evolution of damage surveys, and the inclusion of engineering considerations on damage rankings, so (E)F3 tornadoes have been added to reduce the variability. (E)F0-1 tornadoes were underreported until the 70’s, so the violent tornado count was used in place of the total tornado count. (E)F2 tornadoes were not deemed to be sufficiently violent to be included in the analysis.

The following data were compiled from the Storm Prediction Center’s Annual Tornado Maps and then plotted on Excel. The number of (E)F3+ tornadoes is shown above each data point; a 7-year moving average plot is also shown. 2011 data is from Wikipedia. 2010-2011 data are preliminary and subject to change. Specific dates cited are 12Z-12Z (SPC) days, chart years run from midnight-midnight EST.

Figure 1: (E)F3+ tornadoes from 1950-2010; 2011 preliminary data to May 31 is also included.

The average and standard deviation from 1950-2010, as shown in the chart, are 45.7 and 21.4 (E)F3+ tornadoes, respectively. Despite the inclusion of (E)F3 tornadoes, the data still exhibits high uncertainty and oscillation before the 1970s. A more steady average and standard deviation calculation between 1975-2010, after Fujita had devised and tested his scale, yields 38.1 and 14.3 tornadoes, respectively. This more recent count excludes the anomalous Palm Sunday (1965) and Super Outbreak (1974) events.

From the data, 2011 already appears to be the most violent year since 1974. In the first five months, the 73 EF3+ tornadoes has been the highest since 1974, and has already ranked fifth among years since 1950, after 1974 (131), 1961/1957 (98), 1973 (85), and 1971 (82). The five EF5s (not shown) is ranked second, after 1974 (7), and tied with 1953; the 17 EF4s (also not shown) is ranked fifth, after 1965 (30), 1974 (29), 1957 (23), and 1952 (18). The violent tornado count to date in 2011 is 1.3 standard deviations above the annual 1950-2010 mean, and nearly 2.5 standard deviations above the 1975-2010 mean. 30 of the EF3+ tornadoes were spawned on one day, April 27th, which by itself is a large fraction of the mean. However, even excluding April 27th, the violent tornado count from January-May is running above the total annual mean.

In summary, this tornado season has indeed been a very violent one – the most since 1974 by many metrics, and highly ranked even considering the pre-Doppler radar era, when tornadoes were more commonly given a higher rating. It should be no surprise that given the higher population exposed to these violent tornadoes, more fatalities have been occurring this year than has been observed since the 1950s.

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