Evolution of the U.S. Tornado Database: 1954–2003

Abstract

Over the last 50 yr, the number of tornadoes reported in the United States has doubled from about 600 per year in the 1950s to around 1200 in the 2000s. This doubling is likely not related to meteorological causes alone. To account for this increase a simple least squares linear regression was fitted to the annual number of tornado reports. A “big tornado day” is a single day when numerous tornadoes and/or many tornadoes exceeding a specified intensity threshold were reported anywhere in the country. By defining a big tornado day without considering the spatial distribution of the tornadoes, a big tornado day differs from previous definitions of outbreaks. To address the increase in the number of reports, the number of reports is compared to the expected number of reports in a year based on linear regression. In addition, the F1 and greater Fujita-scale record was used in determining a big tornado day because the F1 and greater series was more stationary over time as opposed to the F2 and greater series. Thresholds were applied to the data to determine the number and intensities of the tornadoes needed to be considered a big tornado day. Possible threshold values included fractions of the annual expected value associated with the linear regression and fixed numbers for the intensity criterion. Threshold values of 1.5% of the expected annual total number of tornadoes and/or at least 8 F1 and greater tornadoes identified about 18.1 big tornado days per year. Higher thresholds such as 2.5% and/or at least 15 F1 and greater tornadoes showed similar characteristics, yet identified approximately 6.2 big tornado days per year. Finally, probability distribution curves generated using kernel density estimation revealed that big tornado days were more likely to occur slightly earlier in the year and have a narrower distribution than any given tornado day.

Tornado Climatology of Finland

“…Specifically, 29% of the tornado cases from 1796-1996 were significant tornado cases, but only 4% of the tornado cases from 1997-2007 were significant. The smaller percentage of significant tornado cases in the recent dataset can be explained by the more efficient collection of tornado reports, especially for F1 and weaker tornadoes, similar to that seen in the United States (Brooks and Doswell 2001;Verbout et al 2006), and the fewer number of intense tornado cases in the shorter 1997-2007 period (Fig. 3).…”

“…Both the increased awareness and the onset of active collecting of tornado reports show in the statistics, as during 1997-2007, a mean of 14 confirmed and probable tornado cases occurred each year, with a mean of 11 tornado days. The tornado database is likely to be more consistent over time for the more intense tornadoes (e.g., Brooks and Doswell 2001;Verbout et al 2006). During 1930During -2007Hales 1988) cases occurred on average every second year, but with a slight downward trend (Fig.…”

“…2b). The annual probability of significanttornado cases was calculated during 1930-2007 because significant tornadoes are less sensitive to changing reporting practice with time (e.g., Brooks and Doswell 2001;Verbout et al 2006). The climatological probability of at least one significant tornado within an 80 km 3 80 km area during a year in several areas in centralsouthern and eastern-central Finland is 5%-7%.…”

“…3). Because stronger tornadoes typically have larger effects on society and influence larger areas (Brooks 2004), observations of the more intense tornadoes tend to be less sensitive to variations in reporting practice over time (e.g., Verbout et al 2006), explaining the large proportion of significant tornado cases in the historical dataset.…”

“…Fujita (1973 found that July and August are the peak months for tornadoes from northern Russia to Germany, France, and northern Italy, whereas, in other parts of western and southern Europe, the peak moves toward the autumn. In the United States, the peak frequency for tornadoes is in May and June (Verbout et al 2006). …”