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

Author:

Verbout Stephanie M.1,Brooks Harold E.2,Leslie Lance M.1,Schultz David M.3

Affiliation:

1. School of Meteorology, University of Oklahoma, Norman, Oklahoma

2. NOAA/National Severe Storms Laboratory, Norman, Oklahoma

3. NOAA/National Severe Storms Laboratory, and Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma

Abstract

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.

Publisher

American Meteorological Society

Subject

Atmospheric Science

Reference22 articles.

1. Some aspects of the international climatology of tornadoes by damage classification.;Brooks;Atmos. Res.,2001

2. Brooks, H. E., and J. P.Craven, 2002: A database of proximity soundings for significant severe thunderstorms, 1957–1993. Preprints, 21st Conf. on Severe Local Storms, San Antonio, TX, Amer. Meteor. Soc., 639–642.

3. Deaths in the 3 May 1999 Oklahoma City tornado from a historical perspective.;Brooks;Wea. Forecasting,2002

4. Climatological estimates of local daily tornado probability for the United States.;Brooks;Wea. Forecasting,2003

5. Bruening, S. L., M. P.Kay, and H. E.Brooks, 2002: A new perspective on the climatology of tornadoes in the United States. Preprints, 16th Conf. on Probability and Statistics, Orlando, FL, Amer. Meteor. Soc., J96–J103.

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