NWC REU 2001
May 28 - August 3



A Statistical Model for Significant Tornado Hazards in the United States

Cathryn Meyer and Harold Brooks



A statistically based model of the hazard associated with significant tornadoes (F2 or greater on the Fujita scale) based on the reports recorded from 1921-1995 in the United States is developed. The model consists of four components. The first is a probability that at least one tornado will occur at a grid point (grid spacing is approximately 80 km on a side) on any day of the year at any location in the United States. The second component is a model of the number of tornadoes reported per day in a single grid box, given that at least one tornado occurs. Third, the intensity of each tornado on the Fujita scale is determined, based on the historical distribution of intensity. Finally, the path length and width of the tornado are modeled using Weibull distributions for each value of the F-scale. The model has been run for 30,000 years, with almost 4,000,000 tornadoes produced in order to develop reliable statistics. The peak areal coverage of tornadoes (at a grid point in southern Oklahoma) is about 3% of a grid box per century, implying a return period for strong and violent tornadoes at any point in that grid box of about 3000 years. The model also shows the dangers of using a short period of record to estimate tornado hazards. Differences in 15-year periods of 50% in mean tornado occurrence are observed in the model, even without changing any of the parameters. This result has significance for the assessment of risk using the 'raw' observations and for detection of changes in the climatology of tornado occurrence.

Paper available upon request.