Using the Destruction Potential Index (DPI) to compare tornado outbreaks in 1998 and 1999

Often the "significance" of a tornado outbreak is largely determined by the number of people killed and injured. While human casualties are of obvious sociological importance, these statistics alone do not necessarily reflect the meteorological significance of a tornado outbreak. Thompson and Vescio (1998) developed the Destruction Potential Index (DPI) to help compare tornado episodes from a meteorological perspective. Quite simply, the potential for destruction by an individual tornado is a function of the tornado's intensity and path area. Thus, the DPI for a tornado outbreak is defined as:

_n

S Ai(Fi+1)

i=1

where n is the number of tornadoes, A the tornado path area (path length multiplied by mean path width) in square miles, and F is the maximum F-scale rating for each tornado. The F-scale plus one is used so that F0 tornadoes can produce non zero values of DPI. For the purpose of this study, the criteria for a tornado outbreak are the occurrence of 10 or more tornadoes in a concentrated area with at least two reaching F3 intensity. Using these criteria, 10 outbreaks occurred during the two year period studied, six in 1998 and four in1999. The most publicized events in 1998 were the east central Florida tornadoes on 22-23 February, the Birmingham area tornadoes on 8 April, and the Nashville Tennessee area tornadoes on 16 April. Although the Florida event resulted in 42 fatalities (DPI=11), it did not qualify as an outbreak for this study because less than 10 tornadoes occurred. Further, the Birmingham Outbreak killed 41 people (DPI=69), but it can be argued that the Nashville outbreak was much more meteorologically significant (DPI=201) despite causing far fewer fatalities (only 5). In 1999, the most significant outbreak by any measure was the Oklahoma City area tornadoes on 3 May. Although this outbreak was confined to Oklahoma and extreme southern Kansas, preliminary indications (tornado data for this episode have yet to be finalized), are that the DPI for this event exceeded 600, which is comparable to the more widespread outbreaks of 26 April 1991 (Andover Kansas; DPI=644) and 13 March 1990 (Heston Kansas; DPI=692). Additional results from these outbreaks and the others in 1998 and 1999 will be discussed in the paper.