A tornado outbreak climatology was established from 1953 to 2002 over the continental United States with special attention paid to tornadoes associated with landfalling hurricanes. The climatology was built upon Galway’s (1977) definition of a tornadic outbreak which states that an event is considered an “outbreak” if the event consisted of 10 or more tornadoes associated with the same synoptic system. A new definition of a tornadic outbreak was constructed from the climatology data set as follows: in one convective day, 1200 UTC to 1200 UTC, a minimum number of tornadoes needed to occur associated with the same synoptic scale system; the minimum number of tornadoes for each year was determined by taking 3.0 % of a linear regression performed on the fifty year tornado data set; the linear regression equation is given by the minimum number of tornadoes = 15.266 * (year) – 29364.43; and each potential “outbreak” day must include at least two F2 or greater tornadoes to be considered significant. The new definition accounted for the average increase in the number of tornadoes reported each year to establish the minimum threshold of tornadoes in an outbreak. In addition, the new definition included a classification for the intensity of the tornadoes.

Turning to tornado outbreaks embedded in landfalling hurricanes, the definition must be slightly altered to account for the shallow mesocyclones that are present in tornadic hurricane conditions. As a result, hurricane tornadoes tended to be weaker, and the intensity threshold must be lowered. Furthermore, landfalling hurricanes can persist for several days, so the convective day restraint needed to be extended to differentiate outbreak occurrences in hurricane environments. Finally, three case studies that met the new definition of an outbreak are compared against several non-hurricane related tornado outbreaks in the U.S. The storms examined were Hurricane Gilbert (1988), Hurricane Danny (1985), and Hurricane Andrew (1992).