An Analysis of Clustered Tornado Events

While any tornado event is dangerous, days with widespread tornado activity over a large area are particularly threatening to life and property. In this study, the subject of tornado “clusters” is investigated. For our purposes, such clusters are defined in the context of SPC probabilistic convective outlooks, which are valid over the “convective day” (12 UTC – 12 UTC) and specify the probability of an event occurring within 25 miles (40 km) of a point. Any tornado probability area of 30% or greater defines a “High Risk” in terms of SPC categorical convective outlooks. A tornado cluster is identified if a polygon of comparable size to a typical outlook can be delineated, for any given convective day, that has >30% areal tornado coverage (within 40km of a point).

Once identified, these tornado clusters are analyzed, both in terms of a long-term climatology (1950-2009) using the Storm Data tornado database, and in terms of a shorter period (2003-2009). The latter corresponds to the time period of the SPC storm environment database, which includes an hourly archive of objectively analyzed convective parameters such as CAPE, bulk shear, storm-relative helicity, LCL height, and other fields. Tornado clusters which meet the criteria defined above are rare, on average occurring less than 10 days a year, but they account for a much larger percentage of tornado events and fatalities. Spatial and temporal trends in the long-term record of these tornado clusters will be discussed, along with an analysis of the convective environments associated with the more recent events.