NWS tornado warnings with zero or negative lead times

During a five-year period of study from 2000 to 2004, slightly more than 10% of all National Weather Service (NWS) tornado warnings were issued either simultaneously as the tornado forms (i.e., with zero lead time) or minutes after initial tornado formation but prior to tornado dissipation (i.e., with “negative” lead time). This study examines why these tornadoes were not warned in advance, and what climate, storm morphology, and sociological factors may have played a role in delaying the issuance of the warning. This data set of zero and negative lead time warnings are sorted by F-scale, geographically by region and weather forecast office (WFO), hour of the day, month of the year, tornado-to-radar distance, county population, and number of tornadoes by day, hour, and order of occurrence. Results show that the more isolated the tornado event, the less likelihood that an advance warning is provided. WFOs that experience many large-scale outbreaks have a fewer proportion of warnings with negative lead time than WFOs that experience many more isolated, one-tornado or two-tornado warning days. Monthly and geographic trends in lead time are directly impacted by the number of multiple tornado events. The impact of tornado-to-radar distance and storm morphology on tornado warning lead time was inconclusive from this particular data set and warrants further study.