On the Significance of Multiple Consecutive Days of Tornado Activity

Motivated by the temporal behavior of recent high-end tornado events, a 30-yr historical record of tornadoes in the United States is examined for multiple-day periods of tornado activity. Comprising the 3129 tornado days during 1983–2012 are 1406 unique, nonoverlapping periods. Only 24% of these periods have lengths of 3 or more days. However, the conditional probability of such a multiday period given an outbreak day (OB; one with 20 or more tornado reports) is 74%, and given a significant tornado day [SIGTOR; one rated Fujita/enhanced Fujita (F/EF) ≥ 3] is 60%. Alternative ways of expressing these conditional probabilities all lead to the conclusion that SIGTORs and/or OBs are more likely to be contained within multiday periods of tornadoes than within 1–2-day periods. Two additional conclusions are offered: 1) SIGTORs and OBs have a relatively higher likelihood of occurrence during the latter half of the multiday periods, and 2) multiday periods have a relatively higher likelihood of occurrence during the warm months of April–July. A hypothesized connection, illustrated using reanalysis data from 2013, is proposed between such behaviors and the characteristics of the larger-scale meteorological forcing. Some speculations are made about possible relationships between multiday periods of tornado activity and convective feedbacks, extended predictability, and modes of internal climate variability.