Tornado Outbreak Day Sequences: historic events and climatology (1875-2003)

Extended Tornado Outbreak Day Sequences, such as the event of 3-11 May 2003, cause enormous damage and loss of life. They test the resource limits within the severe weather forecast, warning, communication, and Emergency Management communities. It is important to understand how frequently these major events occur and explore some of their defining characteristics.

Severe Weather databases from NOAA (Schaefer and Edwards, 1999) and Grazulis (1990) were used to construct a climatology of tornado outbreak sequences. An outbreak sequence is defined as a continuous or near continuous sequence of successive tornado outbreak days. A variety of definitions for an outbreak “sequence” are explored including definitions tailored to fit the recent tornado outbreak sequence from 3-11 May 2003. A recurrence frequency estimate is also made for sequences of varying length and magnitude to illustrate the relative rarity of these major events.

Results indicate that 2003 event, with 9 consecutive days characterized by more than 10 tornadoes and at least one significant (F2-F5) tornado, although rare is not unprecedented. With allowances for increased reporting of tornadoes in recent years, at least 3 other events approached or exceeded the May 2003 event in some measures of intensity and duration. In addition, the clustering of a few less intense neighboring days with unique singular tornado outbreak days such as the 1974 Super Outbreak or 1965 Palm Sunday Outbreak result in a form of outbreak sequence whose intensity dominate outbreak sequences of longer duration.

Select extraordinary historic outbreak sequences from 1917, 1930, and 1949 are also discussed to place the recent 2003 event in historical context. The intensity, duration, spatial extent and evolution of each event are briefly discussed to illustrate the general character of these historic tornado events.