The Tornado Climatology of the NWS St. Louis
County Warning Area

Click >>HERE<< to find out more about how this study was created and its limitations.  Also includes acknowledgements and references.

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Click >>HERE<< to see a list of significant tornado outbreaks in our area.

Click >>HERE<< to see a list of violent tornadoes that have occurred in our area.

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Yearly Averages for the St. Louis CWA (1950-2004):

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Yearly Distribution:

During the 55 year period from 1950 through 2004, a total of 655 tornadoes (all intensities) were identified in the LSX CWA (Fig 2a, below). Significant tornadoes accounted for 173 of the tornadoes (or 26% of the total), resulting in an average 3.20 strong or violent tornadoes per year. Significant tornadoes have occurred in the LSX CWA in 8 out of every 10 years (Figure 2c, below)).   Two days with large outbreaks across the CWA (May 21 and December 18) makes 1957 the year with the most significant tornadoes (12). The database indicates there has been an increase in all tornadoes, but a gradual decline in the number of significant tornadoes in the past 55 years.   The pre-1950 statistics (Grazulis, 1993) are presented in Fig. 2b for comparison. 

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Yearly Distribution of Tornado Days:

The LSX CWA has experienced 312 tornado days (calendar day with an occurrence of at least one tornado) and 110 significant tornado days from 1950-2003, an average of 2.00 days per year (Figure 3c, below). The number of significant days has also shown a gradual decline. The pre-1950 statistics (Grazulis, 1993) are presented in Fig. 3b for comparison.  

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Monthly Distribution:

When the monthly distribution of all tornadoes is shown in figure 4a. for 1950-2004. While the four-month period from March-June contains the greatest percentage of significant tornadoes (69%; 116 out of 173) and of all tornadoes (69%; 445 out of 655), only 26% of all tornadoes which occur during this period are significant (116 out of 445). In contrast, the four-month period from November-February contains only 21% of the significant tornadoes (36 out of 173) and only 13% of all tornadoes (83 out of 655), however 43% of all tornadoes which occur during this period are significant (36 out of 83). Thus for any tornado occurrence, the greatest probability of it being significant, is during the late autumn and winter.  The pre-1950 statistics (Grazulis, 1993) are presented in Fig. 4b for comparison. 

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Hourly Distribution:

Tornadoes are mainly a late afternoon and evening phenomena in the LSX CWA (Figure 5a and c, below), with 70% (120 out of 173) of the significant tornadoes occurring between 300 and 1100 pm CST. Only 5% (8 out of 173) occurred between 300 and 1100 am.  The pre 1950 statistics (Grazulis, 1993) are presented in Fig. 5b for comparison. 

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Monthly vs. Hourly Distribution:

A scatter plot showing the date/time distribution of all tornadoes yields some additional interesting results (Figure 6, below). Tornadoes that occur in the late afternoon and evening hours (300 to 1100 pm CST) during the spring (March through June) account for 46% of all tornadoes (307 out of 655) and 51% of significant tornado occurrences (89 out of 173). Occurrences outside this window tend to be spread out. What is particularly disturbing is only 4 of the 10 violent tornadoes (F-4) are in this cluster. Five of the remaining violent tornadoes occurred in the winter months (from four separate severe thunderstorm events) during the late evening and overnight hours. The most notable event struck the city of St. Louis at 140 am on February 10, 1959. This violent tornado resulted in 21 fatalities and 345 injuries.

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Last Updated: March 16, 2005