A Closer Look At Significant Tornado Days, 1875-2011

In the previous post, I posted a monthly breakdown of significant tornadoes and tornado days from 1875-2011.  Now I will take a closer look at significant tornado days during the same time period, including our most prolific outbreaks.

As mentioned before, a significant tornado is defined as any tornado of at least F2/EF2 strength.  A significant tornado day need only have one tornado of that intensity, but as we will see, not all significant tornado days are equal.  In our history, significant tornado days have produced anywhere from 1 to 20 significant tornadoes.

Using Grazulis data for 1875-1949 and NOAA data for 1950-2011, there have been a total of 602 F2/EF2+ tornadoes on 332 days.  226 of 332 days produced only one significant tornado and 46 days produced only two significant tornadoes, which means that 272 of 332 days produced just one or two significant tornadoes.  Recurrence rates for various numbers of significant tornadoes are below.


332 days with at least 1 significant tornado..........1 day per 0.4 years or 2.4 days per year
106 days with at least 2 significant tornadoes.......1 day every 1.3 years
60 days with at least 3 significant tornadoes.........1 day every 2.3 years
34 days with at least 4 significant tornadoes.........1 day every 4.0 years
20 days with at least 5 significant tornadoes.........1 day every 6.9 years
16 days with at least 6 significant tornadoes.........1 day every 8.6 years
7 days with at least 7 significant tornadoes...........1 day every 19.6 years


Looking at the recent era from 1980-2011, there have been 139 F2/EF2+ tornadoes on 68 days per NOAA records.  43 of 68 days produced only one significant tornado and 15 days produced only two significant tornadoes, which means that 58 of 68 days produced just one or two significant tornadoes.  Recurrence rates for various numbers of significant tornadoes are below.


68 days with at least 1 significant tornado..........1 day per 0.5 years or 2.1 days per year
25 days with at least 2 significant tornadoes.......1 day every 1.3 years
10 days with at least 3 significant tornadoes.......1 day every 3.2 years
10 days with at least 4 significant tornadoes.......1 day every 3.2 years
5 days with at least 5 significant tornadoes.........1 day every 6.4 years
4 days with at least 6 significant tornadoes.........1 day every 8.0 years
3 days with at least 7 significant tornadoes.........1 day every 10.7 years


Using Grazulis ratings for 1875-1949 and NOAA for 1950-2011, here are the top 20 significant tornado days and the number of significant tornadoes they produced.  A list of all days with 2 or more F2/EF2+ tornadoes is below that.


June 2, 1990:  20
April 3, 1974:  17
April 11, 1965:  10
November 22, 1992:   10
March 23, 1917:  7
March 28, 1920:  7
March 10, 1986:  7
June 14, 1880:  6
May 12, 1886:  6
April 17, 1922:  6
March 6, 1956:  6
April 3, 1956:  6
November 26, 1965:  6
October 24 1967:  6
March 12, 1976:  6
May 25, 2011:  6
February 25, 1956:  5
March 6, 1961:  5
April 30, 1962:  5
May 30, 2004:  5