Historic April 27, 2011 Tornado Outbreak Affects Much of the U.S. | 4.28.2011

The catastropic severe weather events that occurred yesterday, April 27, 2011, are a harsh reminder of the power of mother nature and will be remembered for generations to come. Although much information regarding the number of tornadoes, intensities, injuries, and deaths is still being documented, preliminary information (as of April 28, 2011) suggest that this particular tornado outbreak will rank among the nation’s top historic tornado outbreaks.

Looking back at major tornado events - particularly those with violent tornadoes (tornadoes with intensities of F4 and greater) - is a helpful way to establish some context from which April 27, 2011 will be compared. Below is a graphic showing how many violent tornadoes (F4s and F5s) have occurred across the lower 48 states each year from 1950 to 2010. The information is taken from the Storm Prediction Center’s experimental Severe Weather Database (available at During this 60 year period, approximately 10 violent tornadoes have occurred on average each year. Years with much higher totals are pretty easy to spot - these highlight years with major violent tornado oubreaks such as 1974 (the year of the super outbreak of April 3-4), 1965 (April 11 outbreak), and 1952 (March 21 outbreak). Historically speaking, it is also interesting to note that it has been more than 30 years since the U.S. had a year with more than 15 violent tornadoes. It will be interesting to see how 2011 compares, particularly in light of yesterday’s outbreak combined with an extremely active month of April.

From the standpoint of when violent tornadoes have most typically occurred in the U.S., the following graphic can help provide some perspective. The graph below shows the number of violent tornadoes (F4 and F5) that have occurred on each calendar day throughout 1950-2010. The thin blue bars are the daily total number of violent tornadoes, while the dark black line is a 2 week average that helps to reveal the overall trend throughout the months of the year. The analysis is fairly sensitive to outbreaks, which are apparent as individual days with high blue bars. What this image really tells us is that outbreaks of violent tornadoes in the past have occurred most commonly from late March through early May. Late May into early June is also active, however, there are fewer days with many violent tornadoes. In terms of when these events occur, the April 27, 2011 event fits right within the window of time that past historic outbreaks have occurred.

Only time will tell how the April 27, 2011 event stacks up to other historic tornado outbreaks. Early reports of the number of lives lost are difficult to contemplate in an era where warning information and communications have continued to improve. Continuing to understand how to understand these events and how to better prepare for them are a high priority for our research program at the Southern Climate Impacts Planning Program.