SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

Wednesday, 19 April 2017

TORNADO STATISTICS


The United States has the largest number of tornadoes of any country in the world. More than 1100 tornadoes are reported each year on average across the country. The public has the notion that tornadoes cause more fatalities than most other weather events. However, during a recent 30-year span (1981-2010), more people have drowned in floods (an average of 92 per year) than killed by tornadoes (a yearly average of 56 fatalities). Until recently, the number killed by lightning was more, but recently, lightning deaths (an annual average of 55) are comparable with the number of tornado fatalities.

Tornado statistics have been compiled in various states for more than a century. However, statistics from the entire country are only reliable since 1950, when the predecessor to the National Weather Service's Storm Prediction Center (SPC) at Norman, OK began research into tornadoes. This center routinely posts an updated SPC Monthly Tornado Statistics list of the total number of tornadoes and tornado fatalities for the last several years. Identifying the number of tornadoes is not an easy task. Preliminary statistics (PREL) are based upon numbers that have been obtained from the Local Storm Reports submitted by the National Weather Service Offices within hours after a tornado has been reported. A list of final or "actual" statistics (ACT) concerning the number of reported tornadoes is prepared following the checking and verification of more detailed information. The final numbers may vary considerably from the preliminary list, and typically are lower since some multiple preliminary reports of the same tornado are eliminated in the final count. SPC uses a scheme to obtain an " Inflation Adjusted Annual Tornado Running Total." A killer tornado is, by definition, a tornado that has caused at least one fatality.

Inspection of the most recent list (valid through last Wednesday, 12 April 2017) reveals that during the first three and a half months of 2017, a preliminary total of 536 tornadoes had been reported across the United States. This year's totals are more than double the average number of tornadoes recorded to this time of year over the last three years. These numbers may be inflated because of multiple reports of the same tornado. If these preliminary statistics were adjusted using the SPC scheme, 474 tornadoes would have occurred through Monday (17 April), which is 232 above the 1954-2007 long-term median (50th percentile, which represents 50% above or below) values for this time of year of 241. In fact, the accompanying graphic shows the 474 only six tornadoes behind the day-to-day accumulations for the maximum year (1884),

As of last week, 27 deaths have been attributed to tornadoes so far this year from eight killer tornadoes. Over the last three years, an average of two fatalities have been recorded between January and March.

A record number of tornadoes (1817) were reported in 2004, while six years ago in 2011, a total of 1691 tornadoes was confirmed, representing the second highest annual total on record. In contrast, only 656 tornadoes were reported in 1987.

After years of relatively few tornado-related deaths, six years ago (2011) also saw the largest number of fatalities (553) since 1940. The infamous Tri-State on 18 March 1925 claimed approximately 700 lives across Missouri, Illinois and Indiana, leaving the year with a record setting 794 deaths. During the opening years of this current century, the number of tornado-related fatalities had decreased to less than 40 fatalities per year in 2004 (36 fatalities) and 2005 (38 fatalities); in 2009, the number went down as only 21 people lost their lives, which was the lowest since 1986 (15 fatalities). However, the large number of fatalities in 2011 was attributed to several noteworthy tornado outbreaks in April and May across the Southeast and Midwest that contained powerful tornadoes. Between the 25 April and 28 April 2011, as many as 322 people died in tornadoes that hit Alabama, Arkansas, Georgia, Mississippi, Tennessee and Virginia. Four of the tornadoes on the 27th were rated EF-5 on the Enhanced Fujita Scale. Nearly one month later, an EF-5 tornado in May claimed 158 lives as it destroyed a large section of Joplin in southwestern Missouri. During the last two years, 70 people died in 2012, 55 in 2013, 47 in 2014 and 36 in 2015. Additional detailed information concerning tornado activity across the United States for 2011 is available from SPC.

The National Centers for Environmental Information has provided a climatology of U.S. tornadoes (1991-2010) with maps and other graphics that show the number of tornadoes by state and other statistics. The Storm Prediction Center has an interactive website entitled Online Severe Weather Climate that includes the number of tornadoes along with other severe weather statistics within the National Weather Service's radar coverage areas during the last three decades.


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Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2017, The American Meteorological Society.