Above: The F5 tornado that tore through Moore and Oklahoma City, Oklahoma, on May 3, 1999. This photo was taken near Bridge Creek, about 15 miles southwest of Moore. (Courtesy Erin Maxwell via NWS/Norman)
Editor's note: A widespread, dangerous severe weather outbreak is possible this weekend, including on Saturday night in Texas and particularly on Sunday across parts of the lower Mississippi Valley and Southeast. See the weather.com story for more details. We'll be covering this event in Cat 6 starting on Friday. Severe weather is also possible in the Northeast and in Texas/Louisiana on Thursday afternoon and evening.
Arguably, the most intense weather event that takes place on Earth is the rare occurrence of a tornado that reaches EF5 strength on the Enhanced Fujita Scale (or F5 on the original Fujita Tornado Damage Scale). Winds within these monsters have been measured as high as 302 mph by Doppler radar (Bridge Creek, Oklahoma, on May 3, 1999, pictured above) but winds likely have exceeded 300 mph on other occasions in tornadoes that were not so closely observed.
The United States and Canada are the only countries in the world to have verified reports of tornadoes with a classification of F5 or EF5 strength. The reason for this lies with North America’s unique topography. Only in North America does a solid land mass stretch from the subtropics to the arctic with no mountain barriers to inhibit the mixture of air masses from these two regions. When the conditions are just right (normally in the spring) extraordinary supercells can develop in the central portion of the country and can occasionally spin out a tornado of EF5 intensity.
The difference between F5 and EF5 tornadoes
The Fujita classification system changed to a new and more nuanced version in February 2007 when the Enhanced Fujita Tornado Damage Scale was adopted by the NWS. One impetus for this change was the discovery that winds did not have to be as strong as earlier thought to produce a given amount of damage. Another motivation was to establish a more thorough system for rating tornado strength. Both the original and enhanced scales are damage scales, meaning that they assess damage first and then provide an inferred estimate of wind speed.
The original Fujita Tornado Damage Scale (or F-scale) had an upper-limit estimate of 318 mph for a three-second gust within a F5 tornado, but the EF scale does not place a maximum wind value on EF5 tornadoes. Operationally, the threshold for EF5 tornadoes begins at 200 mph for a three-second gust. As can be seen in the chart below, the estimated winds in the enhanced scale are lower for a given F-rating than in the original scale, based on damage surveys and engineering studies. Tornadoes with the same numerical rating in the two scales (e.g., F4 and EF4) are assumed to be similar in strength. Note that tornadoes rated prior to 2007 retain their original F-scale rating.
The new EF scale is, in fact, so complex that it cannot really be published simply as a single table (unlike the old F-scale). There are 28 damage indicators to be taken into account, and the damage to each indicator (such as a motel, a large shopping mall, or a service station canopy) can have 10 or more gradiations. Thus it is difficult to directly compare F5 tornado events from before 2007 to EF5 tornado events since. However, we can assume that the strength of F5 and EF5 tornadoes is roughly comparable; only the estimated wind speeds have changed.
The acknowledged world expert on tornado history is independent researcher Thomas P. Grazulis. He is an acolyte of the famous University of Chicago researcher Theodore Fujita, the pioneer of modern tornado science and the one who developed the original F-scale in 1971. Grazulis is the author of the epic 2000-page book Significant Tornadoes: 1680-1991 (updated through 1995 in an addendum). He is also the founder of the Tornado Project, which released a set of outstanding compilations in the 1990s called “Tornado Video Classics” (now available on DVD).
F/EF5 tornadoes since 1880
In the first table below, I list all of the tornadoes from 1880 to 1949 that Grazulis has classified as F5 strength and described in detail in his book. The second and third tables show F/EF5 tornadoes as rated by the National Weather Service starting with 1950, the first year of the official NWS tornado database.
There has not been an EF5 tornado reported since May 20, 2013—the third longest “drought” of such on record. The longest F5/EF5-free periods are April 1, 1884 to June 15, 1892 (8 years 45 days) and May 3, 1999 to May 4, 2007 (8 years exactly). A new record for longest F5/EF5 “drought” will be established if no EF5s occur before July 5, 2021.
Some observations
Parsing data from the above tables, here are some additional statistics and observations.
—Since 1880 there is, on average, a F/EF5 tornado report about once every 16 months.
—Ten of the 105 F/EF5 tornadoes on record since 1880 occurred on just two days during two spectacular tornado outbreaks: six F5s on April 3, 1974, and four EF5s on April 27, 2011.
—For the period 1991-2010, the U.S. reported an average of about 1250 tornadoes (of all classifications, including EF0) per year, according to the NOAA National Centers for Environmental Information. Only about 0.06% of all tornadoes are classified as F5 or EF5. That’s about one tornado out of every 1,666.
—The months of January and November are the only months never to have reported an F/EF5 tornado. April, May, and June account for 84% of all F/EF5 tornadoes on record since 1880.
—Only 20 states and one Canadian province have experienced a F/EF5 tornado since 1880. All of these tornadoes aside from one (in Pennsylvania) have occurred in the Ohio Valley, Upper Midwest, Southeast, and Plains states. None of the states along the entire Atlantic seaboard has recorded a F/EF5 tornado.
There doesn't appear to be any discernable trend so far as the number of F/EF5 reports are concerned over the 120-year period. However, the 1950s, 1960s, and 1970s seem to be especially active for violent tornadoes. Reviewing the NOAA/NCEI tornado database from 1954 to 2014, we can see a slight reduction of all tornadoes of at least F3/EF3 strength after the 1950s–1970s. However, this reduction does not show up for all tornadoes of at least F1/EF1 strength over the same period. Could it be that when the NWS began classifying tornadoes by the F-scale, it was not as stringent in its damage analysis as it has since been over the past four decades? There is some evidence to support this idea, according to Dr. Harold Brooks, an expert on tornado climatology at the University of Oklahoma.
Dr. Brooks said in an email that the initial F-scale ratings for most tornadoes from 1950 through 1978 were done by students hired in the summer of 1978. "The default rating they started with for each tornado was F2 (Tom Grazulis discovered that in the last couple of years). They were given the text description of the damage and Fujita's canonical pictures and text description of the damage scales. It appears they overrated the tornadoes, relative to the 1978-1999 ratings." An earlier study carried out by Dr. Brooks and Jeffrey Craven showed that the number of U.S. tornadoes rated F2 or stronger from 1957 to 1972 was about 44% higher than one would expect based on the atmospheric environments in that period, as compared to later periods. So even though there were some exceptional outbreaks in the 1950s-1970s, including the 1974 Super Outbreak, it is possible the number of F/EF5s in this period is overestimated.
The Top Five Deadliest F/EF5 Tornadoes
1. The Tri-State Tornado of March 25, 1925
The ”single” deadliest tornado in U.S. history was the famous Tri-State Tornado of March 25, 1925. At least695 people died in Missouri, Illinois, and Indiana when a F5 mile-wide monster carved a course that was apparently 219 miles through the three states. Modern research, however, suggests that this was likely a series of tornadoes developing from a single supercell traversing the area. A 2013 study led by Robert Johns found that the longest truly continuous damage path was 151 miles long, from central Bollinger County, Missouri, to Pike County, Indiana.
At least eight other violent (F2 or stronger) tornadoes killed an additional 52 people in Alabama, Tennessee, and Kentucky over the course of that day, bringing the total killed to 747 and making this the single deadliest tornado outbreak in American history.
2. The Monster Tornado of April 9, 1947
The second-deadliest F/EF5 tornado on record was a monster that plowed for 170 miles through Texas, Oklahoma and Kansas on April 9, 1947, killing 181. The town most impacted was Woodward, Oklahoma, where 107 people perished. At some locations along the tornado’s path it was as much as two miles wide, making it one of the largest tornadoes in U.S. annals. This tornado struck just a few years before the modern tornado warning system was established.
3. The Tupelo, Mississippi F5 of April 5, 1936
The third-deadliest F/EF5 tornado was part of a large outbreak on April 5-6, 1936, that led to 454 deaths across the Southeast from Arkansas to South Carolina. Tupelo, Mississippi, was the worst affected, with 216 were killed by an F5 twister on April 5. Gainesville, Georgia, saw 203 lives lost the following day when a F4 tornado struck the city.
4. The Joplin, Missouri, Twister of May 22, 2011
The fourth-deadliest F5 tornado was that which devastated Joplin, Missouri, on May 22, 2011, killing 158. An additional three to eight fatalities occurred indirectly as a result of the storm, according to various sources. This was the first tornado of any intensity to kill more than 100 people since the Flint, Michigan, twister of June 8, 1953, when 115 perished (another F5 event and the seventh-deadliest single tornado in U.S. history).
5. The New Richmond, Wisconsin “Circus Tornado” of June 12, 1899
The fifth-deadliest F/EF5 tornado was that which destroyed New Richmond, Wisconsin, on June 12, 1899, killing 117. The small town was crowded with people from out of town viewing a circus event. If not for that unfortunate circumstance, the death toll would likely have been much lower.
Keep in mind that the above list is of only the deadliest F/EF5 tornadoes, not the deadliest of any tornado of any strength.
Deadliest single tornadoes in U.S. history (100 fatalities or more)
Below is a list of all the U.S. tornadoes (of any intensity) that have killed 100 or more people. The location given is for the town/city most affected. So many towns were destroyed by the 1925 twister that they cannot all be listed here, but the town suffering the most casualties was Murphysboro, Illinois which lost 234 of its citizens.
On average about 55 Americans are killed every year by tornadoes. This is a big improvement compared to the 1960s and 1970s, when the average annual death toll was closer to 100, and the 1940s and 1950s, when the average was around 160 fatalities per year.
I’m not sure what the total number of U.S. tornado fatalities might be prior to 1950, so I am unable to speculate what percentage of these were caused by F/EF5 tornadoes. However, it is relatively rare that anyone is killed by any tornado (tornado fatalities rank very low on the list of weather-related annual deaths). The opposite can be said so far as F/EF5 tornadoes are concerned. Of the 105 known instances of such since 1880, only 10 did not result in any fatalities.
It has been almost seven years now since an EF5 tornado has been reported in the U.S. or Canada and, on average, one occurs about every year to year and a half. We are long overdue for one of these rare but recurring events to happen again.
KUDOS: To Thomas P. Grazulis, who provided most of the information for the statistics used above in his book Significant Tornadoes: 1680-1991. Rumor has it that he will be publishing an update soon! Also, a big thanks to Mark Stroud of Moon Street Cartography for producing the tables in this blog.
Christopher C. Burt
Weather Historian
