Your Odds of Being Hit By a Tornado

If you simply examined a map of all United States tornado tracks from 1950 through 2014, you might conclude your odds are disconcertingly high in the nation's midsection and South.

Certainly, those are the tornado hot zones well known by most. Oklahoma, Arkansas, Kansas, Texas, Alabama and Mississippi may first come to mind.

Approximations of tornado risk have been calculated by a number of studies over the past several decades.

Some of the earliest work to answer this question was conducted by Joseph Schafer and Donald Kelly in the 1980s. A 1986 study concluded that the tornado risk within any 1-degree latitude (about 69 miles wide) by 1-degree longitude (about 53 miles wide in the midlatitudes) grid box is a maximum over central Oklahoma with about a 0.06% yearly risk. 

Extensive tornado and severe thunderstorm climatology studies were conducted in the early 2000s by Harold Brooks from the National Severe Storms Laboratory, among others.

A 2003 study by Brooks, Charles Doswell and Michael Kay used the concept of a "tornado day," namely, a day in which at least one tornado was reported within 25 miles of any location. This particular 25-mile standard is used in tornado probability forecasts issued by NOAA's Storm Prediction Center today.

While not as granular as you may like – down to your city or street, that is – a tornado within 25 miles is uncomfortably close, right?

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Utilizing 20 years of data from 1980 through 1999, the Brooks et al. (2003) study found a C-shaped area from Illinois to the High Plains southward to Texas and eastward to Mississippi, as well as the Florida Peninsula, with at least one tornado day each year. Maxima in tornado days were found in northeastern Colorado and parts of Central Florida.

Of course, the tornado threat varies during the year, generally speaking, with the return and departure of warm, humid air. Based on the work by Brooks and others, a climatology of seasonal tornado threat was compiled, giving emergency managers and the public an idea when their tornado threat typically peaks.

However, the Brooks et al. (2003) study readily acknowledged the limitations of the tornado database at the time, opting to compute tornado days and use only initial touchdown points of tornadoes.

The advance of technology such as Doppler and dual-polarization radar, cellular networks, smartphones, as well as a far greater number of storm spotters and the general increase in population, means more weak tornadoes (EF0, EF1) are detected or observed today than, say, in the 1960s. This is known as "tornado inflation."

Benefiting from over a decade of additional tornado data, including more standard tornado damage surveys, a 2014 study by Timothy Coleman and P. Grady Dixon took another wag at this.

First, Coleman and Dixon used a statistical technique to incorporate entire tornado path lengths, not simply a tornado's start point, to more accurately depict the risk and potential for destruction.

They also considered only tornadoes of F/EF2 intensity or stronger from 1973 through 2011, the beginning of which corresponded with the adoption of the Fujita scale by the National Weather Service. Unlike their weaker cousins, these so-called "significant" tornadoes have actually been on a slight diminishing trend over the years, despite better technology and larger storm chaser networks, providing a more stable tornado database not affected by tornado inflation.

(MORE: How Tornadoes Are Rated)

From 2000 to 2010, F/EF2 or stronger tornadoes accounted for only 10% of all U.S. tornadoes, but 92% of all U.S. tornado deaths, according to Dr. Greg Forbes, former severe weather expert at The Weather Channel.

What they found was a corridor from central Oklahoma into Arkansas, northern Louisiana, much of Mississippi, West and Middle Tennessee and the northern two-thirds of Alabama with the greatest tornado risk, defying the persistent notion of a Plains "Tornado Alley" maximum. 

Removing the record-setting April 2011 Superoutbreak had little impact on the spatial distribution.

From 1973 to 2010, the study found an average tornado path length each year of over 7 kilometers – which is about 4.3 miles – within 25 miles of any point in parts of central and eastern Arkansas, central Mississippi and northern Alabama. 

For a broad swath of the Central Plains, Mississippi Valley and lower Ohio Valley, Coleman and Dixon found an average yearly tornado path length of at least 3 kilometers – about 1.8 miles – within 25 miles of any point. 

The probabilities of a tornado actually tracking over your one-half acre lot are of course much lower. However, just knowing you live in an area averaging one tornado day and a tornado path of a few miles nearby each year should make you take every tornado warning seriously.

(MORE: Struck Twice...America's F5 Tornado Towns)

Being aware of the forecast, knowing where to take shelter, then taking quick action when a warning is issued can stack the odds of surviving a tornado in your favor, even if the odds of experiencing a direct hit are extremely low.

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