Days With Multiple Tornados More Frequent And Intense

Yearly totals of twisters hold steady - but they’re coming more often in clusters

TwisterOAR/ERL/National Severe Storms Laboratory (NSSL)

First the good news: The number of days with a single tornado is going down in the United States.

Now the bad: The number of days with lots of really intense tornados is going up.

That's the major finding in a new study (login or payment required) by three researchers: James Elsner of Florida State University, Svetoslava Elsner and Thomas Jagger. Elsner and his co-authors detect signals in US tornado data suggesting "an increasing efficiency of the atmosphere to produce tornados," and they suggest further research is needed to find out whether global warming may be contributing to it.

Using the most complete set of tornado data in the United States--reports from 1954 to 2013 held by the US Storm Prediction Center--they found no change in the total number of tornados over time. The mean annual rate of twisters is 505 a year, and the median is 474, with a lot of variation from year to year. (Currently 2011 holds the record high of 896 tornados, and 1999 the record low of 311.)

However, days featuring at least 8, 16, or 32 twisters are increasing. “In particular before 1980 the number of days per year with at least 16 tornados averaged between three and four,” they write. “Since 2000 the average has doubled to seven.” While most years before 1990 saw no days with at least 32 tornados, every year since 2001 has had at least one 32-storm day. The year 2011 saw six days with at least 32 tornados (yikes).

Days on which a single twister occurs are becoming rarer, resulting in an overall decrease in the total number of “tornado days.” There is also a downward trend in the total number of days with at least four tornados.

The researchers also found upward trends in tornado clusters--multiple tornado touchdown locations that are relatively close to one another. (For the tornado geeks out there, this paper uses “partitioning around the mediods” to define a cluster.) From 1954 through 2013, days with at least four tornados saw a 123 percent change in the density of tornados per cluster, which means they more than doubled over time. Tornado density per cluster for days with at least 32 twisters changed by 200 percent--a threefold increase.

“These results are similar to increases in heavy precipitation days during the 20th century over the United States,” the authors note.

As with any active arena of scientific research, some things remain uncertain. Elsner and his team write that events recorded as tornados in the past might not rise past “strong winds” today, thanks to our improved abilities at determining wind speed from analyzing the damage left behind. This factor could account for some of the decline in single-tornado days over the years. However, they firmly conclude that the number of days every year with multiple tornados is on the rise, and that global warming must be considered as a cause.

The research was published in the August 6 issue of the journal Climate Dynamics.

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