Are Big Tornado Days On The Rise?

By Paul Homewood

 

 

There’s a new paper recently published by James Elsner et al, “The Increasing Efficiency of Tornado Days in the United States”.

 

ABSTRACT

The authors analyze the historical record of tornado reports in the United States and find evidence for changes in tornado climatology possibly related to global warming. They do this by examining the annual number of days with many tornadoes and the ratio of these days to days with at least one tornado and by examining the annual proportion of tornadoes occurring on days with many tornadoes. Additional evidence of a changing tornado climate is presented by considering tornadoes in geographic clusters and by analyzing the density of tornadoes within the clusters. There is a consistent decrease in the number of days with at least one tornado at the same time as an increase in the number of days with many tornadoes. These changes are interpreted as an increasing proportion of tornadoes occurring on days with many tornadoes. Coincident with these temporal changes are increases in tornado density as defined by the number of tornadoes per area. Trends are insensitive to the begin year of the analysis. The bottom line is that the risk of big tornado days featuring densely concentrated tornado outbreaks is on the rise. The results are broadly consistent with numerical modelling studies that project increases in convective energy within the tornado environment.

http://myweb.fsu.edu/jelsner/PDF/Research/ElsnerElsnerJagger2014.pdf

 

 The claim that the average number of tornadoes per tornado day is subject to a number of potential flaws, which need to be highlighted.

 

Establishing long term tornado trends can be notoriously fraught with problems. McCarthy & Schaefer analysed some of the issues in their 2003 paper, “Tornado Trends Over The Past Thirty Years.”.

 

Changing Observation Practices

The biggest, but by no means the only problem, is that many, many more tornado reports are filed nowadays. As McCarthy & Schaefer state:

 

This paper looks at the reported frequencies of tornadoes and their characteristics over the contiguous United States since 1970. There was a significant increase in tornado occurrence during two periods in the last 33 years – in the early 1980s when National Weather Service (NWS) warning verification began, and in 1990 when the WSR-88D became operational.

 

The increase in reported tornado frequency during the early 1990s corresponds to the operational implementation of Doppler weather radars. Other non-meteorological factors that must be considered when looking at the increase in reported tornado frequency over the past 33 years are the advent of cellular telephones; the development of spotter networks by NWS offices, local emergency management officials, and local media; and population shifts. Changnon (1982) and Schaefer and Brooks (2000) both discuss these influences on tornado reporting.
The growing “hobby” of tornado chasing has also contributed to the increasing number of reported tornadoes.

 

NOAA address this issue on their Tornado Climatology website, where they say:

 

Today, nearly all of the United States is reasonably well populated, or at least covered by NOAA’s Doppler weather radars. Even if a tornado is not actually observed, modern damage assessments by National Weather Service personnel can discern if a tornado caused the damage, and if so, how strong the tornado may have been. This disparity between tornado records of the past and current records contributes a great deal of uncertainty regarding questions about the long-term behavior or patterns of tornado occurrence. Improved tornado observation practices have led to an increase in the number of reported weaker tornadoes, and in recent years EF-0 tornadoes have become more prevelant in the total number of reported tornadoes. In addition, even today many smaller tornadoes still may go undocumented in places with low populations or inconsistent communication facilities.

With increased National Doppler radar coverage, increasing population, and greater attention to tornado reporting, there has been an increase in the number of tornado reports over the past several decades. This can create a misleading appearance of an increasing trend in tornado frequency. To better understand the variability and trend in tornado frequency in the United States, the total number of EF-1 and stronger, as well as strong to violent tornadoes (EF-3 to EF-5 category on the Enhanced Fujita scale) can be analyzed. These tornadoes would have likely been reported even during the decades before Doppler radar use became widespread and practices resulted in increasing tornado reports.

 

Figure 1

http://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology/trends 

 

 

Elsner correctly excludes these EF-0 tornadoes from his study for this reason. However, when we analyse EF-1 tornadoes, we find a very similar phenomenon to the EF-0’s, namely a rapidly increasing proportion up to around 1990, as Figure 2 illustrates.

 

Figure 2

 

There can only be two reasons for this:-

1) It is artifact of changing observation methods, and other non-climatological reasons. In this case, Elsner’s inclusion of EF-1 tornadoes could significantly skew his results, as the number of tornadoes before 1990 will be substantially underestimated.

(A change from 50 to 70% would add 20% extra tornadoes to the total, assuming the number of EF-2’s+ remained the same).

 

2) The increasing proportion of EF-1’s is due to genuine meteorological factors. In this case, we would need to acknowledge that the average intensity of tornadoes has been reducing. 

 

1950’s

I note that Elsner begins his analysis from 1954, but there are huge issues with the accuracy of tornado data prior to 1970. McCarthy & Schaefer describe this period as the tornado growth period.

This period [their chosen period is 1970 to 2002] does not include the tornado growth period that occurred as official records began to be kept in Tornado Data (through 1957) and Storm Data (in subsequent years.) Also, the years 1950-1969 were a growth period because it was the start of the public awareness and communication revolution that gave tornadoes increased publicity due to television news coverage and graphic depictions of tornadoes and tornado damage.

 

A further complication is that the Fujita Scale was not introduced till 1973, and was applied retroactively to prior years, based on newspaper reports and the like.

Therefore, any comparison of tornado numbers with the period pre-1970 must be treated with extreme caution.

 

Uncategorised Tornadoes

Another issue is that many tornadoes were reported but not categorised at all prior to 1981. For instance, Storm Prediction Center data shows that there were 582 uncategorised tornadoes between 1970 and 1980, an average of 53 a year.

These have subsequently been categorised as EF-0’s, but many could well have been stronger.

 

Multiple Tornadoes

McCarthy & Schaefer comment:

 

The growing “hobby” of tornado chasing has also contributed to the increasing number of reported tornadoes. The capability to easily photograph tornadoes with digital photography, camcorders, and even cell phone cameras not only provides documentation of many weak tornadoes, but also, on occasion, shows the presence of multiple tornadoes immediately adjacent to each other.

 

In the past, such occurrences would almost certainly have been treated as single tornadoes.

 

Recording of Tornadoes across County Boundaries

In 1994, changes were introduced in how tornadoes that cross county lines were treated, which resulted in more “county segments” being recorded. As McCarthy & Schaefer state:

Thus the more segments reported, the higher the probability that a single long-track tornado will appear as two or more individual tornadoes.

 

Population Bias

In the earlier decades, there was a tendency for tornado intensity to be understated because of population bias, as Guyer & Moritz explain:

 

It is well established that the majority of the world’s tornadoes occur in "tornado alley" of the United States plains region. The plains is generally characterized by an agrarian based economy and a relatively low population density.

While the plains region receives the bulk of the tornadoes in the United States, from a tornado climatology standpoint, the tornadoes are of a lesser F-scale rating (Fujita 1971) on average than those of the higher populated areas of the eastern United States. This geographic discrepancy of F-scale climatology has been well documented in previous research literature (Kelly et al. 1978, Doswell and Burgess 1988, Grazulis 1993). As these studies have noted, this is not necessarily attributed to a difference in tornado intensity, but instead, is an artifact of population bias and associated lack of structures.
Doswell and Burgess (1988) discussed the reality of the F-scale as a damage scale versus an intensity scale. They noted tornadoes which occur in open country oftentimes do not damage structures, hence making an F-scale estimation more difficult. Schaefer and Galway (1982) noted a population bias in tornado climatology in the western plains from Oklahoma through Kansas to the Dakotas, finding that tornadoes that strike higher populated areas tend to have a higher rating than those that remain in open country.

 

Because of Doppler radar and other systematic improvements, this bias has largely been eliminated now. As NOAA themselves comment:

Today, nearly all of the United States is reasonably well populated, or at least covered by NOAA’s Doppler weather radars. Even if a tornado is not actually observed, modern damage assessments by National Weather Service personnel can discern if a tornado caused the damage, and if so, how strong the tornado may have been.

 

Tornado Days

Because of these myriad problems, Dr Stanley Changnon, past expert on tornadoes, long advocated the use of “event days” rather than the number of tornadoes, when analysing trends.

Figure 3 shows the number of days each year, with at least one tornado of EF-2 strength or greater.

 

Figure 3

 

Summary

The Elsner paper claims that there is an increasing number of tornadoes on days when tornadoes occur. However, this analysis does not take account of the many changes to observation and evaluation methods which have steadily taken place over the period in question.

The changes may be substantial enough to make a significant difference to the paper’s findings.

Although I have not addressed the subject of “clustering”, which the paper also deals with, this too could be affected by the reporting issues identified.

 

 

Footnote

In the Corfidi et al paper, “Revisiting the 3-4 April 1974 Super Outbreak of Tornadoes”, they show the following graph. It plots the maximum week-long running totals each year of F2+ tornadoes, and suggests that little is changing.

(The graph runs to 2009. The April 2011 outbreak reached a maximum weekly total of 90, between the 22nd and 28th, putting it into a similar ballpark as the April 1974 outbreak).

 

 

 

 

Notes

1) The EF-Scale was introduced in 2007, to replace the F-Scale. All above references to EF-Scale refer to both.

2) All tornado data is sourced from the Storm Prediction Center.

http://www.spc.noaa.gov/wcm/#data