The frequency of strong tornadoes

Given the recent EF5 Moore tornado, the 2011 Joplin EF5, the 2011 Tuscaloosa EF4 and other strong tornadoes in the last few years, a lot of people have been commenting on whether or not we're seeing more violent tornadoes now than in the past.  A few have even tried ascribing recent violent weather to climate change.  I find this particularly amusing, particularly consider that these last 12 months have been the least active 12 months of tornado activity on record.  But what do the trends say--are we seeing more strong or violent tornadoes more frequently?

A lot of people have looked at this particular question, including some excellent blog posts like those from  Jeff Masters.  A timely article in the Bulletin of the American Meteorological Society by Kunkel et al. shows that the frequency of strong (EF1+) tornado reports is not increasing, though weaker (EF0) tornado reports have increased.

Number of tornadoes per year (From Kunkel et al. 2013)

However, they note that these observations are complicated by many different factors, most notably the expansion of population areas over time and the increased prevalence of automated observations.  Because of these factors they looked at changes in the frequency of weather conditions that favor strong tornadoes instead.

Let's dive into these counts a little deeper to illustrate why it's difficult to pull any trend out of these numbers. I pulled a Patrick Marsh and grabbed the tornado count data from the SPC website and made the same sort of plot as the Kunkel et al. plot above, but for all of the different F/EF ratings.  On the left we see the counts of the total tornadoes reported each year in each strength category and on the right we see these counts as a fraction of the annual total.

In terms of raw numbers of tornadoes reported, the number definitely has been increasing over the last 53 years. However, we can see that it really has been those F/EF0 reports that have been driving the increasing trend in tornado reports.  If we look at the percentage plot on the right, we only see an increasing trend for F/EF0 reports--there is virtually no trend in the fraction of F/EF 1, 4 or 5 with slight downward trends (in terms of total fraction in F/EF2 and F/EF3 reports.  This is an interesting breakdown, particularly in the divergence between the reports of F/EF1 and F/EF 2 tornadoes.  This seems to be the breaking point--the total number of F/EF 1 and F/EF 0 tornadoes reported has been increasing while other levels haven't shown a whole lot of change.

A lot of this increase in reporting weaker tornadoes has to do with population growth as we urbanize more areas and a general increase in public awareness.  If you have strong tornadoes (here, F/EF 2 or greater), they're typically more likely to be reported; the damage is a lot more obvious and more easily attributable to tornadoes.  As we increasingly build up our urban areas and become more interconnected, we've become more sensitive to even small disruptions in our infrastructure and as such even damage caused by weak tornadoes gets reported.

I'm also curious as to the sudden jump in the number of  F/EF0 tornadoes that began in the late 1980s and has continued through this day.  I was wondering if this coincided with the roll-out of Doppler radars nationwide that took place in the late 80s and early 90s.  It turns out that a 2005 study by Simmons and Sutter looked at the impact of our Doppler radars on tornado warnings.  They broke down (on a forecast office-by-forecast office basis) the number of tornado reports by F-scale before and after the WSR-88D implementation:

From Simmons and Sutter (2005)

We can use these numbers to plot out the percent change in total number of tornado reports of each F scale category before and after Doppler radar implementation.

The percentage of reports of F0 tornadoes increased by about 11 percent--the only category where this happened.  I really think that the Doppler radar implementation has had a significant role in increasing the number of weak tornadoes reported, as this has allowed meteorologists to see small-scale spinups that might otherwise have been lost in larger thunderstorm wind damage.  This increased detection of weaker tornadoes helps focus storm survey efforts and attributes more damage reports to weak tornadoes than we may have suspected in the past.

So, after that brief look, it's true that the number of tornadoes reported has been increasing over time, but that doesn't necessarily mean that the total number of tornadoes has been increasing over time--we're just getting better and detecting weaker tornadoes and our population is more sensitive to the effects of weaker tornadoes.  In focusing on stronger tornadoes, there's not enough evidence to suggest that the number of reports are increasing.  In fact, as a fraction of total reports, the amount has remained somewhat steady if not decreasing slightly as weaker tornado reports have become far more common.