2011 US Tornado Year Analyzed – no trend indication, still below 1974 for strong to violent tornadoes

Guest Post by Paul Homewood

NOAA have just about finalised their numbers for the 2011 tornado season, although December figures still await confirmation.

It usually takes about three months to confirm the provisional reports as each tornado report has to be physically assessed by NWS personnel, in order to determine the category and, in many cases, even decide whether a tornado has actually occurred.

So let’s take a look at the figures, as they stand currently.

Figure 1

Historical Trends

When observing long term trends, it is important to remember that considerable changes have been made to the way that tornadoes are reported. NOAA have this to say :-

Improved tornado observation practices have led to an increase in the number of reported weaker tornadoes, and in recent years the number of EF0 and EF1 tornadoes have become more prevalent in the total number of reported tornadoes.

With increased national Doppler radar coverage [introduced between 1992 and 1997], 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 true variability and trend in tornado frequency in the U.S., the total number of strong to violent tornadoes (EF3 to EF5 category on the Enhanced Fujita scale) can be analyzed. These are the tornadoes that would have likely been reported even during the decades before Doppler radar use became widespread and practices resulted in increasing tornado reports. The bar chart below indicates there has been little trend in the frequency of the strongest tornadoes over the past 55 years.

Figure 2

http://www.ncdc.noaa.gov/oa/climate/severeweather/tornadoes.html

The effect of changes in observation can be clearly seen when looking at the ratio of the weakest F0 tornadoes to total numbers back to 1950, which rises from 10% to 60%.

Figure 3

http://www1.ncdc.noaa.gov/pub/data/cmb/images/tornado/clim/RatioofEF0s.png

Further background to this issue can be seen here.

Analysis By Category

Figure 4

US Tornadoes Per Year

When the weaker tornadoes are excluded, it is clear that there is very little trend since the 1980’s. It is also very apparent that tornado occurrences were much higher in all categories of F2 and above during the 1970’s, than in the decades since.

Even in 2011, the total of F2+ tornadoes, which amounted to 279, was only slightly above the average of 255 for the whole 1970-79 period.

Are Tornadoes Becoming More Extreme?

In overall terms, Figure 2 indicates that for F3+ categories, 2011 ranked only 6th worst since 1950. But is there any trend towards the most severe categories?

Figure 5

Figure 5 shows the number of F2+ tornadoes by category for each year since 2002 expressed as a percentage of the total of F2 to F5 occurrences. The dotted lines are the averages for the 1970’s. Although 2011 experienced a sharp increase in F3, F4 and F5’s, the pattern over the 10 years as a whole does not seem to indicate any real trend, simply going up and down around the historical averages.

Conclusions

Whilst nobody can predict what 2012 will bring, there fortunately seems to be no evidence to suggest that there is any trend towards an increase in numbers or severity of tornadoes in the US.

Notes

1) The F-scale (Fujita) was uprated to the EF-scale (Enhanced Fujita) in 2007. The intent at the time was that the new scale should be consistent with the old one, and that, therefore, previous years would not be “revalued”. (More on the change here.) Please excuse me then, when I use both terminologies at different times!

2) All tornado statistics have been sourced from the Storm Prediction Centre.