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Long Term Tornado Trends

May 8, 2013

By Paul Homewood

 

As it seems that tornadoes, or the lack of them, are back in the news at the moment, I thought it worthwhile to take another look at the stats at the end of 2012.

 

 

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Figure 1

 

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Figure 2

 

Figure 1 shows all tornadoes above EF1. (See here, why EF1’s are excluded.) The 10-Year Trend is significantly below the level consistently seen up to 1991, although the high totals in 2011 have inevitably caused a small upwards blip.

We see a similar pattern with the stronger EF3+ tornadoes.

I do not claim to know what will happen to tornado numbers in coming years. And anyone who does is lying.

NOAA sum up the situation neatly in their FAQ.

 

Does "global warming" cause tornadoes? No. Thunderstorms do. The harder question may be, "Will climate change influence tornado occurrence?" The best answer is: We don’t know. According to the National Science and Technology Council’s Scientific Assessment on Climate Change, "Trends in other extreme weather events that occur at small spatial scales–such as tornadoes, hail, lightning, and dust storms–cannot be determined at the present time due to insufficient evidence." This is because tornadoes are short-fused weather, on the time scale of seconds and minutes, and a space scale of fractions of a mile across. In contrast, climate trends take many years, decades, or millennia, spanning vast areas of the globe. The numerous unknowns dwell in the vast gap between those time and space scales. Climate models cannot resolve tornadoes or individual thunderstorms. They can indicate broad-scale shifts in three of the four favorable ingredients for severe thunderstorms (moisture, instability and wind shear), but as any severe weather forecaster can attest, having some favorable factors in place doesn’t guarantee tornadoes. Our physical understanding indicates mixed signals–some ingredients may increase (instability), while others may decrease (shear), in a warmer world. The other key ingredient (storm-scale lift), and to varying extents moisture, instability and shear, depend mostly on day-to-day patterns, and often, even minute-to-minute local weather. Finally, tornado recordkeeping itself also has been prone to many errors and uncertainties, doesn’t exist for most of the world, and even in the U. S., only covers several decades in detailed form.

 

It is worth reemphasising just how short the record period is. Given that ocean cycles are around 60 years long, it is difficult to see how we can properly monitor trends with less than at least a century worth of data.

All we can reasonably say is that the long term trend, as measured over a 10-Year average, is lower than it has been for much of the period since 1970.

 

 

References

As usual, all data is from NOAA’s Storm Prediction  Centre.

 

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

2 Comments
  1. Brian H permalink
    May 12, 2013 11:52 pm

    Makes it hard to twist the twister record.

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