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

Guest Post by Paul Homewood

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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.

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

Figure 1

YEAR EF0 EF1 EF2 EF3 EF4 EF5 TOTAL
2006 686 292 93 30 2 0 1103
2007 674 299 93 27 4 1 1098
2008 984 498 151 49 9 1 1692
2009 704 337 83 20 2 0 1146
2010 771 344 124 30 13 0 1282
2011 801 610 196 60 17 6 1690
1970-79 AVERAGE 274 343 188 50 14 3 872
Tornado Occurrences By Category
(The F-Scale was replaced by the EF-Scale in 2007)

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

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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

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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

1970-79 1980-89 1990-99 2002-11
F0 274 331 739 816
F1 343 334 331 376
F2 188 124 109 108
F3 50 33 38 31
F4 14 9 9 7
F5 3 1 1 1
ALL TORNADOES 872 832 1227 1339
F2 AND ABOVE 255 167 157 147

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

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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.

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

  1. belvedere says:

    While there is no trend, it still is a pretty hard tornado season. All the MSN has to do do is drop the global warming cause and just face the fact.. The USA is a shitty country to live in if u hate tornados.

  2. Pat says:

    One can only imagine the number of tornadoes before near ground weather radar systems and the interstate highway act.

  3. Ian H says:

    How come the years 2000 and 2001 have been omitted from the chart? Does this need correcting or is this just a strange chart?

  4. Camburn says:

    Ian H:
    May I suggest that you look at the chart again?

    Us that live here have known for a longggggg time that 1974 was a real dooozy concerning tornadoes. The main media must think that we are all to young to remember this. I was in a strange land during that time but remember reading aout all the tornadoes that occured.

    Thank you for a well written, documented piece Paul.

  5. @Ian H it looks like the final decade was merely shifted to include 2011 meaning there was no where to place 01-02, but the data is available in the first graph indicating they were unremarkable years tornadically speaking.

  6. Bill says:

    So apparently Doppler radar causes F0 and F1 tornadoes!!

  7. Charlie A says:

    Bill says “.. Doppler radar causes F0 and F1 tornadoes!!”

    To which I add —- and trailer parks attract tornadoes!

  8. Ian H says:

    Michael Bergeron: it looks like the final decade was merely shifted to include 2011 meaning there was no where to place 01-02, but the data is available in the first graph indicating they were unremarkable years tornadically speaking.

    I clearly see what has been done. But if you are going to do something a bit odd like this in a chart, it at least needs a mention in the accompanying text explaining what was done and why. Otherwise it just leads to head scratching and possible accusations of cherry picking.

  9. Latitude says:

    Putting up the doppler’s was the most crooked money laundering scam at the time…
    …and barely made the news at all

  10. Luther Wu says:

    I witnessed/survived the OKC May 3rd, ’99 tornado. It was a big nasty F5 with recorded wind speeds over 300 mph and was accompanied by 2 F4 tornadoes on parallel tracks with a total of around 60 total tornadoes that day. Whew.
    I recall talking to a New Jersey transplant a month or so after the event and he said; ” I know that I’m a fully assimilated Oklahoman now, because instead of freaking out and headin’ for the cellar, I grab a beer and go stand out in the yard and watch.”
    That is all.

  11. Eyal Porat says:

    Cooling trend will bring rise to storms.
    One can see it happen already…

  12. Ian H – How come the years 2000 and 2001 have been omitted from the chart? Does this need correcting or is this just a strange chart?

    I wanted to retain a 10 year average.

  13. Ian H – But if you are going to do something a bit odd like this in a chart, it at least needs a mention in the accompanying text explaining what was done and why. Otherwise it just leads to head scratching and possible accusations of cherry picking.

    Fair comment, but if I had used a “12 year decade”, I could rightly have been accused of diluting the higher 2011 figures.

  14. For anyone interested in the 2000 and 2001 numbers, I have added an extra column to Figure 4, which shows 2000-2009. Interestingly this decade now shows the lowest numbers of F2 and above of all.

    http://notalotofpeopleknowthat.wordpress.com/2012/03/31/us-tornado-report-2011/#more-1049

  15. Gary Pearse says:

    Higher activity 1954-74. Lets look at the “Pearse-add-60+/-year” forecast for extreme weather events (floods, droughts, rain, snow….). 2011 is on the threshold of the next cycle: 2014-2034 we will be in a period of strong tornadoes (also coinciding with strong cooling). I’ll be using this simple model in coming threads dealing with extreme weather events. Unfortunately, the desperate and hungry AGW industry will latch on to the elevated tornado energy of this coming period. Be calm and remember this forecast.

  16. observa says:

    As for the Southern Hemisphere don’t forget the capital of Oz for cyclones-

    http://www.bom.gov.au/cyclone/history/wa/onslow.shtml

    All quiet on the Western Front there in recent decades but never let the facts get in the way of a ripping good emote

  17. Gary Pearse says:

    “…no trend… Yes there is a trend, a cyclic trend.

  18. @Gary Pearse

    “…no trend… Yes there is a trend, a cyclic trend.

    Still too early to say at this stage, but if we enter a new cooling phase I suspect you may be right.

  19. Gail Combs says:

    belvedere says:
    March 31, 2012 at 3:58 pm

    While there is no trend, it still is a pretty hard tornado season. All the MSN has to do do is drop the global warming cause and just face the fact.. The USA is a shitty country to live in if u hate tornados.
    ________________________
    In the USA the far-west has earthquakes/volcanoes, the mid-west has tornadoes, the south-east has hurricanes and the North has blizzards. You just have to pick your poison.

  20. u.k.(us) says:

    Gail Combs says:
    April 1, 2012 at 8:22 am
    belvedere says:
    March 31, 2012 at 3:58 pm

    While there is no trend, it still is a pretty hard tornado season. All the MSN has to do do is drop the global warming cause and just face the fact.. The USA is a shitty country to live in if u hate tornados.
    ________________________
    In the USA the far-west has earthquakes/volcanoes, the mid-west has tornadoes, the south-east has hurricanes and the North has blizzards. You just have to pick your poison.
    ============
    You forgot the most damaging of catastrophes: floods.
    Being rather infrequent, they don’t get much hype.

  21. Tsk Tsk says:

    Bill says:
    March 31, 2012 at 4:58 pm

    So apparently Doppler radar causes F0 and F1 tornadoes!!
    ————————–
    Get that man a grant, an IPCC chapter, a movie, and a Nobel!

  22. @Uk(US) –
    You forgot the most damaging of catastrophes: floods.
    Being rather infrequent, they don’t get much hype.

    According to the US Geological Survey

    Only one of four large regions of the United States showed a significant relationship between carbon dioxide (CO2) in the atmosphere and the size of floods over the last 100 years. This was in the southwestern region, where floods have become smaller as CO2 has increased.

    http://notalotofpeopleknowthat.wordpress.com/2011/12/20/katharines-floods/

  23. Agile Aspect says:

    Gail Combs says:
    April 1, 2012 at 8:22 am

    In the USA the far-west has earthquakes/volcanoes, the mid-west has tornadoes, the south-east has hurricanes and the North has blizzards. You just have to pick your poison.

    ;———–

    Or chose the frequency of the poison.

  24. Gail Combs says:

    u.k.(us) says:
    April 1, 2012 at 10:05 am

    Gail Combs says: @ April 1, 2012 at 8:22 am…..

    You forgot the most damaging of catastrophes: floods.
    Being rather infrequent, they don’t get much hype.
    _______________________________
    No I did not forget because I did not forget my Geology Profs’ lesson. People who build homes in rivers are fools, and the one hundred yr flood plain is part of the river.

  25. Michael D Smith says:

    I can see it now… The alarmist version of your chart starts in 1999. See, it IS increasing!!!

  26. u.k.(us) says:

    OK, to clarify:
    And to assure Gail, and Paul, that no harm was meant.
    Per:

    http://ks.water.usgs.gov/pubs/fact-sheets/fs.024-00.html

    “During the 20th century, floods were the number-one natural disaster in the United States in terms of the number of lives lost and property damage.”

  27. IAmDigitap says:

    Well, back to Gore Cherch to pray for more apocalypse.

  28. Brian H says:

    In the USA the far-west has earthquakes/volcanoes, the mid-west has tornadoes, the south-east has hurricanes and the North has blizzards. You just have to pick your poison.

    The north-west, of course, is clam, clue, and corrected! But damp.

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