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US Tornado Trends–Updated to 2011

January 5, 2012
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By Paul Homewood

 

2011 has obviously been a disastrous year for tornadoes in the USA, but does this reflect an increasing trend in either the number or severity of tornadoes?

The NOAA graph below is often used to show trends of severe F3+ tornadoes and certainly indicates that there has been no upward trend up to 2010.

 

image

 

The provisional figure for 2011 is 79, which would be much less than 1974 and on a par with 1965. However, this only gives part of the picture as it only runs from March – August, and excludes weaker tornadoes. Nevertheless comparing total tornado numbers can be misleading.

An NOAA study by McCarthy and Schaefer in 2002 identified a sharp rise in total tornado numbers between 1970 and 2002, but made it absolutely clear that this was due to changes in methods of detection, e.g.

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. The growing “hobby” of tornado chasing has also contributed to the increasing number of reported tornadoes.

(The WSR-88D Doppler system was phased in between 1992 and 1997).

As a result of these changes, the proportion of F0 tornadoes increased from an average of 39% between 1970 and 2002, to 64% for 1998-2002. As the authors stated, “ Finally, it will be seen that the number of strong and violent tornadoes has not varied much since 1970.”

So bearing this in mind, and using data supplied by NOAA’s Storm Prediction Centre, I have analysed all reported tornadoes by category since 1970 and annual averages are :-

 

  1970’S 1980’S 1990’S 2000’S
F0 274 331 739 812
F1 343 334 331 369
F2 188 124 109 108
F3 50 33 38 33
F4 14 9 9 7
F5 3 1 1 1
ALL TORNADOES 872 832 1227 1330
TOTAL F1+ 598 501 488 518

AVERAGE NUMBER OF US TORNADOES PER YEAR

(2000’s are 2000-2011)

 

The following graph shows the trends well.

 

image

 

 

Conclusions

There has been no significant trend in F1+ tornadoes since 1980, and each decade since then has been less than the 1970’s. The 2000’s show more F0 tornadoes, but this is the first decade Doppler systems have been fully operational, so we should now have a good basis for the future.

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5 Comments leave one →
  1. Jon Christianson permalink
    January 10, 2012 2:43 pm

    Thanks for putting this together – it’s a great analysis.

    I’m curious if you have done any similar research at the state-level. Presumably, most, if not all, of your data can be segmented by state. I’m from Minnesota and there has been a lot of press about the recent spike in tornado activity (as 2010 was a record).

    Based on your indicated findings above, I would suspect that the state analysis would mirror the countrywide conclusions, but comparing Minnesota to Texas would be an interesting check nonetheless. If there has been a frequency/severity “drift” northward, that would be insightful.

    • January 10, 2012 4:37 pm

      Jon

      If you click on the “Storm Prediction Centre” link, there is a map “Tornadoes by State”. It is not yet updated for 2011, nor are the CSV files further down.( These include State Names, so you could download these and use a filter to sort out Minnesota ones).

      According to the map the ten year average for Minnesota is 51, up from the thirty year average of 37, but of course this will suffer from the same detection factors as the national totals.

      As a % of the US total there seems to be little change with the thirty year average rising from 3.3% to 3.9%.

      Paul

  2. January 10, 2012 10:25 pm

    Great article about tornado trends.

    A cool infographic just came through my twitter feed about tornadoes and thought I would share.

    http://www.buildingsguide.com/blog/tornado-facts-safety-tips-infographic

    Thanks for the great tornado info, it will help out in my classroom.

  3. January 23, 2013 4:04 am

    You were on the money with a excellent write-up with a handful
    of terrific information and facts

Trackbacks

  1. 2011 US Tornado Year Analyzed – no trend indication, still below 1974 for strong to violent tornadoes | Watts Up With That?

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