US Tornado Trends–Updated to 2011
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.
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 :-
AVERAGE NUMBER OF US TORNADOES PER YEAR
(2000’s are 2000-2011)
The following graph shows the trends well.
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.