U.S. Severe Weather Blog

The rarity of the Oklahoma tornadoes of 10 Feb revisited

One of the questions we get asked a lot about significant tornado events boils down to “How unusual was this?”  The answer is, “That depends.”  It depends on exactly what aspect you’re asking about.  Sometimes, it’s very easy to quantify.  Other times, it’s not.  The case of the 10 February 2009 Oklahoma tornadoes is an excellent example of how difficult it can be to answer.  A lot of the discussion I’ve seen has dealt with the number of tornadoes in the state of Oklahoma during the month of February.  Although that provides a very neat and well-defined answer, it’s probably not all that relevant.  February is a month of very strong gradients in space and time in the occurrence of tornadoes, especially in the southern Plains.  Most of the tornadoes in Oklahoma during that month occur in the eastern part of the state and the late part of the month.

To illustrate, I’ve put together a series of “postage-stamp” maps showing tornado touchdown locations by week for each of the first nine weeks of the year (e.g., 1-7 January, 8-14 January, etc.) for 1950-2007, with the Oklahoma tornadoes of this week overlaid on the 6th week’s map (5-11 February).

Postage stamp maps of tornado locations

Postage stamp maps of tornado locations

In general, there’s a fairly sharp boundary on the northwest side of the locations of tornadoes. For the 6th week (middle panel on the right side), the recent tornadoes are right at the edge of that boundary. Over the next three weeks, however, tornado occurrence becomes much more frequent west of where that boundary is earlier in the year. Thus, a qualitative answer to the question of rarity would be that the tornadoes would have been much more likely if they had been either a couple of weeks later or a couple of hundred miles southeast. Quantitative answers are much harder to develop.  I’ve developed techniques in the past to look at that question, but for events like this that are right at the edges of what we’ve observed in the past, the answer you get is very sensitive to the assumptions you make.

Another point that can be seen is that the 7 January 1992 tornadoes in central Nebraska (see the upper left panel) are a very long distance in space and time from anything else. They are likely to be among the rarest of tornadoes.

Harold Brooks is a research meteorologist with the NOAA National Severe Storms Laboratory in Norman, Oklahoma.

This blog is intended to facilitate communication about the impacts of selected severe weather events, particularly major tornadoes, in the United States. We hope it will allow us to assist interested parties to understand what we at the NOAA Weather Partners in Norman, Oklahoma have been able to gather about the events. It is not intended to provide critical weather forecast information, nor is it intended to provide information on a broad range of research topics associated with severe weather.