That plot shows a progressive rise in tornado rates, but it has no correction for observational bias. (dotearth.blogs.nytimes.com)
Progress in the longer term depends on identifying and correcting model biases, accumulating as complete a set of historic observations as possible, and developing improved methods of detection and correction of observational biases.» (realclimate.org)
At the time, we reported that this would certainly be taken into account in revisions of data set as more data was processed and better classifications of the various observational biases occurred. (realclimate.org)