Last week, here on the blog, I answered the question of whether tornadoes are "getting worse." The answer is clearly no.
At today's AccuWeather B2B tornado seminar in Omaha this morning I was asked if all tornadoes are increasing. I explained the answer is a very qualified no but I am less certain about that answer.
Here is a brand new graph from the National Severe Storms Laboratory's Dr. Harold Brooks. It shows all tornadoes from F-1 to F-5 intensity from 1950 to 2011. It would tend to indicate an increase. This graph does not include the very weak F-0 tornadoes which have clearly increased due strictly to storm chasing, i.e., if chasing didn't exist very few of those would ever be reported.
Here is why I conclude the correct answer is "no" in spite of the opposite being indicated by the graph.
First, as I learned when I was researching Warnings: The True Story of How Science Tamed the Weather, the numbers from 1950-54 are imprecise and many were collected after the fact. I suspect those numbers are too low.
So, I have added an orange box to the graph for the years 1955-2010. I eliminated 2011 because it is such an outlier that it distorts the end of the graph (as a graph from 1955-1974 would have lead one to conclude a rapidly rising rate that didn't really exist).
If you focus on the graph within the orange box, the rate of rise is so small it may not be significant from a statistical point of view. This is especially true when you consider that both storm chasing and Doppler radar might have tended to inflate the numbers during the last 15 years.