A lot of people have looked at this particular question, including some excellent blog posts like those from Jeff Masters. A timely article in the Bulletin of the American Meteorological Society by Kunkel et al. shows that the frequency of strong (EF1+) tornado reports is not increasing, though weaker (EF0) tornado reports have increased.
|Number of tornadoes per year (From Kunkel et al. 2013)|
Let's dive into these counts a little deeper to illustrate why it's difficult to pull any trend out of these numbers. I pulled a Patrick Marsh and grabbed the tornado count data from the SPC website and made the same sort of plot as the Kunkel et al. plot above, but for all of the different F/EF ratings. On the left we see the counts of the total tornadoes reported each year in each strength category and on the right we see these counts as a fraction of the annual total.
A lot of this increase in reporting weaker tornadoes has to do with population growth as we urbanize more areas and a general increase in public awareness. If you have strong tornadoes (here, F/EF 2 or greater), they're typically more likely to be reported; the damage is a lot more obvious and more easily attributable to tornadoes. As we increasingly build up our urban areas and become more interconnected, we've become more sensitive to even small disruptions in our infrastructure and as such even damage caused by weak tornadoes gets reported.
I'm also curious as to the sudden jump in the number of F/EF0 tornadoes that began in the late 1980s and has continued through this day. I was wondering if this coincided with the roll-out of Doppler radars nationwide that took place in the late 80s and early 90s. It turns out that a 2005 study by Simmons and Sutter looked at the impact of our Doppler radars on tornado warnings. They broke down (on a forecast office-by-forecast office basis) the number of tornado reports by F-scale before and after the WSR-88D implementation:
|From Simmons and Sutter (2005)|
So, after that brief look, it's true that the number of tornadoes reported has been increasing over time, but that doesn't necessarily mean that the total number of tornadoes has been increasing over time--we're just getting better and detecting weaker tornadoes and our population is more sensitive to the effects of weaker tornadoes. In focusing on stronger tornadoes, there's not enough evidence to suggest that the number of reports are increasing. In fact, as a fraction of total reports, the amount has remained somewhat steady if not decreasing slightly as weaker tornado reports have become far more common.