F0/EF-0 through F5/EF-5 U.S. Tornado Trends, Plus a Look at Oklahoma and Alabama

Over the past few weeks there have been a number of searches for graphs showing F0/EF-0 through F5/EF-5 tornado trends since records officially began in 1950. I’ve seen this graph in a few places, including on Andrew Revkin’s fantastic post about killer tornadoes last year. That one ran through 2009. Since 2011 data was recently released by the Storm Prediction Center, now seemed like a decent time to cook up an updated version of that graphic.

The most striking feature is the increased number of weak tornadoes. This can largely be attributed both to increased reporting by the public as well as increased storm chasing activities. Early in the modern record, it was not unusual for the majority — or all — tornadoes reported in any given region to be the strong ones that caused problems for humanity.

Once reaching significant tornado status, or F2/EF-2+ the changes over time are much less pronounced overall. The main note is probably what seems like a decrease in F2/EF-2 events starting during the mid-1970s (not counting 2011!). At least some of this is due to increased rating standards and/or overratings of tornadoes earlier in the period.

On a similar note, but given recent news of reports talking about how Tornado Alley has shifted east, I took a quick look at Oklahoma and Alabama tornado history. Both states were examined in some detail in the violent tornado post yesterday, as they led the list for killer violent tornadoes.

Both Oklahoma and Alabama are — and have been — hot spots for big tornadoes throughout recorded history, though the argument in the CoreLogic report says that a place like Alabama are seeing more tornadoes now than it used to.

There’s little doubt that Alabama’s confirmed tornado count has increased in recent decades compared to early in the period. However, there is probably strong question to the early record’s usefulness in charting hard trends due to underreports.

To clear up some of the jumble of yearly trends I’ve taken 10-year averages for total tornadoes in each state and plotted them in the graph above. Oklahoma’s numbers are relatively stable through the period, as are Alabama’s at least through the 2000s. The 2001-2010 period featured considerably more Alabama tornado confirmations than prior periods.

2011 was a very big year for both states. Using it on its own in a trendline is unwise due to its outlier status, but I’ve included it in the graph for simple visual comparison.

This is perhaps a topic to address in more detail at a later date…

SPC tornado data obtained at the Tornado History Project. Graphs can be selected for larger versions. Official 2012 data will be available during spring 2013. Get updates about U.S. Tornadoes on Twitter and Facebook.