Tornado Seasons Like the Ones You’re Used to Could be Changing, New Studies Find

While there isn't a long-term trend in the number of U.S. tornadoes stronger than EF0, several recent studies suggest the time distribution of those tornadoes and their tendency to cluster in outbreaks may be changing.

 

Fewer Tornado Days, But More Active Days

 

When eliminating (E)F0 tornadoes from yearly counts, which have steadily risen over the past few decades due to more extensive spotter networks, the implementation of Doppler radar, and advanced technology such as smartphones and social media, there is essentially no long-term yearly trend in the raw number of (E)F1 and stronger tornadoes.

 

However, the number of days with at least one (E)F1+ tornado in the U.S. has fallen from an average of 150 such days in the early 1970s to around 100 days in the first decade of the 21st century, according to an October 2014 study in the journal Science.

 

However, the study by noted tornado researchers Dr. Harold Brooks of the National Severe Storms Laboratory, Greg Carbin of NOAA's Storm Prediction Center, and Dr. Patrick Marsh, also of NOAA/SPC, found the number of days with a large number of tornadoes is actually increasing over time. 

 

"The frequency of days with more than 30 (E)F1+ (tornadoes) has increased from 0.5 to 1 days per year in the 1960s and 1970s to 3 days per year over the past decade," says the Brooks et al. study. 

 

In essence, we have fewer days with tornadoes, but are packing more of them into the days we have. "Approximately 20 percent of the annual tornadoes in the most recent decade have occurred on the three biggest days of each year," says the Brooks et al. study.

 

Another recent study by Dr. James Elsner not only found a similar clustering of tornadoes into fewer days, but also a spatial clustering of tornadoes on those very active days.

 

"It appears that the risk of big tornado days with densely concentrated clusters of tornadoes is increasing," Elsner says in the July 2014 study.