How Tornado Season Is 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.

(MORE: The Enhanced-Fujita Scale)

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.

Twin tornadoes photographed near Pilger, Nebraska on June 16, 2014.
(Silver Lining Tours)

Large Swings in Monthly, Yearly Numbers

For only the second time since 1950, the first three weeks of March 2015 passed without a single tornado anywhere in the U.S. 

(MORE: 2015 Severe Season Reaches 'Uncharted Territory')

Yet as recently as 2011, almost 1,700 tornadoes ripped across the nation, including 349 tornadoes in a four-day outbreak from April 25-28, the costliest tornado outbreak in U.S. history. 

(MORE: 10 Shocking Things About the April 2011 Outbreak)

While year-to-year variability has long been prevalent in U.S. tornado counts, a 2014 study by Dr. Michael Tippett found volatility, a term he uses for variability in tornado counts, has increased since 2000. 

Furthermore, the Brooks et al. study  found the tendency for more monthly extreme highs and lows in (E)F1+ tornado counts in recent years.

"Excluding the zero-tornado months, there are more extreme months in the most recent 15 years of the database (1999-2013) than in the first 45 years," says Brooks et al. 2014. 

In other words, we've seen extreme high monthly tornado counts (758 tornadoes in April 2011, for example) and extreme low monthly tornado counts (March 2015, for example) more often over the past 15 years, a trend that may continue.

Of course, low tornado count years do not preclude significant tornadoes or tornado outbreaks. Despite the lowest three-year tornado count on record from 2012-2014, we still had destructive outbreaks in March 2012, in May 2013 (Moore and El Reno, Oklahoma), and April 2014 (Vilonia, Arkansas). 

http://www.wunderground.com/news/tornadoes-variability-studies