Tornado 'clusters'? It's a thing

Tornadoes in the USA are now coming more in clusters rather than scattered throughout the year, according to a new study published Thursday. But whether that change is related to global warming is unclear.

In the 1970s, there were about 150 days per year with at least one tornado, said study lead author Harold Brooks, research meteorologist with National Oceanic and Atmospheric Administration's National Severe Storms Laboratory in Norman, Okla. But over the past decade, that figure has decreased to an average of about 100 days a year with at least one tornado.

Sounds like good news, right?

Unfortunately, no. Now, on the days when tornadoes do occur, the twisters happen in greater number, according to the study published by NOAA researchers in the journal Science. For example, in the 1970s, there were only about 0.6 days a year on which more than 30 tornadoes were spotted. But that leaped to about three days per year in the 2000s.

"There is a lower probability of a day having a tornado, but if a day does have a tornado, there is a much higher chance of having many tornadoes," the study found.

That means cities and towns across the nation should expect an increased number of catastrophes, Brooks said.

"Concentrating tornado damage on fewer days, but increasing the total damage on those days, has implications for people who respond, such as emergency managers and insurance interests," Brooks said. "More resources will be needed to respond, but they won't be used as often."

The paper also found the start of tornado season has fluctuated wildly in recent years, with both earlier and later starts compared to the 1950s-1990s. Overall, though, the number of tornadoes per year remains about the same — around 495.

The study, which was based on the past 60 years of data, only counted tornadoes of EF-1 strength or higher, weeding out the weak EF-0 tornadoes that seldom cause damage. (The "Enhanced Fujita" scale rates tornadoes by damage, on a scale from EF-0 to EF-5).

As for a reason behind the change in the number of days with tornadoes, it's not clear just what environmental conditions — a changing climate or otherwise — are driving this clustering effect.

"How such a change would relate to the increase in global temperature, if it relates at all, is unknown at this time," the paper noted. "Nevertheless, if the variability continues to increase, it could lead to an even greater concentration of tornadoes on fewer days."