Significant tornado outbreaks and especially strong tornadoes, like those that recently shook up the South, are more likely to last for three or more days, according to a Purdue University tornado expert.

Tornadoes swept through Arkansas, Oklahoma, Kansas, Missouri, Nebraska, Iowa and Mississippi on Sunday; Mississippi, Alabama and Tennessee on Monday; and North Carolina on Tuesday. The National Weather Service reported multiple deaths due to the violent weather.

Jeff Trapp, a professor of earth, atmospheric and planetary sciences at Purdue, discovered from 30 years of US weather records that an outbreak of 20 or more reported tornadoes had a 74 percent probability of occurring during a period of tornado activity lasting three or more days. During those same periods, a tornado rated 3 or higher on the Enhanced Fujita scale had a 60 percent probability of hitting.

In the past few decades, the US has averaged annually about 1,250 tornadoes of all sizes, The Associated Press reported. They can occur anywhere, anytime, as long as the conditions are right. But expect these multiple-day periods to occur in the warmer months of April through July, Trapp noted.

The Enhanced Fujita scale rates tornadoes from EF0 to EF5 with damage rated as "light," including broken branches and windblown signs, to "incredible," including leveling of strong-frame houses.

These tornadoes can be destructive in more ways than one, the study points out.

"Two extreme tornado events last year led to 32 deaths, injured more than 377 and cost $2 billion in damage and inspired this study," Trapp said in a statement. "Unfortunately, the devastating tornadoes these past few days, tragically, seem to be bearing out the results."

But there is good news in all of this, Trapp adds. Though these storm systems may last longer, they're sluggish and more predictable.

"The encouraging news is that the larger, more slowly evolving and moving systems that appear to contribute to multiple-day tornado periods may be more predictable," he said. "The weather system responsible for the tornadoes this week falls in this category and was revealed in the forecast models at least five days in advance with good fidelity."

Trapp's findings are described in the journal Monthly Weather Review.