Multiple consecutive days of tornado activity spawn worst events

http://www.sciencedaily.com/releases/2014/04/140430143020.htm

Significant tornado outbreaks and especially strong tornadoes are more likely occur within periods of activity lasting three or more days, according to a tornado expert. An examination of 30 years of US weather records found 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 three or higher on the Enhanced Fujita scale had a 60 percent probability of hitting.