Complex sentence: There were four particularly notable tornado outbreaks during the two-week period and produced at least 2 perhaps 3 F5 tornadoes and the third deadliest tornado ever in the United States history.
Split into simpler sentences: There were four particularly notable tornado outbreaks during the two-week period. It produced at least two, or perhaps three F5 tornadoes and the third deadliest tornado ever in the United States history.