Complex sentence: These are the North Atlantic tropical cyclones that are known to have spawned tornadoes in the U.S. From 1995--2010,1,163 tornadoes were associated with tropical cyclones in the U.S., accounting for slightly under 6% of all tornadoes.
Split into simpler sentences: These are the tropical cyclones that are known to have spawned tornadoes. Within the United States 1,163 tornadoes were associated with tropical cyclones in the U.S., accounting for slightly under 6% of all tornadoes.