11.   These rotating storms, known as supercells, are capable of producing tornadoes.

12.   These rotating thunderstorms, called supercells, are capable of producing tornadoes.

13.   These spinning storms, called supercells, are capable of producing tornadoes.

14.   Hurricane experts said the storm was expected to strengthen Monday and might produce a few tornadoes.

15.   Hurricane experts said the storm was expected to strengthen and might produce tornadoes.

16.   Later analysis showed that that storm developed very suddenly, in less than five minutes, in a manner quite different from that thought to produce tornadoes.

17.   While fewer than half of supercells produce tornadoes, when they do sprout a twister they are among the most violent on Earth.