When a hurricane is bad enough its named is retired and replaced with a named of the same gender and first letter. For example Katrina in 2005 was replaced with Katia in 2011.
The names of hurricanes are re-used every 6 years unless they are retired. Names of particularly severe and/or damaging hurricanes are not re-used. The decision whether to remove a name is made yearly at an annual session of the Hurricane Committee. When a name is retired/removed from the list, a new name starting with the same letter is chosen to add to the list in its place.
There has never been a tornado name Sheila as tornadoes are not given names like hurricanes are. The name Sheila is included on the list of tropical cyclones (generic for hurricanes, typhoons, etc.) in the area of Fiji and was used once, but the storm only reached equivalent to tropical storm intensity.
Until the hurricane dies away
Yes. The name Katia was used for hurricanes in 2011 and 2017.
Currently hurricanes are named by the World Meteorological Organization in Geneva. They have 6 lists kept in rotation that are used to name any hurricanes that happen during the year.
Irene is on the list of hurricanes which was used in 1999, 2005, and 2011, etc., until it is retired.
The letters Q, U, X, Y, and Z
No, All hurricanes have different names and do not repeat the same name.
There were four hurricanes named Lili, in 1984, 1990, 1996, and 2002. There were three hurricanes named Lily in 1967, 1971, and 1975. Lili was used for storms in the Atlantic Ocean; Lily was used for hurricanes in the eastern Pacific Ocean. No hurricanes have been named Lilly by that spelling.
Hurricanes in tropical storms are given names from a preselected alphabetical list. Every time a new system becomes a tropical storm it is given the next name on the list. The hurricane we now call Irma was the ninth storm of 2017 to become a hurricane, so it was given the ninth name of the list: Irma. Names are re-used on a six year rotation unless a storm is particularly bad, in which case the name is retired from reuse. Irma was used for the first time in 2017 after the name Irene was retired following the 2011 hurricane season.
These instruments are called dropsondes.
21. The letters Q, U, X, Y, and Z are not used.