The 1998–99 South-West Indian Ocean cyclone season was an annual event of tropical cyclone formation. According to the Joint Typhoon Warning Center it started on July 1, 1998 and ended on June 30, 1999. These dates conventionally delimit the period of each year when most tropical cyclones form in the basin, which is west of 90°E and south of the Equator. Tropical cyclones in this basin are monitored by the Regional Specialised Meteorological Centre in Réunion, which considered the season to run from August 1, 1998 through July 31, 1999.
This disturbance was classified as D2 by La Reunion. Once Cyclone Davina was named, it was reclassified as E1. This system was also classified as Tropical Cyclone 23S by the Joint Typhoon Warning Center.
This was an exceptionally long-lived system that began in the Australian region and moved into La Reunion's area of responsibility on the 8th. La Reunion never considered the system as a tropical depression. This system was also classified as Tropical Cyclone 26S by the Joint Typhoon Warning Center.
This system was formerly named "Hamish" by TCWC Perth, but rapidly dissipated once it crossed 90°E on April 21. La Reunion decided to carry the system as F1, despite the fact that it was still a tropical storm.
A tropical disturbance is named when it reaches moderate tropical storm strength. If a tropical disturbance reaches moderate tropical storm status west of 55°E, then the Sub-regional Tropical Cyclone Advisory Centre in Madagascar assigns the appropriate name to the storm. If a tropical disturbance reaches moderate tropical storm status between 55°E and 90°E, then the Sub-regional Tropical Cyclone Advisory Centre in Mauritius assigns the appropriate name to the storm. A new annual list is used every year so no names are retired.