ALISON

History

Severe Tropical Cyclone Alison developed in the South-East Indian Ocean in early November 1998, forming as a tropical depression northeast of the Cocos (Keeling) Islands around 6–7 November. Despite initially encountering vertical wind shear that slowed its development, the system organised sufficiently to be named Alison and subsequently underwent a period of rapid intensification. It reached its peak intensity on 9–10 November as a Category 4 severe tropical cyclone on the Australian scale, with a minimum central pressure of 955 hPa and maximum sustained 10-minute winds of 155 km/h.

Steered by a north-easterly mid-level flow, Alison tracked south-westward into open ocean, passing close enough to the Cocos Islands to bring gale-force winds and squally conditions to the area, though no structural damage or casualties were reported there. On 11 November, the cyclone encountered a break in a subtropical high-pressure ridge, exposing it to increased wind shear and cooler sea surface temperatures, which caused rapid weakening. Alison fully dissipated on 13 November 1998.

The cyclone's entire track remained well away from the Australian mainland, posing no threat to coastal communities and causing no property damage or injuries onshore. Alison is noted as one of the earliest tropical cyclones to form in a regular Australian season since reliable satellite observation began in 1970.

Sources: Bureau of Meteorology report Wikipedia