Tropical cyclone Dahlia formed in the Indian Ocean west of Sumatra Indonesia on 25 November 2017. TC Dahlia moved to the north of Christmas Island before weakening well offshore of Western Australia and did not develop beyond a category 2 tropical cyclone.
A tropical low formed in the Indian Ocean about 820 kilometres (km) north northwest of Cocos Island on 25 November and remained generally slow moving on the 26 November. The system started to move east southeast from 27 November and slowly developed over the next 48 hours. It was named Tropical Cyclone Dahlia by the Jakarta Tropical Cyclone Warning Center at 8pm AWST 29 November.
Dahlia continued to move steadily east southeast and maintained Category 1 intensity as it passed about 170 km to the north of Christmas Island on 30 November. The system continued to move east southeast over open waters and slowly developed as atmospheric conditions improved, reaching Category 2 intensity as it took a more southeast track into the Perth Tropical Cyclone Warning Centre’s area of responsibility during the morning of 1 December.
The system peaked at 95 kilometres per hour on 2 December before starting to weaken as it interacted with Ex-Tropical Cyclone Cempaka to the south and another tropical low to the east. It weakened below tropical cyclone strength by 8am AWST 3 December as it took a southward track and moved into an area of unfavourable atmospheric conditions.
Gales did not occur at Christmas Island, however it recorded 368.2 millimetres (mm) in the 72 hours to 9am AWST 1 December, including 212.6 mm in the 24 hours to 9am AWST 29 November, its highest daily fall in its 45 years of record. The system did not cause any wind or rainfall impacts on mainland Western Australia.
Tropical Cyclone Dahlia was the first cyclone in the Australian region for the 2017/2018 season.