Severe Tropical Cyclone Thelma was the most intense cyclone to traverse the Northern Australian Region in the last 35 years and was the first Category 5 system (on the Australian scale) observed in the Timor Sea.
A tropical low was first identified in the Arafura Sea north of Arnhem Land on 1 December 1998. A deep low to middle level ridge over central Australia steered the system steadily westward. By 0000 UTC on 6 December the low was located about 280 km north of Melville Island and had deepened into cyclone Thelma . The strong low to middle level ridge which had been steering the system westward over the past few days weakened with the approach of a strong middle to upper level trough into the Northern Territory. The weakened ridge allowed the low to drift slowly southwards. A strong upper westerly jet stream developed over the Gulf of Carpentaria ahead of the amplifying trough system over the Northern Territory. The outflow from the cyclone into this developing westerly jet stream was coincident with the cyclogenesis and rapid intensification of Thelma . The system deepened to Category 5 (on the Australian scale) and estimated maximum intensity with MSW of 227 km/h by 1200 UTC on 8 December as it passed over the northwest end of the Tiwi Islands and about 185 km to the west-northwest of Darwin. A ship, positioned 50 km to the southeast of Thelma at 0230 UTC on 8 December reported MSW to 140 km/h. Radar imagery at 0600 UTC on 8 December indicated an eye diameter of 35 km contracting to 18.5 km by 2100 UTC on 8 December. The main impact on the Tiwi Islands was disruption to communications and power supplies. Vegetation in exposed areas suffered extensive damage and a 3 to 4 metre (m) rise in water level, reported in Milikapiti (Snake Bay) caused minor coastal erosion. In the Darwin area, minor damage occurred to houses and power supplies which were disabled by fallen trees. A peak wind gust of 19 m/s was recorded at Darwin Airport at 0851 UTC on 8 December. Charles Point Automatic Weather Station (AWS) recorded a peak wind gust of 29 m/s at 0550 UTC on 8 December. A record 24 hour rainfall of 425 millimetres (mm) was measured at Howard Springs and a 48 hour total of 432 mm at Darwin Airport causing local flooding in rural areas around Darwin.
Re-establishment of the low to middle level ridge during 8 and 9 December steered Thelma on a more southwestward path through the Timor Sea. SSMI data at 2318 UTC on 8 December indicated a small tight eye surrounded by deep convection enclosed in an outer more diffuse eye. By 0910 UTC on 9 December the TRMM scan showed redevelopment of deep convection and contraction of the outer eye with the inner eye wall weakening, possibly indicative of the system undergoing a concentric eye cycle. This was supported by SSMI data at 1202 UTC on 9 December which indicated further contraction of the outer eye wall as the inner eye wall weakened.
Thelma passed to the near northeast of the Troughton Island AWS on 10 December recording MSW of 137 km/h and maximum gusts to 174 km/h between 0351 UTC and 0437 UTC before failing at 0500 UTC.
While continuing to move southwest, Thelma slowly weakened before crossing the northwest Kimberley coast about 140 km northeast of Kuri Bay around 2200 UTC on 10 December as a Category 3 system. Thelma continued to move in a general southwest direction overland and was finally downgraded to a tropical low at 1500 UTC on 11 December. The low level centre moved back off the northwest Western Australia coast around 1600 UTC on 13 December but did not re-strengthen significantly. The low drifted westward for a day before making final landfall around 130 km northeast of Port Hedland at 1600 UTC on 14 December and slowly dissipated.
Track and intensity
All times in CST.