Tropical Cyclone Orson

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22-23 April 1989

Summary

TC Orson was one of the most powerful tropical cyclones to affect the Western Australian coast. At about 4:45 am on Sunday 23 April 1989 Orson crossed the coast at Cape Preston, about 70 km west of Karratha. Five hours earlier the eye of the cyclone passed over North Rankin gas platform where the minimum pressure recorded was 905 hPa, the lowest ever in an Australian cyclone.

Several Indonesian fishing vessels were reportedly sunk by the cyclone in the vicinity of Ashmore Island, at least four lives were lost. Fortunately Dampier and Karratha were east of the zone of the most destructive winds but nevertheless some damage was sustained including the destruction of the weather radar dome. The maximum wind gust recorded at Dampier was 183 km/h, however Mardie station, about 30 km west of the cyclone path, registered a gust of 211 km/h. Although weakening, Orson caused widespread roof and structural damage at Pannawonica as it passed over the townsite. It subsequently decayed as it accelerated to the south southeast later on the 23rd. The total damage cost was estimated to be in excess of $20M (1989 value).

Track and Intensity

Orson was identified as a tropical low on 17 April Northwest of Darwin and reached cyclone intensity the following day as it moved on a general westerly track. Orson continued developing in the following days developing an eye and reaching category 4 intensity on the 20th. By then Orson began moving to the southwest. Orson reached peak intensity on the morning of the 22nd having a central pressure of 905 hPa. Orson then accelerated to the south towards the Pilbara coast. The centre passed a few kilometres to the west of the North Rankin gas platform at 12:30 am on 23 April. North Rankin's barometer fell to 905 hPa and wind gusts were recorded to 275 km/h before the anemometer was badly damaged.

Orson crossed the coast near Cape Preston at about 4:45 am on 23 April when it was moving at 28 km/h. It then passed near Pannawonica at about 6am and then accelerated across inland Western Australia gradually weakening. Over land the maximum wind gusts measured were 183 km/h at Dampier (56 km to the east of the track) and 211 km/h at Mardie station (30 km to the west of the track). The rapid speed of movement of Orson meant that most localities were only exposed to destructive winds for a relatively short time. This may have lessened the potential for structural damage at Dampier and Karratha.

Impact

Fortunately Orson made landfall in a sparsely populated area and as a result damage was relatively minor. The only lives lost were those of about four Indonesian fisherman in the vicinity of Ashmore Island. Dampier and Karratha were east of the zone of the most destructive winds but nevertheless some damage was sustained including the destruction of the weather radar dome. Although weakening, Orson caused widespread roof and structural damage at Pannawonica as it passed over the townsite. The total damage cost was estimated to be in excess of $20M (1989 value).

Cyclones as intense as Orson pose a significant storm surge threat to the Pilbara coast. Although Dampier recorded a storm surge of 3.1 m, inundation of coastal areas was not significant as the cyclone crossed the coast near the time of low tide (1.6 m). The resultant tide height of 4.7 m is less than the highest astronomical tide level of 5.1m. However, the action of driving seas caused significant erosion of rock fill sea walls at Dampier Port. Areas of fill up to 10 m wide and 20 m deep were gouged out of the walls.

Examination of the damage to the base of North Rankin platform indicated a significant wave height of 12 m and a maximum wave height in excess of 20 m.

Figure 1. Track of TC Orson (click on image to enlarge).

Figure 2. Visible satellite image at 1:30 pm 22 April.

GMS3 image courtesy of Japan Meteorological Agency.

Figure 3. Dampier radar image at 9:15 pm 22 April, approximately eight hours prior to landfall.

Cyclone Parameters at the time of maximum intensity

Date/time: 8 am 22 April 1989

Latitude: 16.4 °S

Longitude: 116.0 °E

Lowest estimated pressure: 905 hPa

Reference:

Report on Cyclone Orson April 1989. Bureau of Meteorology, Department of the Arts, Sport, the Environment and Territories, 1992.