Western Australia Tropical Cyclone Season Summary 1998-99

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Season Overview

The broadscale setting for the 1998/99 cyclone season was a moderate La Nina event following a strong transition from El Nino conditions the previous season. In a La Nina year there are typically more tropical cyclones than normal in the Australian Region and an earlier start to the cyclone season. It proved to be a very active season over the southeast Indian Ocean with 11 TCs compared to an average number of 6. It was an early start with TCs Zelia and Alison in October and November 1998 well off the coast near the Cocos Islands. There were 2 cyclones off the northwest Australian Coast before Christmas, including a coastal crossing on the north Kimberley coast. There were six TCs in all off the Western Australian coast, 3 of these were at the most severe Cat 5 intensity, and there were 3 coastal crossing during the whole season.

TC Elaine weakened before crossing the coast but produced very heavy rain that lead to severe flooding in the mid-west region. The town of Moora was evacuated.

TC Vance was the most powerful coastal impact for the season. It crossed the coast over Exmouth Gulf on 22 March 1999 and produced a wind gust of 267 km/h, the strongest ever recorded on the Australian mainland. The towns of Exmouth and Onslow were severely damaged. As the cyclone moved over inland WA flooding was experienced in many places. The Perth TCWC received many accolades for its excellent warning performance and is credited with saving many lives at Exmouth.

Details of each Tropical Cyclone in the northwest Australian region 1998-99

Tropical cyclone Zelia 8-10 October 1998

Zelia developed from a convective cloud cluster in the near equatorial trough to the southwest of Sumatra. Intensifying and moving to the southeast on the southern side of the middle level ridge it soon encountered low-level southeasterly flow following the passage of a cold front to the south. This increased the shear causing the system to weaken rapidly and change its direction. Zelia then dissipated as a tropical cyclone over water in a high shear environment, but maintained its identity as a tropical depression for several more days

Severe tropical cyclone Alison 7-13 November 1998

A depression with a clear cloud signature was evident to the northeast of the Cocos Islands from the 7th. There was initially significant vertical sheer over the system and it developed only slowly. TC status was reached on the 8th when a separate outflow developed over it. Intensification then became more rapid and the cyclone moved off towards the southwest under the influence of northeasterly steering. On the 11th it moved through a break in the ridge subsequently encountering stronger vertical wind shear and cooler seas, causing it to weaken.

Severe tropical cyclone Billy 2-7 December 1998

TC Billy began as a tropical low to the north of Port Hedland. It reached cyclone intensity on 2 December 1999 and moved on a southwest track towards the west Pilbara coastline. Its structure improved significantly on 3 December; morning visible satellite images on 4 December showed an eye pattern for a few hours. As the cyclone moved further to the southwest very rapid shearing occurred overnight on 4 April. It weakened to below cyclone strength near Barrow Island and eventually crossed the coast near Onslow as a tropical low.

Severe tropical cyclone Thelma 6-12 December 1998

Severe tropical cyclone Thelma was the most intense cyclone to traverse the Northern Region in the last 35 years. It was also the first category five observed in the Timor Sea. Thelma brushed past the Tiwi Islands as it was intensifying to a category 5 but was a sufficient distance from communities to not cause any major damage. Thelma reached its peak in the Timor Sea, but started to weaken as it crossed the north west Kimberley coast in a sparsely populated region. No deaths or injuries were reported.

Tropical cyclone Cathy 22-28 December 1998

A persistent tropical low was located in the region around 10°S 90°E. The low developed into cyclone Cathy to the east of Cocos Island on 24 December. It moved southwards initially then was steered on a westward track as it intensified during Christmas Day. As it moved further westward it moved into a unfavorable upper wind shear pattern. By the 27th it had weakened below tropical cyclone strength.

Severe tropical cyclone Damien 21-28 January 1999

Tropical cyclone Damien commenced as a low in the eastern Indian Ocean on 21 January. The low moved steadily westward north of the mid-latitude ridge and intensified in a low-shear environment. Shear increased as the cyclone passed to the south of the Cocos Islands. The cyclone weakened and Moved more towards the northwest during this period. It was re-named Birenda by La Reunion RSMC when it passed across the 90°E on the 28th.

Severe tropical cyclone Elaine 15-20 March 1999

A low was evident within the monsoon trough in the Timor Sea from 8am on 12 March. It moved westward in a favourable environment and intensified to cyclone strength by the morning of the 16th. Further intensification to severe cyclone strength occurred in the following 24 hours as it continued on a path parallel to but well offshore from the WA coastline. Elaine changed direction to a more southerly course during the 18th March. As it moved further south it entered into an unfavourable high wind shear zone and started to weaken. By the morning of the 19th deep convection had been stripped from the centre. A resurgence of convection occurred in southern quadrants later on that day. By 8pm its movement changed towards the southeast and it weakened further to a tropical low by the morning of the 20th. The low crossed the coast between Geraldton and Kalbarri around 9am on the 20th. Heavy rain associated with the tropical low caused severe flooding in the town of Moora. Houses and commercial properties in the town were inundated. By the floodwaters and approximately 1000 people were evacuated.

Severe tropical cyclone Vance 16-23 March 1999

As the circulation associated with tropical cyclone Elaine moved slowly west, another low within the monsoon trough, which persisted over the north coast of Australia near Darwin for several days, moved into the Timor Sea. The low-level circulation, favourably situated beneath the upper ridge in a region of weak wind shear quickly intensified to become cyclone Vance. Vance moved quite steadily westsouthwest at first, then towards the southwest during the 20th and 21st March. It intensified steadily and was declared a severe Category 3 cyclone late on 19 March. It was further upgraded to a Category 5 (the highest category) overnight on 20 March. The Perth TCWC posted the first cyclone warning for the Exmouth/Onslow area at 10am on Saturday 20 March.

Late on 21 March the cyclone changed track to a due southerly course towards Exmouth/Onslow area. The eye of the cyclone passed across Exmouth Gulf, approximately 25 kilometres to the east of Exmouth and 80 kilometres to the west of Onslow during Monday morning 22nd March. A record wind gust speed for the Australian mainland of 267 kiolmetres per hour was recorded at the Learmonth Meteorological Office, 35 kilometres south of Exmouth shortly before midday.

At Onslow the maximum gust recorded was 174 kilometres per hour. The combination of very high seas and high tides caused severe erosion of the beachfront at Exmouth. At Onslow the storm tide left 3 large barges stranded on the edge of Beadon Creek. The cyclone crossed the southern part of Exmouth Gulf around 1pm then moved further inland and began to weken. By morning of 23March it was near the town of Mount Magnet and was moving southeast at 50 kilometres per hour. There was damage to property in the town of Cue when winds struck the town at around 4am. Vance was downgraded to a Cat. 1 and passed to the northeast of Kalgoorlie around 3pm. Rain from the decaying cyclone caused flooding in the southern Goldfields. The main highway and the rain link to the eastern states were both cut by floodwaters. It moved into waters of the Great Australian Bight in the early hours of the 24th. The remnants of Vance caused gale force winds over parts of South Australia and Victoria later that day.

Severe tropical cyclone Frederic 25 March - 1 April 1999

A low in the monsoon trough formed off the north Kimberley coast on 25 March. A favourable upper air pattern and a region of weak vertical wind shear provided a suitable environment for the low to intensify. The low moved to the westsouthwest under the steering influence of a mid-level ridge to the southsoutheast. It attained cyclone strength on 27 March, the third tropical cyclone in the space of only 2 weeks to affect waters off the northwest coast of Australia. As Frederic moved further west it experienced easterly shear, that temporarily slowed its rate of development. The shear reduced overnight on 28 March and an eye became visible by the morning of the 29th. Frederic remained a severe cyclone as it traversed westward across the Indian Ocean. It crossed out of the WA region of responsibility on the 1st of April and then weakened rapidly due to wind shear.

Severe tropical cyclone Gwenda 2-7 April 1999

A tropical low formed in the weak monsoon trough just north of Darwin on 2 April 1999. Middle level ridging pushed the low westward over the following days through the Timor Sea. The low was sheared but a gradual organization and intensification occurred as the low spent 2 nights over the Timor Sea. Tropical cyclone Gwenda formed in the Indian Ocean to the southwest of Timor Overnight on 4 April 1999. It was the only cyclone to form in Australian waters during the month of April. During its lifetime it was a very intense cyclone of average size.

Gwenda intensified very rapidly late on 5 April. By the evening of the 6th the central pressure was estimated to be near 900 hPa (Category 5). It moved on a due southward course towards the Pilbara coast overnight on the 6 April and on the morning of the 7th the track changed towards the south-southeast towards the town of Port Hedland. The cyclone moved steadily towards Port Hedland during the day. During the evening as the cyclone approached to within 100 kilometres of Port Hedland the cyclone experienced very strong wind shear and weakened rapidly. The cyclone crossed the coast 50 kilometres to the east of Port Hedland around midnight. At Port Hedland gale force winds were reported for a period of about 5 hours commencing at 9pm in the evening. The strongest average winds recorded were 78 kilometers per hour. The maximum gust to 102 kilometres per hour occurred at 11.30pm. Following landfall the cyclone weakened further and eventually merged into the heat trough.

Tropical cyclone Hamish 20-21 April 1999

A tropical low was analysed to the southwest of Sumatra from the 16th of April. Under the favorable upper air outflow and weak steering flow of the low intensified to cyclone Hamish by the afternoon of 20 April. Hamish moved to the southwest but approached a zone of high level vertical wind shear to the south. The influence of the shear caused rapid weakening of the cyclone during the night of 21st April.

Tracks of tropical cyclones, 1998-99.

Season tracks 1998/99