Severe Tropical Cyclone Orson was the fourth most intense

cyclone In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...

ever recorded in the Australian region. Forming out of a

tropical low A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...

on 17 April 1989, Orson gradually intensified as it tracked towards the west. After attaining Category 5 intensity on 20 April, the storm began to track southward and accelerated. The following day, the cyclone reached its peak intensity with winds of 250 km/h (155 mph 10-minute sustained) and a

barometric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...

of 904 hPa (mbar). Orson maintained this intensity for nearly two days before making

landfall Landfall is the event of a storm moving over land after being over water. More broadly, and in relation to human travel, it refers to 'the first land that is reached or seen at the end of a journey across the sea or through the air, or the fact ...

near Dampier. The cyclone rapidly weakened after landfall as it accelerated to the southeast. After moving into the

Great Australian Bight The Great Australian Bight is a large oceanic bight, or open bay, off the central and western portions of the southern coastline of mainland Australia. Extent Two definitions of the extent are in use – one used by the International Hydrog ...

on 24 April, the storm dissipated. Despite Orson's extreme intensity, damage was relatively minimal as it struck a sparsely populated region of

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...

. Five people were killed offshore and damages amounted to (). The storm damaged a new gas platform, delaying the project for nearly two weeks. The most severe impacts took place in Pannawonica, where 70 homes were damaged. Following the storm, cleanup costs reached A$5 million (US$4.1 million). Due to the severity of the storm, the name ''Orson'' was

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after the season.

Meteorological history

Cyclone Orson originated out of a

, monitored by the

Australian Bureau of Meteorology Australian(s) may refer to: Australia * Australia, a country * Australians, citizens of the Commonwealth of Australia ** European Australians ** Anglo-Celtic Australians, Australians descended principally from British colonists ** Aboriginal Aus ...

, that formed northwest of

Darwin, Northern Territory Darwin ( ; Larrakia: ) is the capital city of the Northern Territory, Australia. With an estimated population of 147,255 as of 2019, the city contains the majority of the residents of the sparsely populated Northern Territory. It is the smalle ...

on 17 April 1989. The system tracked southwest throughout the day before turning due west and strengthening into a

tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...

, at which time it received the name ''Orson''. At this time, the

Joint Typhoon Warning Center The Joint typhoon Warning Center (JTWC) is a joint United States Navy – United States Air Force command in Pearl Harbor, Hawaii. The JTWC is responsible for the issuing of tropical cyclone warnings in the North-West Pacific Ocean, South P ...

(JTWC) also began monitoring the storm as Tropical Storm 28S. The forward motion of the storm gradually slowed as it intensified and on 19 April, Orson attained Category 3 status on the

Australian tropical cyclone intensity scale Tropical cyclones are ranked on one of five tropical cyclone intensity scales, according to their maximum sustained winds and which tropical cyclone basins they are located in. Only a few scales of classifications are used officially by the mete ...

, classifying Orson as a severe tropical cyclone. Later that day, as the storm attained Category 4 status, an

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

developed. By this time, Orson began to turn towards the southwest and on 20 April, the storm intensified into a Category 5 cyclone with winds of 210 km/h (130 mph 10-minute sustained). The JTWC also reported significant strengthening during the same period. They assessed Orson to have attained an intensity equivalent to a

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on the Saffir–Simpson hurricane scale on 22 April with winds of 260 km/h (160 mph 1-minute sustained). Around this time, the storm tracked directly over the North Rankin gas platform. The platform was in the 40 km (25 mi) wide eye of Orson for roughly 40 minutes. A weather station there recorded a

of 904

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(

mbar The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea leve ...

; 26.69

inHg Inch of mercury (inHg and ″Hg) is a non- SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury in heigh ...

) and wind gusts of 250 km/h (155 mph) before the station was damaged. This was, at the time, the lowest pressure ever recorded in the Australian region since records began. It was later surpassed by Severe Tropical Cyclone Gwenda in

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when that storm attained a pressure of 900 hPa (mbar). By this time, Cyclone Orson was roughly 555 km (345 mi) in diameter. Continuing on a southerly track, accelerating ahead of an approaching

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Norther ...

, Cyclone Orson made landfall, near Dampier, around 4:45 am AWST on 23 April (2045 UTC 22 April). with winds of 220 km/h (140 mph 10-minute sustained). The JTWC also reported that Orson had weakened, with winds at landfall estimated at 230 km/h (145 mph 1-minute sustained). Tracking at 28 km/h (17 mph), the weakening storm passed over Pannawonica. Less than 12 hours after landfall, the storm weakened below Category 3 status. By this time, the JTWC was no longer monitoring the system. Around 5:00 am AWST on 24 April (2100 UTC 23 April), Orson weakened to a tropical low while situated over southern Western Australia. Continuing to accelerate to nearly 50 km/h (31 mph), the remnants of the storm moved over the

late on 24 April. Several hours after moving back over water, the storm dissipated. The Australian Bureau of Meteorology uses 10-minute sustained winds, while the Joint Typhoon Warning Center uses one-minute sustained winds. The conversion factor between the two is 1.14. The Bureau of Meteorology's peak intensity for Orson was 250 km/h (155 mph) 10-minute sustained, or 290 km/h (180 mph) one-minute sustained. The JTWC's peak intensity for Orson was 260 km/h (160 mph) one-minute sustained, or 220 km/h (140 mph) 10-minute sustained.

Preparations and impact

As Cyclone Orson approached the coast of Western Australia, residents were urged to prepare for the storm; people proceeded to clean up litter, secure outdoor items and make sure their disaster kits were stocked. All 200 personnel from a A$1.5 billion gas platform off the coast were evacuated ahead of the storm. (Accessed via Lexis-Nexis) Since Cyclone Orson made landfall in a sparsely populated region, its effects were relatively light compared to its intensity. More than 20 fishermen were reported missing during the storm. On 23 April, a rescue mission with three aircraft recovered roughly 20 fisherman, while one was still missing. Offshore, the storm killed four Indonesian fishermen after their ships sank in swells up to produced by the storm. The North Rankin gas platform sustained minor damage despite wind gusts reaching 270 km/h (165 mph) and waves estimated at . The large swells also delayed the find of a major oil field that contained more than of oil. The waves knocked a drill rig used to find oil out of position; it would take several days for the drill rig to be repositioned. After an assessment of damage, it was found that the drill rig snapped off and broke the chains of two anchors before drifting nearly from the platform. The repositioning and cleanup of the drill rig delayed the project by nearly two weeks. The damages from Cyclone Orson increased the total cost of the platform to roughly (). Upon making landfall, Orson produced a

storm surge A storm surge, storm flood, tidal surge, or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low-pressure weather systems, such as cyclones. It is measured as the rise in water level above the n ...

of . This came during low tide, having a height of . Severe erosion was recorded along coastal areas, some losing nearly of rocks. (Accessed via Lexis-Nexis) Wind gusts in Dampier reached and a station near where Orson made landfall recorded a wind gust of . Harbour officials stated that several ships were knocked off their moorings and washed up onshore. In Karratha, the local

weather radar Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly puls ...

sustained roughly A$900,000 (US$760,000) in damages. A nearby airport was also damaged. The jetty at

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was severely damaged and eventually removed. The most severe damage took place in the mining town of Pannawonica, where 70 homes were damaged by the storm. Numerous trees and power lines were downed along the storm's path. Before dissipating, the storm left one additional person missing after contact was lost with his yacht. Later reports confirmed that the missing person drowned during the storm. Twenty people were also injured during the storm, 60 were left homeless and about 1,000 were affected. Total damages from the storm were estimated at A$20 million (US$16.8 million) and repair costs reached A$5 million (US$4.1 million). Due to the severity of the storm, the name ''Orson'' was

after the season.

References

External links

Track of Cyclone Orson
(provided by Australia Severe Weather) {{Featured article Orson, cyclone Orson Orson Orson Orson 1989 disasters in Australia

Meteorological history

Preparations and impact

See also

References

External links