Cyclone Orson

Severe Tropical Cyclone Orson was the fourth most intense cyclone ever recorded in the Australian region.[2] Forming out of a tropical low 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 of 904 hPa (mbar).[1] Orson maintained this intensity for nearly two days before making landfall near Dampier. The cyclone rapidly weakened after landfall as it accelerated to the southeast. After moving into the Great Australian Bight on 24 April, the storm dissipated.

Category 5 Australian region cyclone in 1989

Severe Tropical Cyclone Orson
Category 5 severe tropical cyclone (Aus scale)
Category 5-equivalent tropical cyclone (SSHWS)

Satellite image of Cyclone Orson near peak intensity
Formed 17 April 1989
Dissipated 24 April 1989
Highest winds 10-minute sustained: 250 km/h (155 mph)
1-minute sustained: 260 km/h (160 mph)
Lowest pressure 904 hPa (mbar); 26.7 inHg
(Fourth lowest pressure in Australian basin[1])
Fatalities 5 direct
Damage $16.8 million (1989 USD)
Areas affected Western Australia
Part of the 1988–89 Australian region cyclone season

Despite Orson’s extreme intensity, damage was relatively minimal as it struck a sparsely populated region of Western Australia. Five people were killed offshore and damages amounted to A$20 million (US$16.8 million). 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 retired after the season.

. . . Cyclone Orson . . .

Map plotting the storm’s track and intensity, according to the Saffir–Simpson scale

Map key

  Tropical depression (≤38 mph, ≤62 km/h)
  Tropical storm (39–73 mph, 63–118 km/h)
  Category 1 (74–95 mph, 119–153 km/h)
  Category 2 (96–110 mph, 154–177 km/h)
  Category 3 (111–129 mph, 178–208 km/h)
  Category 4 (130–156 mph, 209–251 km/h)
  Category 5 (≥157 mph, ≥252 km/h)

Storm type
Extratropical cyclone / Remnant low / Tropical disturbance / Monsoon depression

Cyclone Orson originated out of a tropical low, monitored by the Australian Bureau of Meteorology, that formed northwest of Darwin, Northern Territory on 17 April 1989. The system tracked southwest throughout the day before turning due west and strengthening into a tropical cyclone, at which time it received the name Orson.[3] At this time, the Joint Typhoon Warning Center (JTWC) also began monitoring the storm as Tropical Storm 28S.[4] 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, classifying Orson as a severe tropical cyclone.[2] Later that day, as the storm attained Category 4 status, an eye developed. By this time, Orson began to turn towards the southwest and on 20 April,[3] the storm intensified into a Category 5 cyclone with winds of 210 km/h (130 mph 10-minute sustained).[2]

The JTWC also reported significant strengthening during the same period. They assessed Orson to have attained an intensity equivalent to a Category 5 hurricane on the Saffir–Simpson hurricane scale on 22 April with winds of 260 km/h (160 mph 1-minute sustained).[4] 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.[5] A weather station there recorded a barometric pressure of 904 hPa (mbar; 26.69 inHg)[1] 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.[5] It was later surpassed by Severe Tropical Cyclone Gwenda in 1999 when that storm attained a pressure of 900 hPa (mbar).[6] By this time, Cyclone Orson was roughly 555 km (345 mi) in diameter.[7]

Cyclone Orson intensifying on 21 April

Continuing on a southerly track, accelerating ahead of an approaching cold front, Cyclone Orson made landfall, near Dampier, around 4:45 am AWST on 23 April (2045 UTC 22 April).[5] with winds of 220 km/h (140 mph 10-minute sustained).[2] The JTWC also reported that Orson had weakened, with winds at landfall estimated at 230 km/h (145 mph 1-minute sustained).[8] Tracking at 28 km/h (17 mph), the weakening storm passed over Pannawonica.[5] Less than 12 hours after landfall, the storm weakened below Category 3 status.[2] By this time, the JTWC was no longer monitoring the system.[8] Around 5:00 am AWST on 24 April (2100 UTC 23 April), Orson weakened to a tropical low while situated over southern Western Australia.[2] Continuing to accelerate to nearly 50 km/h (31 mph), the remnants of the storm moved over the Great Australian Bight late on 24 April.[3] Several hours after moving back over water, the storm dissipated.[5]

The Australian Bureau of Meteorology uses 10-minute sustained winds, while the Joint Typhoon Warning Center uses one-minute sustained winds.[9] The conversion factor between the two is 1.14.[10] 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.[2][10] 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.[8][10]

. . . Cyclone Orson . . .

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. . . Cyclone Orson . . .