Cyclone Gwenda
Encyclopedia
Severe Tropical Cyclone Gwenda was tied for the most intense Australian tropical cyclone
on record, with a barometric pressure of 900 hPa
(mbar
). Forming out of a tropical disturbance over the Arafura Sea
on 2 April 1999, the precursor to Gwenda tracked slowly westward and gradually became more organised. On 4 April, the system developed into a Category 1 cyclone and was named Gwenda. It began to undergo explosive intensification
the following day, and in a 30-hour span ending early on 7 April, the storm's maximum 10-minute sustained wind speed increased from 75 km/h (45 mph) to 225 km/h (140 mph) and its barometric pressure decreased to 900 hPa (mbar). The Joint Typhoon Warning Center
reported that the storm had peaked as a high-end Category 4 equivalent on the Saffir–Simpson Hurricane Scale.
Increasing wind shear
and an acceleration in forward speed caused Gwenda to quickly weaken. Less than 18 hours after peaking, the storm made landfall
roughly 45 km (28 mi) east of Port Hedland, Western Australia
with winds of 100 km/h (65 mph). After moving ashore, Gwenda abruptly stalled before dissipating on 8 April. Although it was once an extremely intense cyclone, the factors that caused its deterioration also prevented significant damage. Rainfall from the storm peaked at 205 mm (8.1 in). Minor structural damage was reported, and only localised flooding was recorded. Following its usage, the name Gwenda was retired at the end of the season.
. Drifting westward, the system gradually became better organised, and early on 2 April, the Australian Bureau of Meteorology classified it as a tropical low. Over the following two days, the low continued to mature; on 4 April, the Bureau of Meteorology upgraded it to a Category 1 cyclone and named it Gwenda. Around the same time, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert
as deep convection
became concentrated around the centre of circulation
and the system's outflow
significantly improved. Located within an environment of low to moderate wind shear, the storm was expected to intensify.
By 5 April, the JTWC began issuing advisories on Gwenda, classifying it as a weak tropical storm and designating it as Cyclone 32S. Tracking southwest in response to a subtropical
ridge
to the south, Gwenda began to rapidly intensify
. Its forward motion significantly decreased as it turned due south before curving towards the southeast, and in a 30-hour span, maximum winds around the centre of the storm increased from 75 km/h (45 mph) to 225 km/h (140 mph). The barometric pressure decreased by 90 hPa (mbar), making Gwenda one of the fastest intensifying storms on record. At the end of the intensification phase on 7 April, the Bureau of Meteorology classified the storm as a Category 5 cyclone, the third of the season, with a pressure of 900 hPa (mbar). The JTWC also reported a substantial increase in intensity, classifying Gwenda as a high-end Category 4 equivalent on the Saffir–Simpson Hurricane Scale with 1-minute sustained winds of 240 km/h (150 mph).
Upon attaining peak intensity, Gwenda displayed a well-defined 30 km (18.6 mi) eye
surrounded by deep convection. At this time, some monitoring satellites estimated that it had attained winds of 260 km/h (160 mph), equivalent to a Category 5 hurricane. While Gwenda was active, the Bureau of Meteorology stated that its intensity peaked with winds of 215 km/h (125 mph) and a pressure of 915 hPa (mbar). Hours after attaining this intensity, increasing wind shear began to impact the cyclone, causing convection to become elongated and the eye less defined. Rapid weakening commenced as Gwenda turned southeastward towards the Pilbara coastline.
The cyclone continued to deteriorate as it approached Western Australia
, with convection displaced ahead of its centre. In addition to the wind shear, Gwenda's forward speed suddenly increased, leading to further disorganisation. Late on 7 April, the centre of Gwenda made landfall as a Category 2 cyclone roughly 45 km (28 mi) east of Port Hedland with winds of 100 km/h (65 mph). The JTWC estimated the storm to have been slightly stronger at landfall, with winds near 120 km/h (75 mph). Shortly after moving inland, the JTWC issued their final advisory on the weakening storm. The Bureau of Meteorology continued to monitor Gwenda as it abruptly stalled just onshore. However, convection associated with Gwenda continued to stream southeastward due to high wind shear. The storm's remnants persisted for several hours before dissipating early on 8 April.
The Australian Bureau of Meteorology uses 10-minute sustained winds
, while the Joint Typhoon Warning Center uses one-minute sustained winds. The Bureau of Meteorology's peak intensity for Gwenda was 225 km/h (140 mph) 10-minute sustained, or 260 km/h (160 mph) one-minute sustained. The JTWC's peak intensity for Gwenda was 240 km/h (150 mph) one-minute sustained, or 220 km/h (130 mph) 10-minute sustained.
two weeks earlier, which had destroyed or severely damaged 40% of the homes in Exmouth, residents in Western Australia
heeded warnings of the impending cyclone. By 6 April, the Bureau of Meteorology had issued storm warnings for areas between Port Hedland and Exmouth
. State Emergency Service
s throughout the threatened areas were put on high alert. Several offshore oil platforms were shut down and evacuated on 6 April and did not reopen until 15 April. Hours before the storm made landfall, the State Emergency Services stated that "Gwenda potentially had the destructive power of Cyclone Vance", and urged residents to seek appropriate shelter. Shortly before the storm struck, Len Broadbridge, director of the Western Australia Bureau of Meteorology declared, "Port Hedland is now in grave danger."
Despite the cyclone's strength, its effects were relatively minor. Winds up to 100 km/h (62.1 mph) were recorded in Port Hedland, leading to minor structural damage. The police in Port Hedland reported severe damage to one house, but no deaths or injuries. Emergency crews were sent to the northwest coast of Australia to assist in cleaning up damages, but found no "major incidents" or significant damage as a result of the storm. A group of five people on a camping trip in the Outback were stranded by the storm, when heavy rains swept away their vehicle. The group "would have perished", but one of its members hiked non-stop for twenty four hours to a manganese mine where he contacted rescuers.
In a 12-hour span, Port Hedland recorded 86 mm (3.4 in) of rain, well-above the average April precipitation total of 23 mm (0.905511811023622 in). Towns in the Pilbara region of Australia received heavy rainfall, amounting to 130 mm (5.1 in) in some places. Carlindi picked up the greatest rainfall total of 205 mm (8.1 in). This rain caused localised flooding, especially in Nullagine
where the river traversing the town broke its banks. The rainfall contributed to a long-term cooling effect in northern Western Australia, leading to below-average temperatures for much of April.
At its peak Gwenda was the strongest storm on record to form near Australia, surpassing Cyclone Orson
in 1989. Gwenda held this record until 2003, when its maximum winds were eclipsed by Cyclone Inigo
. Despite the minimal damage, the name was retired from the circulating lists of tropical cyclone names for the Australian Region.
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
on record, with a barometric pressure of 900 hPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
(mbar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
). Forming out of a tropical disturbance over the Arafura Sea
Arafura Sea
The Arafura Sea lies west of the Pacific Ocean overlying the continental shelf between Australia and New Guinea.-Geography:The Arafura Sea is bordered by Torres Strait and through that the Coral Sea to the east, the Gulf of Carpentaria to the south, the Timor Sea to the west and the Banda and Ceram...
on 2 April 1999, the precursor to Gwenda tracked slowly westward and gradually became more organised. On 4 April, the system developed into a Category 1 cyclone and was named Gwenda. It began to undergo explosive intensification
Rapid deepening
Rapid deepening, also known as rapid intensification, is a meteorological condition that occurs when the minimum sea-level atmospheric pressure of a tropical cyclone decreases drastically in a short period of time. The National Weather Service describes rapid deepening as a decrease of...
the following day, and in a 30-hour span ending early on 7 April, the storm's maximum 10-minute sustained wind speed increased from 75 km/h (45 mph) to 225 km/h (140 mph) and its barometric pressure decreased to 900 hPa (mbar). The Joint Typhoon Warning Center
Joint Typhoon Warning Center
The Joint Typhoon Warning Center is a joint United States Navy – United States Air Force task force located at the Naval Maritime Forecast Center in Pearl Harbor, Hawaii...
reported that the storm had peaked as a high-end Category 4 equivalent on the Saffir–Simpson Hurricane Scale.
Increasing wind shear
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
and an acceleration in forward speed caused Gwenda to quickly weaken. Less than 18 hours after peaking, the storm made landfall
Landfall (meteorology)
Landfall is the event of a tropical cyclone or a waterspout coming onto land after being over water. When a waterspout makes landfall it is reclassified as a tornado, which can then cause damage inland...
roughly 45 km (28 mi) east of Port Hedland, Western Australia
Port Hedland, Western Australia
Port Hedland is the highest tonnage port in Australia and largest town in the Pilbara region of Western Australia, with a population of approximately 14,000 ....
with winds of 100 km/h (65 mph). After moving ashore, Gwenda abruptly stalled before dissipating on 8 April. Although it was once an extremely intense cyclone, the factors that caused its deterioration also prevented significant damage. Rainfall from the storm peaked at 205 mm (8.1 in). Minor structural damage was reported, and only localised flooding was recorded. Following its usage, the name Gwenda was retired at the end of the season.
Meteorological history
Severe Tropical Cyclone Gwenda originated from a weak tropical disturbance that formed on 1 April over the Arafura SeaArafura Sea
The Arafura Sea lies west of the Pacific Ocean overlying the continental shelf between Australia and New Guinea.-Geography:The Arafura Sea is bordered by Torres Strait and through that the Coral Sea to the east, the Gulf of Carpentaria to the south, the Timor Sea to the west and the Banda and Ceram...
. Drifting westward, the system gradually became better organised, and early on 2 April, the Australian Bureau of Meteorology classified it as a tropical low. Over the following two days, the low continued to mature; on 4 April, the Bureau of Meteorology upgraded it to a Category 1 cyclone and named it Gwenda. Around the same time, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert
Tropical Cyclone Formation Alert
A Tropical Cyclone Formation Alert is a bulletin released by the U.S. Navy-operated Joint Typhoon Warning Center in Honolulu, Hawaii or the Naval Maritime Forecast Center in Norfolk, Virginia, warning of the possibility of a tropical cyclone forming from a tropical disturbance that has been...
as deep convection
Atmospheric convection
Atmospheric convection is the result of a parcel-environment instability, or temperature difference, layer in the atmosphere. Different lapse rates within dry and moist air lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to...
became concentrated around the centre of circulation
Eye (cyclone)
The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the second most severe weather of a cyclone...
and the system's outflow
Outflow (meteorology)
Outflow, in meteorology, is air that flows outwards from a storm system. It is associated with ridging, or anticyclonic flow. In the low levels of the troposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud on weather...
significantly improved. Located within an environment of low to moderate wind shear, the storm was expected to intensify.
By 5 April, the JTWC began issuing advisories on Gwenda, classifying it as a weak tropical storm and designating it as Cyclone 32S. Tracking southwest in response to a subtropical
Subtropical cyclone
A subtropical cyclone is a weather system that has some characteristics of a tropical and an extratropical cyclone. As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical or extratropical cyclones. They were officially recognized by the National...
ridge
Ridge (meteorology)
A ridge is an elongated region of relatively high atmospheric pressure, the opposite of a trough....
to the south, Gwenda began to rapidly intensify
Rapid deepening
Rapid deepening, also known as rapid intensification, is a meteorological condition that occurs when the minimum sea-level atmospheric pressure of a tropical cyclone decreases drastically in a short period of time. The National Weather Service describes rapid deepening as a decrease of...
. Its forward motion significantly decreased as it turned due south before curving towards the southeast, and in a 30-hour span, maximum winds around the centre of the storm increased from 75 km/h (45 mph) to 225 km/h (140 mph). The barometric pressure decreased by 90 hPa (mbar), making Gwenda one of the fastest intensifying storms on record. At the end of the intensification phase on 7 April, the Bureau of Meteorology classified the storm as a Category 5 cyclone, the third of the season, with a pressure of 900 hPa (mbar). The JTWC also reported a substantial increase in intensity, classifying Gwenda as a high-end Category 4 equivalent on the Saffir–Simpson Hurricane Scale with 1-minute sustained winds of 240 km/h (150 mph).
Upon attaining peak intensity, Gwenda displayed a well-defined 30 km (18.6 mi) eye
Eye (cyclone)
The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the second most severe weather of a cyclone...
surrounded by deep convection. At this time, some monitoring satellites estimated that it had attained winds of 260 km/h (160 mph), equivalent to a Category 5 hurricane. While Gwenda was active, the Bureau of Meteorology stated that its intensity peaked with winds of 215 km/h (125 mph) and a pressure of 915 hPa (mbar). Hours after attaining this intensity, increasing wind shear began to impact the cyclone, causing convection to become elongated and the eye less defined. Rapid weakening commenced as Gwenda turned southeastward towards the Pilbara coastline.
The cyclone continued to deteriorate as it approached Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
, with convection displaced ahead of its centre. In addition to the wind shear, Gwenda's forward speed suddenly increased, leading to further disorganisation. Late on 7 April, the centre of Gwenda made landfall as a Category 2 cyclone roughly 45 km (28 mi) east of Port Hedland with winds of 100 km/h (65 mph). The JTWC estimated the storm to have been slightly stronger at landfall, with winds near 120 km/h (75 mph). Shortly after moving inland, the JTWC issued their final advisory on the weakening storm. The Bureau of Meteorology continued to monitor Gwenda as it abruptly stalled just onshore. However, convection associated with Gwenda continued to stream southeastward due to high wind shear. The storm's remnants persisted for several hours before dissipating early on 8 April.
The Australian Bureau of Meteorology uses 10-minute sustained winds
Maximum sustained wind
The maximum sustained winds associated with a tropical cyclone are a common indicator of the intensity of the storm. Within a mature tropical cyclone, they are found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are...
, while the Joint Typhoon Warning Center uses one-minute sustained winds. The Bureau of Meteorology's peak intensity for Gwenda was 225 km/h (140 mph) 10-minute sustained, or 260 km/h (160 mph) one-minute sustained. The JTWC's peak intensity for Gwenda was 240 km/h (150 mph) one-minute sustained, or 220 km/h (130 mph) 10-minute sustained.
Preparations, impact and records
Already suffering from the effects of Cyclone VanceCyclone Vance
Cyclone Vance was a tropical cyclone that struck Western Australia during the active 1998–99 Australian region cyclone season, and was also one of six tropical cyclones to form off the coast of Australia during that season. When making landfall the Learmonth Meteorological Office recorded the...
two weeks earlier, which had destroyed or severely damaged 40% of the homes in Exmouth, residents in Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
heeded warnings of the impending cyclone. By 6 April, the Bureau of Meteorology had issued storm warnings for areas between Port Hedland and Exmouth
Exmouth, Western Australia
-Further reading:* Western Australia. Ministry for Planning. Exmouth-Learmonth structure plan. Perth, W.A. : Western Australian Planning Commission...
. State Emergency Service
State Emergency Service
A State Emergency Service is an Australian volunteer organisation that provides emergency help during and after declared disasters. The SES is also the primary or secondary agency for emergencies, such as storm damage,flood damage, building damage, traffic hazards and road crash rescue...
s throughout the threatened areas were put on high alert. Several offshore oil platforms were shut down and evacuated on 6 April and did not reopen until 15 April. Hours before the storm made landfall, the State Emergency Services stated that "Gwenda potentially had the destructive power of Cyclone Vance", and urged residents to seek appropriate shelter. Shortly before the storm struck, Len Broadbridge, director of the Western Australia Bureau of Meteorology declared, "Port Hedland is now in grave danger."
Despite the cyclone's strength, its effects were relatively minor. Winds up to 100 km/h (62.1 mph) were recorded in Port Hedland, leading to minor structural damage. The police in Port Hedland reported severe damage to one house, but no deaths or injuries. Emergency crews were sent to the northwest coast of Australia to assist in cleaning up damages, but found no "major incidents" or significant damage as a result of the storm. A group of five people on a camping trip in the Outback were stranded by the storm, when heavy rains swept away their vehicle. The group "would have perished", but one of its members hiked non-stop for twenty four hours to a manganese mine where he contacted rescuers.
In a 12-hour span, Port Hedland recorded 86 mm (3.4 in) of rain, well-above the average April precipitation total of 23 mm (0.905511811023622 in). Towns in the Pilbara region of Australia received heavy rainfall, amounting to 130 mm (5.1 in) in some places. Carlindi picked up the greatest rainfall total of 205 mm (8.1 in). This rain caused localised flooding, especially in Nullagine
Nullagine, Western Australia
Nullagine is an old goldrush town in Western Australia's Pilbara region. It is located on the Nullagine River 296 km south-east of Port Hedland and 1,364 km north-north-east of Perth on the old Great Northern Highway....
where the river traversing the town broke its banks. The rainfall contributed to a long-term cooling effect in northern Western Australia, leading to below-average temperatures for much of April.
At its peak Gwenda was the strongest storm on record to form near Australia, surpassing Cyclone Orson
Cyclone Orson
Severe Tropical Cyclone Orson was the fourth most intense cyclone ever recorded in the Australian region. 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...
in 1989. Gwenda held this record until 2003, when its maximum winds were eclipsed by Cyclone Inigo
Cyclone Inigo
Cyclone Inigo was tied for the most intense recorded cyclone in the Australian region. It developed from a tropical low that crossed eastern Indonesia in late March 2003. Becoming a named tropical cyclone on 1 April, Inigo rapidly intensified as it tracked southwestward, reaching a minimum central...
. Despite the minimal damage, the name was retired from the circulating lists of tropical cyclone names for the Australian Region.
See also
- 1998–99 Australian region cyclone season1998–99 Australian region cyclone seasonThe 1998–99 Australian region cyclone season was an event in the ongoing cycle of tropical cyclone formation. It began on 1 November 1998 and ended on 30 April 1999...
- Lists of tropical cyclones
- List of the most intense tropical cyclones