User:ChocolateTrain/Cyclone Wallace

Severe Tropical Cyclone Wallace was a late-season tropical cyclone of moderate intensity which brought mild impacts to isolated parts of Australia and Indonesia. The ninth named tropical cyclone and sixth severe tropical cyclone of the 2018-19 Australian region cyclone season, Severe Tropical Cyclone Wallace originated from a weak tropical low which first appeared on 1 April 2019 over the Arafura Sea, a few hundred kilometres to the northeast of Darwin. The system was slow to develop initially, but began to intensify as it moved generally southwestwards away from the Arafura Sea, where it had meandered for several days. The tropical low reached cyclone intensity by 18:00 UTC on 5 April while passing between Timor and the Kimberley region of Western Australia. The system continued to strengthen as it moved through the western Timor Sea and into the Indian Ocean, albeit at a much slower rate than is typical for tropical cyclones, largely the result of high vertical wind shear. At 18:00 UTC on 8 April, the system strengthened into a severe tropical cyclone while located well to the northwest of Western Australia's Pilbara coastline. This coincided with the system's peak in intensity, characterised by sustained winds of 120 km/h (75 mph) and a minimum central barometric pressure of 980 hPa (28.94 inHg). The system was assessed as a low-end Category 3 severe tropical cyclone on the Australian scale by the Australian Bureau of Meteorology, and was equivalent to a minimal Category 1 hurricane on the Saffir-Simpson hurricane wind scale according to wind speed estimates from the United States' Joint Typhoon Warning Center. The storm weakened steadily thereafter in a generally unfavourable environment, falling below tropical cyclone status by 10 April. The remnant tropical low dissipated to the southwest of the Cocos (Keeling) Islands by 16 April.

Although Severe Tropical Cyclone Wallace was located well out to sea during its period of strongest winds, the system brought mild impacts to localised regions of Australia and Indonesia while intensifying early in its lifetime. In particular, Browse Island, off the coast of Western Australian, experienced sustained gale-force winds equivalent to Category 1 strength on the Australian scale for a short period on 6 April. Croker Island, located 230 km (145 mi) northeast of Darwin, recorded 159.0 mm (6.3 in) of rainfall over a 48-hour period as the storm passed nearby as a tropical low. No injuries or fatalities are known to have been caused by the cyclone, either directly or indirectly, and no infrastructural or economic damage has been attributed to the storm.

Origin and initial development
Following the breakdown of the active monsoon trough which had contributed to the formation of Severe Tropical Cyclones Trevor and Veronica two weeks prior, the Bureau of Meteorology forecast the potential for an increase in cross-equatorial flow from the northern Pacific Ocean in the next week. It was highlighted that such an event may lead to a final burst in monsoonal activity in Australian tropical waters in the last month of northern Australia's traditional wet season. On 31 March, a weak monsoon trough began to develop in the Arafura and Coral Seas north of Queensland, generating atmospheric conditions which were more supportive of tropical low formation, and subsequently spawning a shallow and short-lived tropical low southeast of Papua New Guinea. As the monsoon trough extended westwards towards Timor on 1 April, the revival of convective activity in the region led to the formation of a weak tropical low a few hundred kilometres to the northeast of Darwin, embedded within the developing monsoon trough. Upon formation, the Bureau of Meteorology assigned the nascent system the official identifier code 21U. Monsoon troughs which form late in the wet season typically do not migrate far southwards onto the Australian continent, and hence spend the majority of their existence over warm tropical waters, increasing the likelihood of embedded tropical lows developing into tropical cyclones.

The newly formed tropical low remained predominantly slow moving and erratic over the following few days, showing only gradual signs of organisation in the marginally favourable atmospheric environment. Overnight on 3 April, the system began to deepen more rapidly as it assumed a steadier track to the west-southwest, developing sustained deep convection in the western semicircle of the low-level circulation. The moist atmosphere, strong poleward upper outflow channel and very warm sea surface temperatures of 29-30 degrees Celsius (84-86 degrees Fahrenheit) continued to assist organisation into 4 April, at which point development began to stall. The influence of strong vertical wind shear of 30-35 knots (55-65 km/h; 35-40 mph) on the northwestern side of the low was causing the displacement of the deep convection, confining it to the west and south of the system and leaving the low-level circulation centre exposed. The speed and direction of movement of the tropical low acted to offset some of the negative effects of the wind shear, however, and fuelled by the very warm sea surface temperatures, intensification proceeded, albeit slowly. The Joint Typhoon Warning Center assessed the system as having developed sustained gale-force winds (one-minute mean) at 00:00 UTC on 5 April—equivalent to a tropical storm on the Saffir-Simpson hurricane wind scale—and designated the storm as Tropical Cyclone 23S accordingly. The storm acquired gale-force winds (ten-minute mean) in the southwestern quadrant of the circulation at 15:00 UTC, although it was still officially classified as a tropical low as the gales did not extend more than halfway around the system. The low reached tropical cyclone strength on the Australian scale soon afterwards, at 18:00 UTC on 5 April, while located about halfway between Timor and the northern coastline of Western Australia. The cyclone was named Wallace by the Bureau of Meteorology, becoming the ninth named tropical cyclone of the 2018-19 Australian region cyclone season, and the first to form over the Timor Sea since Severe Tropical Cyclone Frances in April 2017.

Intensification
Following its official classification as a tropical cyclone, Wallace continued to intensify at a faster rate than it had previously, attaining ten-minute sustained winds of 85 km/h (50 mph) by 6 April. The renewed phase of steady strengthening proved fleeting, however, as strong vertical wind shear and upper-level convergence on the eastern side of the storm once again nullified the otherwise favourable conditions and arrested the intensification trend. No more strengthening occurred during the following 24 hours as the storm struggled to maintain organisation, with its deep convection and central dense overcast being continually displaced to the west by the hostile winds, as clearly apparent on visible satellite imagery. At 00:00 UTC on 7 April, a marginal decrease in the wind shear strength allowed Wallace to increase in intensity slightly to Category 2 on the Australian scale, with ten-minute sustained winds of 95 km/h (60 mph). Once again, however, the strengthening was short-lived.

Late on 7 April, Tropical Cyclone Wallace shifted from its generally westwards motion, executing a slow turn to the southwest under the influence of a strong mid-level ridge anchored over the Australian mainland, to the system's southeast. Around the same time, the storm saw an increase in favourable upper-level divergence, and deep convection increased to the west of the circulation centre as a result. A few hours later, at 00:00 UTC on 8 April, the vertical wind shear which had been hindering the development of the cyclone decreased considerably to 13 knots (24 km/h; 15 mph). Despite this shear strength still being moderate, the cyclone began to intensify, reaching a wind speed of 100 km/h (65 mph), given the otherwise highly conducive environment comprising sea surface temperatures near 31 degrees Celsius (88 degrees Fahrenheit), high moisture content in the low- and mid-level layers of the troposphere, and a strong poleward outflow channel.

During the period of less damaging atmospheric dynamics, microwave satellite imagery indicated that Wallace had been able to develop a more robust inner core. As a result, the system became better able to sustain greater deep convection despite the wind shear, allowing a more circular and symmetrical central dense overcast to develop over the circulation centre. Further intensification ensued during the night of 8-9 April, local time, while within the diurnally favourable period for tropical convection. A dimple feature became evident in enhanced infrared satellite imagery around 12:00 UTC on 8 April, indicating the system was developing a primitive eye. The cyclone reached its peak intensity at 18:00 UTC on 8 April as a low-end Category 3 severe tropical cyclone, located approximately 530 km (330 mi) to the north-northwest of the town of Karratha. The Bureau of Meteorology estimated ten-minute sustained winds to be at 120 km/h (75 mph), gusting to 165 km/h (105 mph), with a minimum central barometric pressure of 980 hPa (28.94 inHg). Concurrently, the Joint Typhoon Warning Center classified the cyclone as the equivalent of a low-end Category 1 hurricane on the Saffir-Simpson hurricane wind scale, with one-minute sustained wind speeds equal in speed to the ten-minute sustained winds. The Bureau of Meteorology's analysis of the system using the Dvorak technique yielded a peak current intensity number of 4.5 at this time. Gale-force winds were estimated by the Bureau of Meteorology to extend to a maximum radius of 80 nautical miles (150 km; 90 mi) from the storm's centre, and hurricane-force winds to a radius of 15 nautical miles (30 km; 15 mi). Wallace was the sixth severe tropical cyclone of the 2018-19 Australian region cyclone season, the first such occurrence since the 2014-15 season.

Weakening and dissipation
Only six hours after reaching Category 3 strength, Severe Tropical Cyclone Wallace began to weaken under the influence of increasing vertical wind shear that was allowing dry air to become entrained into the circulation. The system was downgraded to Category 2 on the Australian scale at 00:00 UTC on 9 April and simultaneously to tropical storm strength on the Saffir-Simpson hurricane wind scale as the visual presentation of the system on satellite imagery rapidly deteriorated. The central dense overcast became detached and displaced significantly from the system's core, leaving the circulation centre exposed. As a consequence, the wind field became increasingly asymmetrical, with the majority of sustained gales present only in the southern quadrants of the system, underneath the remaining deep convection. The storm weakened to Category 1 at 12:00 UTC on 9 April, however being in a diurnally favourable period acted to temper the weakening trend until the following morning (local time). The system became devoid of sustained gales in the northern semicircle at 00:00 UTC on 10 April, and was downgraded by the Bureau of Meteorology to a vigorous gale-force tropical low while located approximately 600 km (375 mi) to the north-northwest of the town of Exmouth on Western Australia's North West Cape. Despite gale-force winds of 75 km/h (45 mph) persisting in the southern quadrants, the Bureau of Meteorology's definition of a tropical cyclone requires that sustained gales be present more than halfway around the system—a condition that the rapidly weakening Wallace no longer fulfilled. Noting the completely exposed low-level circulation centre and the forecast increase in the already high along-track wind shear, the Joint Typhoon Warning Center also issued their final advisory on the cyclone as this time; however, the system's classification as a tropical storm was maintained until 18 hours later.

Ex-Tropical Cyclone Wallace proceeded to track generally westwards across the Indian Ocean, away from the coastline of Western Australia, while continuing to weaken. The shallow remnant low passed to the south of the Cocos (Keeling) Islands on 14 April, before dissipating as a tropical low on 16 April over the central Indian Ocean as it approached the eastern border of Météo-France La Réunion's area of responsibility.

Impacts
Severe Tropical Cyclone Wallace did not produce any major impacts on any land areas, primarily as it did not make landfall and was located far from land during its period of highest intensity. As such, no damage or economic losses were attributed to the system, and no fatalities or injuries are reported to have occurred as a result of the cyclone. During the system's formative stages as a tropical low and a Category 1 tropical cyclone, however, Wallace did track near enough to land areas to cause some minor impacts.

Indonesia
The tropical low which developed into Cyclone Wallace originated in the region of Indonesia's Maluku Islands, and tracked near the island of Timor (comprising the country of East Timor and part of Indonesia's East Nusa Tenggara province). As such, the Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) issued a number of weather advisories on the system while its impacts were being felt on land areas. On 5 April, the BMKG indicated that strong winds were being experienced in the provinces of East Nusa Tenggara, West Nusa Tenggara, Maluku and Papua due to the proximity of the system, in addition to moderate rainfall in these areas and southeastern Sulawesi. At 00:00 UTC on 6 April—six hours after intensification into a Category 1 tropical cyclone on the Australian scale— sustained winds in parts of East Nusa Tenggara were estimated at approximately 45 km/h (30 mph). Waves heights of up to 4.0 m (13 ft) in waters south of Kupang and the islands of Savu and Rote were also noted by the BMKG, increasing to 6.0 m (20 ft) further off the coast of East Nusa Tenggara, in the Indian Ocean.

Australia
Wallace's precursor tropical low tracked southwestwards across the Arafura and Timor Seas while strengthening early in its lifetime, bringing it close to northwestern areas of the Northern Territory's Top End and Western Australia's Pilbara region. Thunderstorms and periods of heavy and occasionally torrential rainfall were experienced to the northeast of Darwin at Croker Island throughout 3–4 April, with the island's airport recording as total of 159.0 mm (6.3 in) of precipitation in the 48 hours leading up to 9:00 a.m. on 5 April (local time), including 52.0 mm (2.0 in) in one hour on 4 April. Other significant rainfall totals throughout the event included 80.6 mm (3.2 in) at Troughton Island, 62.6 mm (2.5 in) at Truscott-Mungalalu Airport, and 56.2 mm (2.2 in) at Darwin Airport. Browse Island, situated approximately 335 km (210 mi) west of the Western Australian town of Kalumburu, was the only land area to experienced sustained gale-force winds. At 11:00 UTC on 6 April, sustained winds peaked at 63 km/h (39 mph)—the threshold of Category 1 intensity on the Australian scale—while Tropical Cyclone Wallace passed nearby on the northwest, at that stage at the upper end of Category 1 intensity. Troughton Island also observed a wind gust of 70 km/h (43 mph) just before 21:00 UTC on 5 April.

The Cocos (Keeling) Islands received light rainfall on 14 April as Ex-Tropical Cyclone Wallace passed to the south. West Island, the capital of the Cocos (Keeling) Islands, recorded a total of 11.2 mm (0.4 in) of precipitation in the 24 hours leading up to 9:00 a.m. on 15 April (local time).

Warnings and advisories
The Bureau of Meteorology issued a total of 29 tropical cyclone forecast track maps throughout Severe Tropical Cyclone Wallace's duration as the official regional specialised meteorological centre for the Australian region. The United States' Joint Typhoon Warning Center also issued advisories on the system in an unofficial capacity.

Official tropical cyclone advice and forecast track map products were first issued by the Bureau of Meteorology at 00:00 UTC on 4 April, when the system was situated approximately 350 km (220 mi) north of Darwin as a tropical low. The first tropical cyclone watches associated with the system were also issued at this time, covering the area from Cockatoo Island to the northernmost section of Western Australia's coastline, east of Kalumburu, indicating the potential for sustained gale-force winds to develop within the watch area within 24 to 48 hours. The Bureau of Meteorology increased the tropical cyclone watch to a tropical cyclone warning at 00:00 UTC on 5 April, accounting for the potential for gales to occur within 24 hours, and began the issue of three-hourly updates accordingly. Cape Leveque and Beagle Bay were also placed under a cyclone watch at this time, with this converted to a cyclone warning nine hours later. As the system tracked further west-southwestwards away from the Kimberley coastline, damaging gale-force winds were deemed unlikely to impact coastal communities, and all cyclone warnings were subsequently discontinued at 00:00 UTC on 6 April—a total of 48 hours after they were first issued.

Media coverage
Upon developing into a tropical cyclone, Wallace was the subject of several news articles from various media companies. Approximately two weeks prior to the development of the system, Severe Tropical Cyclone Veronica had approached the Pilbara coastline of Western Australia as a Category 4 severe tropical cyclone on the Australian scale, before becoming slow-moving just of the coast for an extended period. Veronica brought significant rainfall and flooding, damage to farms and port infrastructure, and was responsible for up to AU$2 billion (US$1.4 billion) in delays to iron ore exports. Concerns were initially held that Cyclone Wallace may make landfall on the same section of coastline, with the potential to lead to more severe impacts in the region. Numerical weather model runs early in the cyclone's development stage predicted that such a scenario was possible, with the Bureau of Meteorology noting on numerous occasions in their tropical cyclone technical bulletins that a landfall in the Pilbara or North West Cape could not be ruled out. As time progressed, however, it became clear that such an outcome was increasingly unlikely, and Wallace was consequently forecast to remain out over open ocean.