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Cyclone Seroja

Category 3 Australian region cyclone in 2021

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Through Thunderstorm, Australia and Convection

Due to the presence of Tropical Cyclone Odette in Seroja's vicinity, interaction was anticipated as the storm moved away from Indonesia and Timor-Leste. Its intensity fluctuated as it moved southwest, with its strengthening being highly hindered due to interaction with Odette. This caused the system to weaken as Seroja moved closer to it, due to a phenomenon known as the Fujiwhara effect. Eventually, Seroja began to restrengthen and weaken Odette, with Seroja absorbing Odette into its circulation on 10 April. Due to Odette, Seroja was steered to the southeast towards Australia, before strengthening even further. At around 8 p.m. local time on 11 April, Seroja made landfall on the western coastline of Western Australia as a Category 3 severe tropical cyclone, slightly south of the coastal town of Kalbarri, bringing heavy rain and hurricane-force wind gusts (about 170 km/h, or 110 mph). Later that day, Seroja began accelerating southeastward while weakening. On 12 April, Seroja emerged off the southern coast of Western Australia while beginning to undergo an extratropical transition, before being absorbed into another larger extratropical cyclone to the south. The name Seroja means lotus in Indonesian.

As of May 2021, it is estimated that at least 272 people were killed by the storm, with 183 people in Indonesia, 42 in Timor-Leste, and one in Australia. At least 72 people from Indonesia and 30 from Timor-Leste are still missing. The cyclone damaged or destroyed more than 20,000 houses and five bridges in Indonesia's East Nusa Tenggara province while more than 12,000 people were evacuated to government-owned shelters. Around 9,000 people were displaced on Timor-Leste, while at least 10,000 homes have been submerged. Damage reports from Kalbarri started coming in soon after the storm made landfall. The storm is estimated to have caused over $490.7 million (2021 USD) in damages, a majority across Indonesia. The damage caused by Cyclone Seroja in Indonesia was described as catastrophic.

Efforts to relieve the devastating effects of the cyclone came a short time after Seroja left Indonesia. They included almost a million USD in donations, methods to help fight COVID-19, evacuations, local governments sending aid to their respective areas, and more. Timor-Leste also was given outside assistance from other organizations and nations. The Indonesian and Western Australian government's responses were criticized due to the slow nature of their respective responses; in the case of Indonesia, this resulted in some high-ranking officials being fired.

Meteorological history

On 29 March, the BoM began to mention that Tropical Low 22U was developing to the south of Timor. By 3 April, the tropical low fully developed as it was within an active trough. The low was located in a generally favorable environment with deep moisture, low vertical wind shear, and defined outflow. The low drifted close to the coast of Timor very slowly with persistent spiraling bands of convection occupying the storm's circulation, producing prolific rainfall in the surrounding regions on 3–4 April. At this time, the low pressure system was located inside the Area of Responsibility (AoR) of TCWC Jakarta. By the early morning of 4 April, the presentation of its structure had improved with spiral bands of deep convection and tight curvature at its center. Although there were fluctuations in central convection, a favorable environment of deep moisture, low vertical wind shear, and good outflow meant further development of the system was expected. Meanwhile, the Joint Typhoon Warning Center (JTWC) issued their first warning on the storm as Tropical Cyclone 26S at 15:00 on UTC 4 April. The tropical low slowly gained strength, intensifying to a Category 1 tropical cyclone, and was given the name Seroja by the Tropical Cyclone Warning Centre (TCWC) Jakarta at 20:00 UTC on 4 April, about 95 km (59 mi) north-northwest of Rote Island.

During 5 April, Seroja continued to move at 3 km/h (1.9 mph) in a west-southwest direction, away from the Indonesian coastline. The storm intensified to a Category 2 tropical cyclone at 19:00 UTC, with 10-minute sustained winds of 105 km/h (65 mph) and its central pressure deepening to 982 mb (29.00 inHg). Long term uncertainty in both track and intensity persisted in the forecasting of Seroja's track, due to interaction with Tropical Low 23U, to the far west of the storm. Seroja held this strength for roughly 12 hours, before unexpectedly weakening to a Category 1 tropical cyclone yet again on 6 April, due to southeasterly wind shear, and dry air partially exposing the low level circulation (LLC) and degrading its thunderstorm activity. Seroja fluctuated in strength for several hours, intensifying slightly on 7 April, as it continued in a generally southwesterly direction. Seroja became disorganized on satellite imagery as convection became dislocated to the south on 8 April, while it began to closely interact with Tropical Cyclone Odette (formerly Tropical Low 23U) just to the northwest, as a result of the Fujiwhara effect, complicating the forecasts for the tropical cyclone and causing it to weaken again. Seroja resumed intensification by the next day, re-intensifying into a Category 2 tropical cyclone at 13:16 UTC on 9 April, as the storm began to absorb Odette and as wind shear started to ease off. By 10 April, Seroja had absorbed most of Odette's remaining thunderstorms and began to accelerate to the southeast towards Western Australia, maintaining a relatively organized structure. Later that day, Seroja fully absorbed Odette into its circulation.

Seroja continued to steadily intensify as it approached the Gascoyne Region; the JTWC upgraded Seroja to a Category 1–equivalent tropical cyclone on the Saffir–Simpson scale, with sustained winds of 120 km/h (75 mph), at 18:00 UTC on 10 April. By early on 11 April, Seroja had intensified into a Category 3 severe tropical cyclone, the third severe tropical cyclone of the season, as a ragged eye began to emerge from the cyclone's central dense overcast. Seroja continued rapidly accelerating to the southeast at 53 km/h (33 mph), with its eye becoming more defined on infrared and Doppler weather radar images before making landfall near Gregory, Western Australia, or just north of Geraldton at peak strength with 1-minute sustained winds of 130 km/h (81 mph), 10-minute sustained winds of 120 km/h (75 mph), and a minimum central pressure of 971 hPa (28.76 inHg), with observations of gusts as high as 150 km/h (93 mph). Following landfall, Seroja quickly moved to the southeast over Australia, while gradually weakening. Early on 12 April, Seroja's extratropical remnant entered the Great Australian Bight, while beginning to transition into an extratropical cyclone, before being absorbed into a larger extratropical cyclone to the south later that day.

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