Severe Tropical Cyclone Ilsa was a powerful tropical cyclone that struck Western Australia in April 2023. The sixth named storm, and the fifth severe tropical cyclone of the

2022–23 Australian region cyclone season The 2022–23 Australian region cyclone season is the period of the year when most tropical cyclones form in the Southern Indian Ocean and Pacific Oceans between 90°E and 160°E. The season officially started on 1 November 2022 and will finish ...

, Ilsa formed from a tropical low off the coast of Indonesia on 6 April. It fluctuated in intensity and became a Category 1 tropical cyclone on 11 April, after deep convection became symmetric around the low-level circulation center. Ilsa then rapidly intensified the following day and reached its peak intensity as a Category 5 severe tropical cyclone 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 ...

. Ten-minute sustained winds were estimated as , with a central barometric pressure of . One-minute sustained winds reached at this time, equivalent to a Category 5 hurricane on the Saffir–Simpson hurricane wind scale. Additionally, record-breaking ten-minute sustained wind speed of were measured at

Bedout Island Bedout Island is a small Australian island in the eastern Indian Ocean. It lies 42 km offshore from Larrey Point and the mouth of the De Grey River, and 96 km north-east of Port Hedland, on the Pilbara coast of north-west Western Au ...

, beating the previous record of Cyclone George in 2007. Ilsa made landfall roughly northeast of Port Hedland, Western Australia. Inland, Ilsa weakened to a low-end tropical cyclone with winds. Overall, Ilsa caused around A$4 million in damage, and caused 8 deaths after two boats capsized off the coast of Western Australia.

Meteorological history

An active burst of the Madden–Julian oscillation in conjunction with a westward-propagating equatorial Rossby wave led to the formation of a tropical low on 6 April. The low—designated 23U by the Bureau of Meteorology (BoM)—slowly moved southwest around a mid-level

ridge A ridge or a mountain ridge is a geographical feature consisting of a chain of mountains or hills that form a continuous elevated crest for an extended distance. The sides of the ridge slope away from the narrow top on either side. The line ...

above north-eastern Australia, as convection persisted near the system's circulation. By the next day, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert on the system, citing the slightly favourable environmental conditions encompassing poleward

outflow Outflow may refer to: *Capital outflow, the capital leaving a particular economy *Bipolar outflow, in astronomy, two continuous flows of gas from the poles of a star *Outflow (hydrology), the discharge of a lake or other reservoir system * Outflow ...

and warm sea surface temperatures (SST) of subduing high vertical wind shear. Convection continued to strengthen west of the exposed low-level circulation centre (LLCC) as it scattered under a strong

temperature gradient A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degree ...

. During the following day, the JTWC initiated advisories on the system and classified it as ''Tropical Cyclone 18S''. Ilsa 2023-04-12 0510Z

More consolidation took place, with spiral rainbands present in all quadrants of the storm wrapping around the LLCC obscured by a cold dense overcast (CDO). Easterly wind shear decreased as a result of the intense convective mass causing upper-level winds around it. Over the subsequent days, the low steadily drifted to the southwest, but due to a surge of unfavourable wind shear, intensification was delayed. However, levels of wind shear later diminished, and by late on 11 April, the BoM reported that the symmetrical tropical low had developed into a Category 1 tropical cyclone and named it ''Ilsa''. The expanding CDO completely obscured the circulation as westerly dry air attempted to enter the system from the north. Consequently, Ilsa became a Category 1-equivalent tropical cyclone on the Saffir–Simpson scale (SSHWS). A burst of convection occurred near Ilsa's center, and the BoM reported that it briefly strengthened into a Category 2 cyclone. On the same day, Ilsa rapidly intensified into a Category 3 severe tropical cyclone within a favourable environment of low shear and warm SST. Ilsa then underwent steady intensification under persistent wind shear, as SST and

ocean heat content In oceanography and climatology, ocean heat content (OHC) is a term for the energy absorbed by the ocean, where it is stored for indefinite time periods as internal energy or enthalpy. The rise in OHC accounts for over 90% of Earth’s excess the ...

increased along the way, with cloud top temperatures of at least . A mid-level trough passed south of the system, eroding the steering ridge, and by 12 April, Ilsa changed course to be more southerly. Ilsa displayed an

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feature in microwave imaging, with vortical hot towers indicating eye development. The eye in the symmetric system later became visible on satellite imagery as rainbands tightened after intensifying to a Category 3-equivalent tropical cyclone. Later the next day, the BoM assessed the storm to have attained ten-minute sustained winds of , ranking it as a Category 4 severe tropical cyclone. Owing to the substantial radial outflow and very warm SST, despite moderate wind shear, Ilsa intensified into a Category 4-equivalent tropical cyclone on the SSHWS. Later that day, Ilsa strengthened further to its peak intensity as a Category 5-equivalent tropical cyclone with 1-minute sustained winds of . An automated weather station on

Rowley Shoals The Rowley Shoals is a group of three atoll-like coral reefs south of the Timor Sea, about west of Broome on the northwestern Australian coast, centered on , on the edge of one of the widest continental shelves in the world. Each atoll cover ...

recorded wind gusts up to . Later that same day, Ilsa had become a Category 5 severe tropical cyclone with sustained winds of and the lowest atmospheric pressure of . Around that time, the eye was measured from its initial to . Ilsa made landfall in a coast east-northeast of Port Hedland, with 10-minute sustained winds of . Following this, the JTWC discontinued the issuing of advisories of Ilsa, as the storm accelerated inland and rapidly deteriorated from land interaction and high vertical wind shear. While over Western Australia, Ilsa weakened to a Category 4 severe tropical cyclone with winds. The storm continued to weaken with a deteriorating cloud signature. During 14 April, another mid-level trough enhanced over Western Australia, steering Ilsa south then southeast. Ilsa fell below tropical cyclone strength at 12:00 UTC. Ilsa was last noted on 15 April, with its remnants moving east before dissipating over

Central Australia Central Australia, also sometimes referred to as the Red Centre, is an inexactly defined region associated with the geographic centre of Australia. In its narrowest sense it describes a region that is limited to the town of Alice Springs and i ...

Preparations and impact

Gale-force winds with gusts up to , and heavy rain might occur on 9 April between Kalumburu and

Kuri Bay Kuri Bay is a remote coastal bay in the far north of Western Australia in the Kimberley region, about north of Broome. It is a designated point on the Western Australian Forecast Areas, utilised by the Bureau of Meteorology as a boundary for ...

in the state's north. Squally thunderstorms are also anticipated in far northern

Kimberley Kimberly or Kimberley may refer to: Places and historical events Australia * Kimberley (Western Australia) ** Roman Catholic Diocese of Kimberley * Kimberley Warm Springs, Tasmania * Kimberley, Tasmania a small town * County of Kimberley, a ...

. Residents across the coast of Pilbara evacuated as high tides, large waves and inundation were possible, along with a highway closure along the Pilbara coast. In anticipation of Ilsa, the BoM issued a warning for Western Australia’s Kimberley and Pilbara regions. Additional emergency workers, aircraft and essential supplies have also been sent to the region. On 12 April, the Department of Fire and Emergency Services (DFES) issued a cyclone yellow alert, and urged residents to be prudent and to take action and prepare to shelter. At least 700 residents were being evacuated to cyclone shelters prior to the storm. Port Hedland mayor Peter Carter said damaging winds could turn flying debris into "missiles in the air". Western Australia's Emergency Department on 15 April assessed damage from a tropical cyclone that hit the northwest of the state. At least eight personnel have been sent to the Great Northern Highway fires near Pippingarra. Pardoo Roadhouse, located northeast of Port Hedland and near the landfall point, sustained major damage valued at A$4 million. Five accommodation sheds were entirely blown away by the storm at the Pardoo Roadhouse, and emergency officials have been unable to locate them. Apart from Pardoo, acting emergency services minister Sue Ellery called the storm damage "fairly minimal." Two boats containing 19 men total illegally fishing off the coast of Western Australia were caught in the cyclone. One of the boats was found shipwrecked on Bedwell Island, while the other boat sank. The 11 people on the shipwrecked boat were rescued, while one survivor was found from the sunken boat. The other 8 fishermen died.

Retirement

As a result of the damage associated with the storm, the name ''Ilsa'' was removed from the list of Australian region cyclone names and will never be used again for a tropical cyclone in that basin. It was replaced by ''Isabella'' for future seasons.

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