Cyclone Ilsa
Severe Tropical Cyclone Ilsa was a powerful tropical cyclone that struck Australia's Western Australia in April 2023. The sixth named storm, and the fifth severe tropical cyclone of the 2022–23 Australian region cyclone season, 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. 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, 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 

On 6 April, an active burst of the monsoon trough led to the development of a tropical low in the Timor Sea. Deep convection persisted in the system's southern semicircles. By the next day, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert (TCFA) on the system. During the following day, the JTWC initiated advisories on the system and classified it as Tropical Cyclone 18S.

Easterly vertical wind shear decreased as a result of the intense convective mass causing upper-level winds around it. Over the next subsequent days, the low steadily drifted to the southwest, but due to unfavorable environmental conditions, intensification was delayed. Late on the next day, the BoM reported that the tropical low had developed into a Category 1 tropical cyclone and named it Ilsa. By that time, it had developed an obscured low-level circulation center. Satellite imagery showed that a CDO was obscuring the LLCC, and Ilsa intensified further into a Category 1-equivalent cyclone. A burst of convection occurred near Ilsa's center during six hours later, and the BoM reported that it briefly strengthened into a Category 2 cyclone. Ilsa rapidly intensified, into a Category 3 severe tropical cyclone, within a favorable environment with low shear and warm sea surface temperature (SST). Ilsa shows an eye feature seen in microwave imaging, with IIsa later becoming a Category 2-equivalent cyclone. The eye in the symmetric system later became visible on satellite imagery as the rainbands wrapped around the center 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. On 13 April, the JTWC also assessed Ilsa to have attained an intensity equivalent to a Category 4-equivalent tropical cyclone on the Saffir–Simpson hurricane scale. 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 recorded wind gusts up to . Later that same day, Ilsa had become a Category 5 severe tropical cyclone with sustained winds of .

A record-breaking ten-minute sustained wind speed of  was measured at Bedout Island, beating the previous record of Cyclone George in 2007.  The cyclone crossed the coast about  northeast of Port Hedland on 16:00 UTC, with sustained winds of . Shortly after the landfall, the JTWC discontinued warnings on the system. 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.  Ilsa was last noted on 15 April.

Preparations and impact 

Gale-force winds with gusts up to , and heavy rain might occur on 9 April between Kalumburu and Kuri Bay in the state's north. Squally thunderstorms are also anticipated in far northern Kimberley. 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.

See also 

Tropical cyclones in 2023
Weather of 2023
Cyclone Ilona (1988)
Cyclone Orson (1989) – a powerful tropical cyclone that had a similar path and intensity.
Cyclone John (1999)
Cyclone George (2007) 
Cyclone Laurence (2009) 
Cyclones Heidi and Lua (2012) – two severe tropical cyclones which both made landfall around Pardoo within the span of two months. 
Cyclone Kelvin (2018) – the previous severe tropical cyclone to make landfall between Broome and Port Hedland prior to Ilsa.

References

External links

 Australian Bureau of Meteorology
 Joint Typhoon Warning Center
 Tropical Cyclone Warning Center Jakarta 
 Papua New Guinea National Weather Service

2023 meteorology
2022–23 Australian region cyclone season
April 2023 events in Australia
2023 disasters in Australia
Tropical cyclones in 2023
Tropical cyclones in Western Australia
Category 5 Australian region cyclones
2020s in Western Australia