ILSA

History

Severe Tropical Cyclone Ilsa developed from a tropical low that formed off the Indonesian coast on 6 April 2023, its genesis assisted by an active pulse of the Madden–Julian Oscillation combined with a westward-moving equatorial Rossby wave. Designated 23U by the Bureau of Meteorology, the low drifted slowly southwest around a mid-level ridge, with convection repeatedly building and scattering as bursts of vertical wind shear held back development for several days. Once shear relaxed, the system became symmetric and was named Ilsa as a Category 1 cyclone late on 11 April.

Intensification then became explosive. Ilsa climbed through Category 2 and 3 within a low-shear environment over very warm water, with cloud-top temperatures falling to around −92 °C. A mid-level trough eroded the steering ridge and the cyclone turned onto a more southerly heading on 12 April. An eye appeared, first in microwave imagery and then on conventional satellite pictures, contracting from roughly 41 km to 26 km across. Ilsa reached Category 5 intensity with estimated ten-minute mean winds of 230 km/h and a central pressure of 915 hPa — equivalent to a Category 5 hurricane, with one-minute winds near 260 km/h. An automatic weather station at Bedout Island recorded a ten-minute mean wind of 219 km/h, an Australian record surpassing the mark set by Cyclone George in 2007, while Rowley Shoals measured gusts to 235 km/h.

Landfall occurred on the Pilbara coast about 120 km east-northeast of Port Hedland, with sustained winds near 215 km/h. Ilsa accelerated inland and weakened quickly, dropping to Category 4 and then below cyclone strength on 14 April, its remnants dissipating over central Australia by 15 April.

Warnings covered the Kimberley and Pilbara, with highway closures and about 700 people moved into cyclone shelters. Damage onshore was concentrated around Pardoo, where the roadhouse lost its roof and solar panels and five accommodation sheds vanished entirely; station losses were estimated at about A$15 million. Two vessels carrying 19 men fishing illegally were caught offshore — 12 were rescued but eight died. The name Ilsa was subsequently retired and replaced with Isabella.

Sources: Bureau of Meteorology report Wikipedia