Cyclone Narelle is now larger and 'more severe' as it crosses the Western Australian coast

By Steve Turton, The Conversation
Edited by Sadie Harley, reviewed by Robert Egan

Credit: Bureau of Meteorology, Himawari-9 satellite, CC BY-SA

Severe Tropical Cyclone Narelle has demonstrated remarkable persistence throughout its journey across northern Australia. The cyclone originated near the Solomon Islands on March 16, propelled by the rapid ascent of moist air that formed a low-pressure zone.

Last Friday, Narelle crossed the Cape York Peninsula, emerging as a compact category 4 system, and has since maintained a steady westerly trajectory across the Gulf of Carpentaria, the Northern Territory, and the Kimberley region of Western Australia.

As of today, Narelle has intensified into a dangerous category 4 cyclone, making landfall near Exmouth in the far northwest of WA. According to the Bureau of Meteorology, the cyclone has traveled over 5,700 kilometers since its formation near the Solomon Islands.

It is infrequent for a single tropical cyclone to impact Queensland, the Northern Territory, and Western Australia. The last instances were Severe Cyclone Ingrid in 2005 and Cyclone Steve in 2000. The anticipated trajectory of Narelle appears to closely mirror that of Cyclone Steve, with expectations for its final dissipation in the Great Australian Bight.

A large and severe cyclone

Narelle presents a more severe threat than Cyclone Steve. It has expanded to twice the size it was upon reaching far north Queensland last week, as indicated by the increased area of strong gales surrounding its eye. Currently, damaging winds extend 200–260 km from the center, destructive storm-force winds cover 110–210 km, and the highly destructive core of hurricane-force winds encompasses a width of 90–130 km.

The enlarged core of the cyclone represents a considerable risk to settlements along its path, with threats stemming from both severe winds and intense rainfall. Additionally, there is a heightened probability of dangerous storm surge and ocean inundation, particularly for coastal areas exposed to the cyclone's path. The impact will be exacerbated if the cyclone passes during high tide.

To date, wind and storm surge damage has been limited, as Narelle has traversed more sparsely populated regions. The most significant impacts thus far have been heavy rainfall and flooding across Northern Territory catchments, which were already saturated from weeks of monsoonal conditions. The western regions of the continent may not experience the same level of luck.

An unusually predictable path

The trajectory, forward speed, and intensity of Narelle have displayed exceptional predictability in comparison to many cyclones within the Australian region. Prevailing easterly winds associated with a subtropical high-pressure ridge over southern Australia have propelled the cyclone along at speeds of 15–25 km per hour over the last week.

Narelle is currently being guided around the northwestern edge of the same high-pressure system, contributing to its more southerly southwest track. It is anticipated to strengthen over warmer ocean waters.

The cyclone is expected to continue moving in a southerly direction while maintaining its intensity as a dangerous category 4 system until later today, at which point it is projected to weaken to a still severe category 3 system near Shark Bay.

The towns of Onslow and Exmouth are bracing for severe impacts as the core winds pass over them, with predictions of wind gusts reaching up to 250 km per hour. Further south, Carnarvon anticipates winds of up to 200 km per hour this afternoon.

Weakening over land

The world heritage-listed Ningaloo Reef is expected to suffer significant impacts from the cyclone as its core winds sweep the entirety of the reef. This scenario compounds challenges for the reef, which was already severely affected by last year’s marine heatwave, resulting in catastrophic coral bleaching and extensive mortality in certain areas.

Coral reefs that are already compromised by bleaching are likely to experience prolonged recovery times if they are subsequently impacted by a powerful tropical cyclone.

Narelle will retain its classification as a severe category 3 system as it moves through Shark Bay, likely on Friday, but it will begin to weaken as it traverses land south of the tourist destination Denham.

An incoming upper trough from the Southern Ocean is expected to interact with the cyclone, prompting a more rapid southeastward movement. As it approaches the major town of Geraldton, it is projected to downgrade to a category 2 cyclone.

Changes in the wind speed and direction forecast near the cyclone from the west will result in the strongest winds shifting to its eastern flank, leading to a loss of its tropical cyclone characteristics.

Residents of Kalbarri and Northampton, small towns situated north of Geraldton, may feel heightened anxiety as they recall the events of April 2021.