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Rare clash of tropical tempests expected off WA coast ‘like two planets going close to each other’

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“The real threat to WA will come after that,” Mr Domensino said, adding meteorologists and fans of wild weather will be paying attention.

“Any weather nerd is going to be watching this very closely,” he said. “It’s very unusual to see [the Fujiwhara effect] as close to Australia.”

The range of possible coastal crossings is from Carnarvon north of Shark Bay to Mandurah, about 1000 kilometres to the south, taking in Perth as well.

The Bureau of Meteorology was forecasting the cyclone will make landfall between Perth and Coral Bay late on Sunday or early Monday.

Bureau duty forecaster Noel Puzey said the event was “like two planets going close to each other”.

“They sort of can rotate around each other and then move away again, or one might be absorbed by the other, but it doesn’t double the effect or anything,” he said.

Andrew Burton, a Bureau manager for cyclones, said the two systems were about 700 kilometres apart as of the late afternoon, local time, and were beginning to interact.

He said the tropical low, dubbed 23U, could be “sling-shot” northwards towards the equator, an unusual path for systems to travel.

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Even if the two systems didn’t combine, their interaction would help to intensify cyclone Seroja by reducing the wind-sheer effects that had been weakening that system for the past day.

If the low does reach cyclone category-1 strength, it would be called cyclone Odette. It poses a threat of brief but very intense weather for the region from Coral Bay to Exmouth, an area popular with campers during the school holidays.

“It’s usually a period of very benign weather for the west coast,” Mr Burton said.

While unlikely to hit Perth, the WA capital has been buffeted by cyclones before, such a cyclone Alby which stuck on in early April 1978. Rainfall will be in the order of 30-50 millimetres in about three hours as the system movers inland, potentially affecting the wheat belt, Mr Burton said.

WA’s Department of Fire and Emergency Services has warned of prolonged, unpredictable and potentially dangerous weather meaning travellers must stay alert and be prepared to change their plans sooner rather than later.

Professor Walsh said the interaction reflected well-understood fluid dynamics that play out whenever vortices near or absorb each other.

The events were more likely where cyclones were more common, such as the north-west Pacific, the world’s most active basin for such storms. With relatively limited statistics on Fujiwhara cyclones, it’s difficult to say whether they were becoming more or less common, he said.

Similarly, it is challenging to forecast how such events will change as the planet heats up with climate change. “Predicting changes in rare events is pretty tricky,” he said.

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