The cyclone brewing off the coast of Western Australia has now been joined by two other tropical lows nearby, adding to an already rare weather event.
Tropical Cyclone Seroja, currently a category one system, is about 670 kilometres north-west of Karratha and 650km north-north-west of Exmouth.
It is moving west-south-west at a speed of 29 kilometres per hour and is forecast to strengthen to a category two or three as it tracks over open waters, before crossing the coast between Jurien Bay and Carnarvon late on Sunday or Monday.
It could bring a brief but intense period of dangerous weather to the west coast, including destructive winds, very high rainfall and higher than normal tides.
'Two spinning tops on the table'
A second tropical low (known as 23U) that has formed nearby is expected to strengthen to cyclone intensity, and the two systems are on track to interact with one another over the next couple of days in a rare phenomenon known as the Fujiwhara Effect.
"It's like having two spinning tops on the table," said BOM duty forecaster Noel Pusey.
"They're their own entity, so they can interact, but they also tend to move around each other in a little bit of a dance.
[Embed Wind Map]"You don't get doubling up of the system or anything, but you get interaction between the two.
"Eventually I think there will be just one left standing and that will be Seroja and that will be the one that comes down and crosses the WA coast later.
"It's quite an unusual crossing for late in the season."
BOM said 23U would most likely cross the Exmouth coast on Saturday.
Third tropical low edges closer
Overnight a third tropical low edged closer to the two existing systems but Mr Pusey said it was unlikely to add to the interaction.
"There's a tropical low out well to the west near Cocos Islands which looks like it's slowly developing at the moment and may bring some weather to the Cocos Island region over the weekend," he said.
"But that's a completely separate system."
The bureau's James Ashley described the situation as an extremely rare weather event, which he had not witnessed in the past two decades.
"I've been working here in Perth for over 20 years and I've never seen three systems so close to being cyclones at one time," he said.
Holidaymakers urged to 'leave now'
The Department of Fire and Emergency Services (DFES) has issued a warning to tourists spending the school holidays in the Mid West-Gascoyne and the Pilbara.
Acting Commissioner Craig Waters has urged people to reconsider their travel plans.
"We know there are many holidaymakers in the area and others making their way there, many of whom will not have experienced a cyclone before," Commissioner Waters said.
"If you're in a tent or caravan, you are simply not protected against the damaging winds that may hit the region.
"The size of this potential impact area is another reason to be prepared, because you may need to travel some distance before you are out of harm's way."
He has urged people to familiarise themselves with the cyclone warning alerts.
His comments were backed by DFES Assistant Commissioner Paul Ryan, who said people needed to act quickly.
"We're also advising that the North West Coastal Highway will possibly be closed near Minilya on Friday and you may not be able to travel south," he said.
"You will also have tropical cyclone Seroja impacting the coastline the next day so you need to leave now because if you don't leave now, you'll be stuck."
People living near Coral Bay and Jurien Bay have also been urged to prepare their properties for severe weather.
It is possible a cyclone Blue Alert will be issued for people in and around Exmouth later on Thursday.
BOM said the south-west corner of the state, including Perth, was likely to get some rainfall associated with Tropical Cyclone Seroja over the weekend.
Wave heights could reach 10m off west coast
Forecasters are warning of treacherous ocean conditions on the west coast, particularly around the Mid West, with modelling predicting wave heights of over 10 metres off the Abrolhos Islands and already elevated water levels made worse by storm surges.
UWA Professor of Oceanography Chari Pattiaratchi said there could be significant erosion to beaches north of Geraldton.
"We're just coming past La Nina so we have elevated water levels already so the storm surge is going to act on top of that," he said.
"Together with the big swell we are saying that there will be devastating effects on the beaches ? lots of erosion.
"In previous cyclones we've had five metres vertically being taken away."
He said Shark Bay was on track to record water levels not seen in nearly 100 years.
"So the last big recorded storm surge was in 1921 and we say that this is probably going to be comparable to that," he said.
"[In 1921] a lot of places in Denham and some of the stations were flooded, and on the southern part of Shark Bay the water levels extended to more than 8 kilometres."
While the Mid-West coast looks to be the worst hit, he said elevated water levels could stretch as far as Perth and high water levels could remain for two or three days.
© ABC 2021
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