A rare out-of-cycle cyclone that has formed into a category one system inthe Indian Ocean is set to bring dangerous weather to Western Australia, including Perth, from Sunday.
Tropical Cyclone Mangga will impact the Cocos Keeling Islands today before tracking towards WA.
The official Australian cyclone season ends on April 30 and the Bureau of Meteorology (BOM) said while some cyclones were known to form in early May, it was rare to see one so late in the month.
The Cyclone Mangga was located about 515 kilometres north-west of the Cocos Keeling Islands just before midnight and was expected to pass to the west of the islands later today, potentially resulting in heavy rainfall and a period of gale-force winds.
The BOM said it was expected to weaken tomorrow as it continued to move south-east towards WA's north-west coast and was "highly unlikely to be a tropical cyclone" come Sunday.
Senior forecaster Luke Huntington said the combination of a tropical low and a cold front occurring at the same time, especially this time of year, was out of the ordinary.
"We don't normally see this system at all, really, so it's quite unprecedented," he said.
"If it was going to happen, it would be earlier in the summer period when we do get the tropical cyclones and lows … but to have it this late in the season, it is quite unusual.
"It's just a coincidence the cold front came up at the same time and it's just going to deepen this low as it comes down towards the west coast, and create all that rain and the damaging winds."
Torrential rain and dangerous winds are expected to come from the state's north-west before extending south-east over the remainder of the state on Sunday and Monday.
"We can expect widespread damaging winds of around 100 kilometres an hour and in some regions it could even exceed 125kph," Mr Huntington said.
"There's also some quite significant rainfall associated with it as well … in isolated areas, particularly along the west coast, some areas could receive even up to 100 millimetres."
'Dangerous swells' along with wind, rain
BOM spokesman Neil Bennett said yesterday it was "an evolving and dynamic system" that would still pack a punch on Sunday as it tracked down the west coast.
"It'll produce severe weather, in terms of wind, some heavy rainfall in places and significant wave heights," he said.
"We're looking at dangerous swells, severe weather warnings for the winds and possibly some heavy rainfall as well … right up and down the west coast."
BOM has cautioned that it was still too early to pinpoint which areas would be hardest hit, but a prolonged period of strong winds was expected.
It said at this stage, rainfall totals in Pilbara coastal areas were likely to be about 20–40mm, with isolated falls from Karratha to Kalbarri of up to 100mm.
Heavy rain is also possible along coastal areas from Kalbarri to Albany, including Perth, with totals of about 20–30mm and some isolated falls up to 70mm.
At least 5–10mm is forecast for much of the Wheatbelt, with areas closer to the west coast expecting 10–20mm.
Peak wave heights in excess of 8 metres are predicted for much of the west coast on Monday and they have the potential to cause significant beach erosion, while storm tides could lead to coastal inundation.
Severe weather to hit traffic
The severe weather will coincide with the first weekend since regional travel restrictions were eased in WA, with an influx of people expected on the roads.
But fine weather is forecast for most of the state until Sunday, providing plenty of opportunity to tie down loose items around the home and clear out gutters.
In WA, late autumn and winter rainfall usually comes in the form of cold fronts.
The BOM has described this system as a "rare event" — the result of the remnants of a tropical low interacting with a cold front coming up from the south.
"This sort of system has happened in the past, but it's something that would probably only happen once every two or three years, maybe even once every five to 10," Mr Bennett said.
A similar event occurred in June 2012, which led to more than 600 calls for assistance and more than 170,000 homes losing power.
Squally winds and rain are also forecast for Monday but conditions should gradually ease throughout the day.ABC