The weather system brewing off NSW could be the first east coast low in three years

Australia Broadcast Center

15 hours ago

New South Wales residents are being told to prepare for a powerful low-pressure system the Bureau of Meteorology says will "drench and batter" the east coast over the coming week.
Forecasts are even flagging the potential for it to develop into an "east coast low" — one of the most dangerous weather systems the region experiences.
If it does, it would be the first east coast low in three years.
So what exactly are east coast lows and what makes them so dangerous?
Broadly, an east coast low is an intense low-pressure system that develops on or near the east coast of Australia.
But they're no ordinary storms.
What sets them apart from the more typical low-pressure systems, according to Bureau of Meteorology community information officer Daniel Hayes, is their impact.
"So, something bringing extreme rainfall, storm-force winds and large waves."
To be able to bring this impact, the system needs to tick a few boxes.
Firstly, according to Mr Hayes, it must be strong.
"An east coast low is first and foremost an intense low-pressure system that forms quite quickly. So it has to get to be a strong low-pressure system, such that it can have a large impact," he said.
Secondly, it must be near the coast.
"To have an impact it has to be within a couple of hundred kilometres of the coast. If it's too far out, it doesn't matter how strong it is, it won't have nearly as much impact."
And thirdly, it needs to hang around for an extended period of time — generally "at least 12 hours", according to Mr Hayes.
This means its effects can last much longer — producing long periods of heavy rainfall and flooding, damaging winds and very large swell.
With the exception of rare southern-tracking tropical cyclones, east coast lows are the most significant weather systems to impact south-east Queensland and eastern parts of New South Wales, Victoria and Tasmania.
Though they are different from tropical cyclones, which are born in warm tropical waters, they can produce winds up to category two cyclone level.
"They can be very similar [in their impacts], because largely they have the same mechanisms for causing an impact: heavy rainfall, strong winds, and they can develop large and powerful surf," he said.
A look at past events reveals east coast lows have been behind some of Australia's worst natural disasters, including the deadly 1998 Sydney to Hobart race in which six sailors died.
Latest forecasts show the low-pressure system developing off the New South Wales coast has the potential to form into an east coast low, with the impacts lasting several days.
However, Mr Hayes said it may not reach to east coast low status due to its impact potentially being softened by another low-pressure system nearby.
"It's complicated by the fact that we do have another low-pressure system developing off the coast as well, and the two are going to interact somewhat," he said.
"They may end up cancelling each other out to some extent and reducing the overall impact."
The interaction between the two low-pressure systems is known as "dumbbelling", with the systems rotating around each other.
Regardless of whether it does get named a true east coast low, Mr Hayes said severe weather was still likely with the potential for damaging winds, heavy rainfall and flooding and powerful waves.
"We are expecting to see significant impacts out of this.
Mr Hayes said rain and showers, with the potential for thunderstorms, were forecast to develop around the northern New South Wales coast from Monday afternoon.
But he said the main impacts would begin on Tuesday, as the system deepened and pushed further south toward Sydney and Wollongong.
A severe weather warning has been issued for areas stretching from Lismore to just south of Woolongong, with damaging winds averaging 60 to 70 kilometres per hour, with peak gusts up to 100kph from Tuesday.
These winds are expected to linger throughout Wednesday, potentially even lasting into Thursday.
Mr Hayes said significant rainfall was also forecast for some parts of the coast, particularly on the southern side of the system around Illawarra, the NSW south coast and southern parts of Sydney.
"The rainfall is likely to be somewhat concentrated on the southern side," he said
"So while we might very well see 50 to 150 millimetres of rain in the 48 hours from Tuesday into Wednesday, we could see some areas that do go higher than that and push up to the 200 millimetre range," he said.
"On the northern side, we are likely to be seeing that rain starting to taper off after today."
East coast lows are most common during late autumn into winter.
Their frequency depends on the definition used, but on average, there are around two severe events per year, which generate daily rain totals in excess of 100mm.
However, it has been three years since Australia has seen one reach the threshold.
Research indicates that east coast lows are projected to become less frequent with climate change, particularly during the cooler months of the year.
However, those that do occur are likely to be more intense and potentially more hazardous, with rising sea levels exacerbating coastal impacts.