Up to 250 millimetres of rain could soak parts of south-east NSW during the next 24 hours as another major weather event unfolds across the state.
The drenching is the result of a similar weather pattern to last weekend, however on this occasion the heaviest rain will land further south, prompting the Bureau of Meteorology to issue a flood watch for at least a dozen rivers from the Hawkesbury to the South Coast as rain falls on wet catchments and produces significant runoff.
For Sydney, the system could easily bring another 100mm, boosting the city's 2024 rain total close to the annual average less than halfway into the year.
And while rain is arriving in abundance, our ski slopes remain mostly bare, ensuring a limited opening to the snow season this King's birthday long weekend.
Three-day drenching underway across central and southern NSW
After last weekend's soaking, another extensive cloudband has now formed over NSW, triggering a flood watch from the Hawkesbury-Nepean, through Sydney and the Illawarra to Clyde.
The first drops from the current system commenced falling on Wednesday as a pool of polar air, called an upper low, drifted across the state's west — although rain remained light and patchy yesterday due to a relatively dry atmosphere.
Rain then became steadier overnight across the east as the upper low moved towards the coast and interacted with warmer, moister air off the Tasman Sea.
Rain will increase even further today as the clash of cold and warm air masses initiates the development of a low pressure system offshore – repeating the pattern from last weekend when an offshore low generated Sydney's wettest June day in more than three decades.
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The heaviest falls on Thursday should impact the region from about Sydney to Ulladulla where more than 100mm is possible in just 24 hours.
Some modelling even indicates parts of the Illawarra could see up to 250mm, more than a month's worth of rain and enough to trigger areas of flash flooding.
As opposed to last week's system which quickly tracked south to Tasmania, on this occasion the Tasman low will remain slow moving for several days, leading to a continuation of rain on Friday and Saturday from about Sydney to the Victorian border.
While not quite reaching east coast low status, fairly widespread 72 hour totals of 100 to 300mm are likely across much of Sydney, the Illawarra and South Coast.
Following the state's second wettest May since 1996, the ongoing rainbands should ensure above average rain for NSW again this June.
Sydney on track for one of the wettest years on record
Sydney's rainfall at Observatory Hill has been above average every month this year, except for March, and the city's total for 2024 now exceeds 1,000mm, well above the average to early June of just over 600mm.
Heavy falls this week will boost Sydney's 2024 rainfall even closer to the annual average of 1,222mm, and with more than 6 months remaining in the year a platform is now set for one of the wettest years on record.
Analysing previous year's rainfall at Observatory Hill shows that even without any further rain this month, the first half of 2024 would still rank as the second wettest since 1990 – only behind the all-time record wet 2022 which had already produced more than 1,500mm by the end of May.
The result of another wet year for Sydney is not only green gardens and flowing creeks, but also a full supply of water as Warragamba Dam again threatens to spill during the coming days.
Snow season on hold as mild temperatures bring alpine rain
The traditional King's birthday long weekend opening of the snow season will arrive with one critical missing ingredient – enough snow for skiing or snowboarding.
So why isn't snow falling on the Alps when the coast is under water? The problem with Tasman lows is their opposing sectors of cold and warm air, and the exact positioning of these contrasting air masses can mean the difference between epic dumps of snow and heavy rain.
Unfortunately, the current system's coldest air is well north of the Alps, meaning snow will be confined to the highest peaks.
The resulting rain will not only erode the existing thin cover, but also prevent snowmaking as temperatures hover just above zero.
While a snowless opening weekend is not unusual, and many previous bumper years have no snow in early June, the short-term forecast offers little hope of improvement.
The bread and butter of an Australian ski season is cold fronts, however modelling indicates the next front arriving over south-east Australia on Wednesday should produce most of its precipitation in the pre-frontal warm air – and that means more rain.
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