By Tom Saunders, ABC meteorologist
Photo: Bureau of Meteorology / Supplied
A major rain event is engulfing most of eastern Australia for the next 48 hours, prompting the Bureau of Meteorology (BOM) to issue flood watches from southern Queensland to the NSW South Coast.
Greater Sydney could be soaked by up to 200 millimetres from late Thursday to early Saturday, potentially leading to major flooding along the Hawkesbury-Nepean River, while Brisbane and Canberra also face the prospect of heavy rain.
The sudden shift after recent warm and dry weather is due to polar air moving north meeting a humid tropical airstream off the Tasman and Coral seas, a pattern seen numerous times during 2022 - Sydney's wettest year on record.
Rainband forming over southern Queensland to track south
The clash of tropical and polar air is already leading to a thickening cloudband today over southern Queensland and northern NSW.
By midnight, falls of 100mm are possible south of a line from about Blackall to Roma to Brisbane, including the NSW Northern Rivers and far northern slopes and ranges.
The rainband will then broaden across NSW on Friday as an inland trough of low pressure deepens, however a second trough will also form along the coastline, leading to torrential falls in the east from southern Queensland to the Victorian border.
Later on Friday, a low-pressure system could even form around Sydney that would further intensify rainfall overnight, including peak three hourly totals in excess of 50mm from Greater Sydney to the southern coast and ranges.
"The heaviest rain and strongest winds are likely to be tied to wherever that low may develop but there is still an element of uncertainty," Miriam Bradbury from the BOM said.
The rainband will then contract to south-east NSW on Saturday, although pockets of heavy rain from thunderstorms are still possible this weekend right along the eastern seaboard up to Cape York Peninsula
Modelling currently indicates an average of 50mm to 200mm from Brisbane to the Victoria border and north into the Queensland's Central West, however, the nuances of such a complex system will result in localised heavier totals.
A frequent result of a moist onshore flow over Australia's east is enhanced uplift due to the close proximity of the Great Dividing Range to the coast - it would therefore be no surprise to see pockets closer to or even exceeding 300mm.
Widespread flood risk including Sydney and the Hawkesbury
While this event may not technically meet East Coast Low (ECL) thresholds due to the possible absence of a closed circulation near the coast, the set-up is essentially identical with humid maritime air clashing with an inland cold pool to lower the surface pressure.
The similarity to an ECL this week ensures the impacts will be nearly as severe, although the rain should be mobile enough to prevent record or near record flooding, as opposed to an ECL's slow movement capable of producing several days of torrential rain.
The main concern from this system is flooding, as the amount of rain predicted should easily be enough to generate rapid river rises, especially in areas where totals exceed around 100mm.
"Rainfall of this magnitude can quickly lead to flash flooding, cutting off our roads, making them impassable and blocking our drainage systems, we may also see riverine flooding developing through Friday," Ms Bradbury said.
The BOM has issued an initial minor flood warning for the Wilson's River at Lismore in northern NSW.
Heavy rain and thunderstorms in the catchment could see minor flooding in the town from this evening.
A number of hinterland communities near Lismore also became isolated today as heavy rains swelled creeks and flooded causeways.
Nashua, in Byron Shire, copped 219mm of rain in the 24 hours to 9am on Thursday morning.
A flood watch has also been issued for many rivers from the Condamine-Balonne to the Gwydir, including the risk of major flooding along the Hawkesbury-Nepean where a near full Warragamba Dam will be unable to hold back flows.
For Western Sydney, a major flood would follow three previous major floods through 2021 to 2022, culminating in the highest river levels in more than 40 years.
Another flood continues the recent shift to a rain dominate period after a three-decade hiatus in floods through Western Sydney from 1990 to 2021.
Flooding is also possible from Friday along creeks and rivers through Metropolitan Sydney, including the Parramatta River.
Strong winds and big surf to pound coast
The lowering pressure during the coming days will additionally strengthen winds and increase seas and swells.
Even with the absence of an ECL, which can bring gusts well above 100kmh, strong winds could eventuate with damaging gusts to around 90kmh, most likely late on Friday and early Saturday along the southern and central NSW coast.
While winds are unlikely to reach typical gale force ECL speeds, power outages are still possible as heavy rain softens the soil and heightens the risk of trees coming down in gusty winds.
The strong winds will also whip up huge waves - combined seas and swells should rise to 4 metres south from the Hunter, generating maximum waves closer to around 8m.
- This story was first published by the ABC.