Expect “absolutely incredible” rainfall totals as weather systems collide causing a peak in rainfall amounts overnight through to Tuesday.
Each state and territory except WA should expect a heavy rainfall warning by today evening.
BOM warns very dangerous weather conditions continue to lash the areas ravaged by floods as another 50mm to 100mm is forecast in the already soggy catchment areas on Monday.
“We’ve got major flooding across the Hawkesbury, Nepean and Colo Rivers and moderate flood warnings to the mid-north, but we’re expecting another peak today,” noted Jonathan How, a senior meteorologist at the weather Bureau.
“So pretty treacherous conditions right along the coast.”
Another 50mm to 100mm of rainfall is expected on Monday in various parts of Western Sydney, the Hunter, Blue Mountains and in the Mid-North Coast stretching to Brisbane through to the Gold Coast, as highly isolated rainfall is expected again.
The highest falls overnight was received on the Gold Coast hinterland around North Tambourine which received 263mm. Brisbane CBD recorded 125mm.
The gauge in NSW recorded highest falls at 245mm in Nambucca Heads along the Mid-Northern coast.
Within the Blue Mountains, Kurrajong Heights received 174mm, Springwood 116mm, North Richmond 95mm, Warragamba 93mm and Penrith 88mm.
In Sydney’s CBD, the gauge stood at 28mm by the time the rain stopped.
‘Absolutely incredible’ totals received so far
The overnight overalls were remarkable though the totals received since 9am on Thursday were more extreme.
Comboyn to the south of Port Macquarie received a whopping 889mm of rainfall between 9am on Thursday and 9am on Monday.
Mr How noted that it was likely they would hit the 1 metre of rainfall by the next day or so.
“They could get to a metre over the next day or so, which is absolutely incredible.”
The highest rainfall received in Sydney within the same period was 465mm at Blackheath.
Western Sydney received 353mm at Warragamba with Penrith Lakes receiving 290mm and 263mm at Richmond.
“So we’re basically talking three to four months worth of rain over three to four days,” Mr How noted.
“Since records began, we haven’t seen these types of numbers before.”
According to Mr How, the heavy rainfall hasn’t been restricted to the past few days. Yarras also named Mount Seaview in the Mid-North Coast has already recorded 911mm in March.
Weather systems colliding
Starting today evening, expect even wetter conditions as a tropical cloud arrives from the west, colliding with the coastal trough believed to have brought in the rains.
“We’ve had days of torrential rain and now this tropical system coming through,” Mr How said.
“It’s a collision of two masses to create a huge rain/thunderstorm band that’ll affect every state and territory except WA.”
Falls amounting to 120mm were received overnight across Central Australia.
Moving into Monday night, the heavy showers will extend further east to areas like Dubbo as well as the Northern Tablelands.
The consequential highest falls of the collective deluge are forecast to be received later tonight, and continue into Tuesday morning.
“By tonight, we’re likely to have every state and territory in Australia, except WA, under some sort of heavy rain warning,” Mr How said.
Warnings are set to cover various areas including the NT, Queensland, South Australia, the ACT, New South Wales, eastern Victoria and most probably the north-eastern parts of Tasmania.
“A huge section of Australia will be under this warning when the two systems collide tonight into Tuesday morning,” Mr How noted.
It’s not unusual for weather systems to collide, but recording this much rainfall in the lead-up is certainly not common.
Mr How said it was ironical for Sydney that the rainfall totals received so far in Nepean and Hawkesbury do not exceed what was received in February 2020.
“We did see a pretty similar event, in some regards, in February 2020, but the main difference was that coming off the drought, the soils were much drier and so had a much bigger capacity to absorb water,” he said.
“This time around, unfortunately because of a wetter La Niña summer, runoff is much greater.”
Recent analysis indicate that flood levels during this weather event will be below the 1867 flood event in the Nepean and Hawkesbury, but could surpass the 1961 flood event.
While this event is set to be unprecedented particularly for parts of New South Wales, it would not outdo other rainfall totals for 2010 and 2011 across Australia, according to Mr How.
Even though this rainfall would be welcome news for residents in areas still in need of good showers, other areas particularly the northern parts of Brisbane such as Rockhampton, have once again missed out.
When will it end?
The silver lining on the tropical cloud band is set to drive the coastal trough off and into the Tasman Sea.
Even with the low-pressure system inside the rainband, showers are expected to clear by Wednesday morning for most parts of Queensland and the New South Wales.
Wet weather will continue in Victoria and Tasmania through to Wednesday though the system is likely to clear completely off Australia by Thursday, Mr How told Storm Assist.
However, since heavy rainfall has been received in recent days, we can expect to see swollen rivers even after Wednesday, rolling high into the weekend.
“Even though the rain will stop and it will be mostly sunny in Sydney by Wednesday, the flooding risk will persist for some time to come,” Mr How noted.
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