South-east faces flooding, storms after second-driest September on record

Rain descends on Port Phillip Bay on Thursday afternoon, with Melbourne soaked by 20mm in one hour between about 2 and 3pm. (Supplied: EC Bird)

A major rain and storm event intensifying across south-east Australia could trigger flooding during the next 48 hours. 

The sudden soaking follows a severe lack of rain in September — Melbourne only picked up 18mm last month, but was soaked by 20mm in just one hour on Thursday afternoon.

And for many areas, even heavier rain is still to come as a stream of tropical moisture feeds into a low-pressure system over the southern Murray Basin. 

The system is notable for its slow movement, which should ensure moderate to heavy falls continue today, with a focus across northern and eastern Victoria, along with the south-east corner of New South Wales.

Isolated thunderstorms are also firing up near the leading edge of the rainband, and with abundant moisture and an overriding jet stream, storms could become severe and produce intense rainfall, with a lesser risk of damaging winds and large hail.

Thunderstorms are possible today across much of south-east Australia and they could produce flash flooding and damaging wind gusts. (ABC News)

In the meantime, rain in South Australia has now mostly eased back to showers, although some coastal regions, including Adelaide, could still pick up another 10mm or more by tonight.

While the main band of precipitation has been shifting slowly east, as the low drifts towards Bass Strait tomorrow, it could re-circulate rain back across southern Victoria.

It means moderate to possibly heavy falls and thunderstorms are still possible tomorrow across a large swathe of Victoria, along with central and eastern NSW.

And again on Saturday, severe thunderstorms with flash flooding, gusty winds and hail are possible, most likely on the NSW western slopes and ranges.

A flooded road in West Footscray following heavy rain on Thursday afternoon. (Supplied: Violetta Kara)

By Sunday the low should be slowly retreating in the Tasman Sea, although a few showers and storms are still possible in Gippsland and south-east NSW.

Adelaide – showers are likely through this morning and afternoon, but modelling indicates they should mostly clear to the east in the evening with only the odd shower returning tomorrow.

Melbourne — further showers are likely today, although perhaps not at the intensity of the past 24 hours. The weather tomorrow depends on the exact location of the low, but the risk of heavy falls returning to Melbourne is plausible. All up, this event could bring more than 50mm to the outer metropolitan region, while the inner suburbs and CBD could pick up from 20 to 50mm, including what fell yesterday.

Canberra — Rain is almost guaranteed both today and tomorrow, including the risk of severe thunderstorms. Modelling is indicating 20 to 60mm during the next 36 hours, before rain mostly clears tomorrow night.

Sydney — rain should arrive later today from the west and could intensify tomorrow with possible thunderstorms. While moderate falls are possible, modelling is favouring the heaviest rain will remain just to the west and south of Sydney.

All up, the weather system is likely to drop at least 25mm south of a line from about Adelaide to Sydney, including pockets near 100mm over southern and north-east Victoria.

There are some indications parts of Gippsland could even see closer to 150mm, definitely enough to trigger river rises despite dry catchments after below-average rain through winter and September.

For north-east Victoria, winter rain was plentiful and catchments are still moderately wet.

As a result, a Flood Watch is in force for minor to moderate flooding across 16 separate catchment regions covering Gippsland, North East and Central Victoria.

A Flood Watch has been issued for up to 16 catchments covering Central, North East and Gippsland in Victoria. (ABC News)

The moist tropical airstream is also producing a secondary area of rain across the western interior near another trough of low pressure.

This outback rain could amount to 50mm, which is extremely unseasonable considering the WA interior averages only about 10 to 20mm through the entire month.

Well over 50mm of rain could fall between Thursday and Sunday over parts of Victoria, while 20 to 50mm totals are expected over parts of NSW, SA and the western interior. (ABC News)

Australia just experienced its second-warmest September on record, with the majority of the nation at least 2 degrees Celsius warmer than normal.

But in such a large country there is always variability, and pockets of every state and territory did see September records fall, including Canberra, where temperatures landed at 2.35C above the city's long-term mean.

September temperatures were in the top 10 per cent of years across the majority of Australia, and the highest on record over parts of the interior and south. (Supplied: Bureau of Meteorology)

Sydney, Perth, Adelaide and Hobart all had their second-warmest Septembers on record, but the most extraordinary anomalies were over the outback, where temperatures were as much as 5C above the long-term mean.

Alpine Australia also experienced unprecedented early spring heat, including Thredbo, where temperatures were more than 1.5C warmer than the previous warmest September.

While inland WA received record-high rainfall in September, pockets of south-east of Australia recorded their lowest-ever rain totals. (Supplied: Bureau of Meteorology)

For south-east Australia, it was also the second-driest September, but again, there were pockets that did see record low falls, including around central SA, the Victoria coast and along the central stretch of the NSW ranges.

But the wet start to October could be a sign of wetter months ahead, with modelling tipping further rain during the coming weeks, along with a general improvement in rain prospects as we transition into summer.