A north-west cloud band will engulf Australia this week, signalling a rapid transition away from the monotonous high pressure systems which have dominated recent weather charts.

At its peak, this week’s cloud band will stretch more than 4,000 kilometres from Western Australia to the east coast, delivering welcome rain after an extended dry spell.

Ultimately the system should bring rain to more than 80 per cent of Australia, arriving just in time to prevent parts of WA and South Australia from seeing their driest autumn on record.

Why north-west cloud bands have been missing in 2024?

A north-west cloud band is a continental scale band of continuous cloud that stretches from north-west to south-east Australia.

Unlike most weather systems which bring variable rain to localised regions, north-west cloud bands are different.

Their enormous lateral expanse and consistent depth can bring relatively uniform rainfall over multiple states — the type of steady soaking rain farmers love.

The more extensive bands which extend from the northern Indian Ocean to the Tasman Sea have a peak frequency between March to September, however, despite being nearly halfway through the season, they have been notably absent from the satellite picture this year.

So, where have they gone in 2024?

The process for a north-west cloud band starts with thunderstorms bubbling away off the north-west coast of WA, which lifts moisture vertically from warm tropical waters high into the atmosphere.

If the moisture is then carried south by upper-level winds, and this stream encounters the denser air associated with a cold front, this leads to further lifting and a broad ribbon of cloud and rain develops.

Their absence in 2024 is the direct result of a reduction in cold fronts, which links back to the dominance of blocking high pressure systems, however colder than normal water temperatures off WA’s northern coast have most likely also played a role in reducing numbers this year.

Week of rain to replace stubborn high pressure systems

Rain from the cloud band will start falling near the WA west coast on Tuesday, then track east across central and south-east Australia from Wednesday to Friday, before reaching the eastern seaboard at the weekend.

The system will deliver rain to every state and territory along the way — for some regions the first significant precipitation in two months.

The best falls, about 10 to 30 millimetres, will land in a broad swathe from the Kimberley through the central outback to the Murray Darling Basin.

Moderate rain is also likely over south-west WA and parts of Tasmania, which will face the brunt of the associated cold front, however other southern areas, including many parts of SA can expect less than 10mm.

A solid week of rain for Australia, although parts of SA are likely to see below 10mm.  (ABC News)

For the New South Wales coast, totals are less certain, since the system won’t arrive until the weekend, however some modelling indicates the north-west cloud band could interact with a developing low pressure trough in the Tasman Sea to cause heavy rain.

While north-west cloud bands rarely bring flooding, their presence is vital to replenish soil moisture across southern Australia for winter crops. And for areas that miss out on the best falls on this occasion, there’s hope of follow-up rain arriving quickly.

A stronger cold front reaching WA on Sunday could potentially lead to a second cloud band, however it’s currently uncertain whether or not this system will reach eastern states and evolve into a fully-fledged north-west cloud band.

The impact of this second system could ensure Perth is soaked by nearly 100mm of rain across the coming week, the city’s wettest seven days since last June.

Wet finish to near-record dry autumn in WA and SA

Adelaide is in desperate need of rain after receiving just 16mm this season.(Supplied: Patrick Wynne)

This autumn has been extremely dry in most southern parts of Australia, apart from NSW and south-east Queensland.

Both Perth and Adelaide were facing their driest autumns on record before this week, indicating how desperate the lack of rain was becoming.

To Monday 9am, Perth had only received 24mm this autumn, well below the city’s average of 179mm, and below its current record of 42mm in 1983.

Compounding the severe drought across Perth is relentless heat with the city’s maximums on track to average just below 28 degrees Celsius this autumn, 3C above normal and easily the highest on record.

Adelaide has received even less rain than Perth, just 16mm this season, and may still break an autumn record if at least 7mm does not fall by Friday 9am.

While most of southern Australia has been dry this autumn, NSW was the clear exception — Sydney’s estimated (due to a station malfunction) seasonal total of 586mm is more than 200mm above average.

Posted , updated