A large, complex system of low pressure is establishing itself over the southeastern mainland this week, in an unusual set-up for the last week of autumn.

As Weatherzone meteorologist Ben Domensino wrote on Monday morning, a slow-moving upper-level cut-off low pressure system will deliver wet, stormy and windy weather to a broad area, and has already begun to do so.

What is a cut-off low and why is this system unusual?

Cut-off lows are common over eastern Australia in autumn and winter, but it's unusual to see them stick around for so long. This week’s system appears in no hurry to go anywhere fast.

As Weatherzone meteorologist Yoska Hernandez explains, a cut-off low forms when a pool of colder air and low pressure in the upper atmosphere becomes separated, or "cut off", from the main belt of strong westerly winds that usually flow across the Southern Ocean, south of Australia. 

"Once cut off from this fast-moving flow, the low can become slow moving and linger over the same region for several days. This can lead to prolonged periods of cloud, rain, showers and gusty winds," Hernandez added.

Words like "vagrant" or "wandering" are often applied to these upper atmospheric cold pools, because their trajectory (and the associated wet and/or severe weather) is notoriously hard to plot due to a lack of strong, consistent steering winds. 

Where will it rain this week? 

Moderate and even heavy rainfall totals have been recorded in the 24 hours to 9am Tuesday across parts of New South Wales, Victoria, South Australia, Tasmania and the ACT. 

Further significant rainfall can be expected throughout this week – including in parts of southeast Queensland and central and northeastern New South Wales which have been very dry in recent months – due to a surface low which has formed in response to the upper low. 

As the week progresses and the weekend approaches, the surface low will cross the coastline into the Tasman Sea and it is likely to produce heavy rain over parts of the NSW coast and nearby ranges, with the help of well-above average sea surface temperatures.

Image: Mean sea level pressure and precipitable water for this coming Friday night, showing the position of the low off the NSW coast. Source: Weatherzone.

Does this system have snow potential?

The official Australian ski season begins in under two weeks, on Saturday, June 6. Currently there is no natural snow cover anywhere in Australia, and that’s highly unlikely to change until the end of this week at the earliest.

One feature of cut-off lows is that they don’t usually bring cold temperatures to a large area.

Indeed, you’ll often see rain (or no precipitation and mild temperatures) in Australia’s alpine region. On the menu for mainland Australia’s alpine resorts this week is rain, possibly heavy at times.

Towards the end of the weekend, winds look likely to swing more westerly, and that’s when the high country could get a dusting of white. Most models are also hinting at the chance of further snowfalls at Australia’s highest elevations during the middle of next week.

This much is clear: with low pressure dominating the weather across the southeast for the next week or so, there’s strong potential for wet and possibly stormy weather across the most heavily populated corner of the country.