Twin rainbands offer hope of drought relief across southern Australia

Australia Broadcast Center

2 days ago

Despite hopeful forecasts in autumn, widespread soaking rains have so far failed to materialise across much of southern Australia this winter.
The relatively dry start to the season has maintained the record drought in South Australia and Victoria, while less severe rainfall deficits have now also spread north through western and central New South Wales.
It is a similar story across Western Australia, with farmers struggling through below-average rain this cool season, despite moderate relief during the past few weeks.
But there is now hope on the horizon. Weather modelling is showing increasing potential for the best rains of the year during the next 10 days.
The longer-term prospects also remain positive, with outlooks for the remainder of 2025 favouring consistent wetter conditions to become the norm.
Frequent cold fronts this winter have brought bursts of showers and plenty of alpine snow. However, many agricultural regions, especially inland areas, are still waiting for meaningful relief.
So why have typically reliable fronts not delivered more rain this winter?
The missing ingredient has been the absence of pre-frontal tropical moisture critical for inland cloud bands.
Without the inclusion of humid air from the north, fronts will often only supply showers along the coast and ranges as limited Southern Ocean moisture is quickly wrung out further inland.
Although rainfall through June and July has been mixed, fortunes could be about to swing dramatically.
Two rainbands are set to sweep east across southern states during the next week, and the second system has the potential to form into a full-blown inland low with widespread rain.
But let us start with system one. After an initial weakening front brushes the far south-east today, a more vigorous system will reach WA through Sunday and Monday, and bring rain south of a line from about Shark Bay to Eucla.
Around 20 millimetres should fall in coastal areas from about Lancelin to Esperance, including around Perth, while the wheat belt can expect closer to around 10mm.
The front will then shift rain across south-east states early next week, although, in a repeat of previous systems this winter, many inland districts, such as the Riverland, Murraylands, Mallee and western Riverina, may only see a millimetre or two.
However, front number one is just a warm-up to a stronger system later in the week.
Modelling is in healthy agreement that a powerful front will reach south-west WA on Wednesday and bring further widespread showers. Although, again, that is nothing we have not already seen multiple times this winter.
However, on this occasion, as the system moves into the Bight on Thursday, it is likely to be preceded by a plume of tropical moisture — the genesis for a rainband developing across the interior.
The key is what happens to this rainband from Friday, with modelled scenarios currently ranging from a steady path over south-east states, which would produce only light rain, to the development of a major low-pressure system.
Under the low scenario, the rainband would both slow down and increase in intensity, resulting in possibly several days of widespread rain extending from tropical Queensland to Tasmania.
Below is one model's current output showing total rainfall over the next 10 days, noting the solid falls across the Murray Basin, and while most models have a less generous spread of precipitation, the system represents the best potential for significant drought relief so far in 2025.
The Bureau of Meteorology's (BOM) ensemble forecast, which involves running a model 99 times to determine the probability of an outcome, is also favouring a wet week.
Of those 99 forecasts, more than 80 indicate wetter-than-normal conditions across a large swathe of northern, central and south-east Australia.
This outcome is expressed as a greater than 80 per cent chance of above-median rainfall, shown as the darkest green shading in the map below.
Although there are no guarantees in long-range forecasting, the BOM's ensemble prediction discussed above also favours a wet three months from August to October.
The ensemble is most consistent that above-median rain will arrive through Australia's east, including a greater than 80 per cent probability in parts of northern NSW and western Queensland.
The BOM's modelling is also backed by a leading global model, European Centre for Medium-Range Weather Forecasts (ECMWF), which is even more confident of a wet three months.
The modelling displays a 70–90 per cent chance of above median falls from Tasmania, through the eastern states to the Top End.
While these forecasts appear to suggest the relentless dry spell should come to an end, probability outlooks still leave the window open for unexpected results.
For example, there may be a five-in-six chance you will not roll a one on a standard die, but it is not a guarantee — and if a one is rolled, that does not imply the initial probability was incorrect.