One day has brought cold fronts, strong winds and sharply cooler temperatures. Just days later, parts of the region have surged into unusually warm conditions, with temperatures reaching the low to mid-30s in some inland areas.
The swings have been striking, but they are also a reminder that spring can bring rapid changes across Australia’s south-east.
The Bureau of Meteorology says the broader spring pattern is leaning warmer than usual, while rainfall is expected to be below average across parts of south-eastern Australia. El Niño is also strengthening, although the Bureau cautions that it is only one of several factors controlling Australia’s weather.
From polar blast to September heat
The change has been particularly noticeable during September.
On September 15 and 16, a strong cold front crossed south-eastern Australia. Strong to damaging winds affected South Australia, Victoria, Tasmania and southern New South Wales. Some locations recorded wind gusts above 80 km/h, including 144 km/h at kunanyi/Mount Wellington and 133 km/h at Mount Hotham. Melbourne Airport recorded a gust of 104 km/h, its strongest September gust since 2002.
Then the weather pattern changed quickly.
By the weekend of September 19, temperatures across parts of south-eastern Australia were forecast to climb 6 to 12 degrees above average. Coastal areas were expected to reach the high 20s, while some inland locations moved into the low to mid-30s.
Sydney provided another example of the rapid swings. Temperatures in the city’s outer west reached about 35°C on September 21 before a cool change brought a sharp drop within hours.
These rapid changes can make September feel less like a steady transition into summer and more like several seasons arriving within the same week.
Why spring weather can change so quickly
Spring is a transition season, and large weather systems can move across the southern states quickly.
Cold fronts can push north and east, bringing cooler air, rain and strong winds. Behind them, high-pressure systems and northerly winds can pull warmer air towards the south-east.
That is part of what produced the recent contrast between the cold conditions and the later heat.
The Bureau’s latest weekly outlook also shows how quickly the pattern can change. Its September 27 update described a mixed week, with showers and storms affecting different parts of the country while another warm period was developing across the south-east.
For residents, this means a warm afternoon does not necessarily indicate that the cooler weather has disappeared for the season.
The bigger spring signal is warmer conditions
Although individual cold fronts will continue to occur, the seasonal outlook points towards a warmer-than-average spring overall.
The Bureau’s September-to-November forecast shows above-average maximum temperatures are likely across most of Australia outside parts of the tropical north. South-eastern Australia has one of the highest chances of unusually warm maximum temperatures.
Minimum temperatures are also expected to be above average across most of the country, with particularly high chances of unusually warm nights across much of southern Australia.
This distinction is important.
A warmer seasonal outlook does not mean every day will be hot. Cold fronts can still produce sudden cold spells, especially in Victoria, Tasmania and southern New South Wales.
Instead, it suggests that when the whole three-month period is considered, temperatures are more likely to sit above the long-term average.
Rainfall is another part of the story
Temperature is only half of the picture.
The Bureau expects spring rainfall to be below average across parts of south-eastern Australia, including southern New South Wales, Victoria and Tasmania. The dry signal is particularly strong early in the season.
That does not mean these areas will remain dry every week.
A single cold front can still produce heavy rain, thunderstorms or strong winds. In fact, September has already produced significant rainfall events in parts of the south-east.
The seasonal outlook simply means the total amount of rain over the broader period is more likely to be below the long-term average.
This combination of warmer temperatures and lower rainfall can become important for agriculture, water supplies and bushfire conditions later in the season.
El Niño is adding another layer
The current weather story is also unfolding as El Niño strengthens in the Pacific.
The Bureau says El Niño is now established and is likely to strengthen further. It is commonly associated with warmer and drier conditions across parts of Australia, particularly during winter and spring.
But the Bureau has stressed that the strength of an El Niño event does not automatically determine how strong its impact will be in Australia. Other climate influences can alter the final weather pattern.
This year’s situation is a good example.
Despite El Niño, winter rainfall was unusually mixed across Australia. Some areas of the south-east experienced dry conditions, while other areas received substantial rainfall.
That is why forecasters look at the entire climate system rather than using El Niño alone to predict individual weather events.
What September’s swings could mean for the weeks ahead
The recent cold-to-hot swings should not be taken as proof that Australia is heading into constant extreme heat.
Instead, they show how quickly spring conditions can change while the broader seasonal background becomes warmer.
The Bureau expects above-average temperatures to continue being a feature across much of Australia during spring. At the same time, southern and eastern areas are expected to have a greater chance of below-average rainfall.
That combination could make weather planning more complicated.
A cool morning can still turn into a very warm afternoon. A hot spell can be followed by a strong cold front. And a generally drier seasonal pattern does not prevent individual storms or heavy rainfall events.
For people living in Victoria, southern New South Wales, Tasmania and South Australia, keeping an eye on short-term forecasts will therefore remain important even when the seasonal outlook points in a particular direction.
Spring is moving towards summer, but not in a straight line
September’s wild temperature swings are a reminder that Australia’s transition into summer rarely happens smoothly.
Cold fronts will continue to push across the south-east, while warmer air can arrive quickly between systems. The difference this year is that the broader climate outlook is pointing towards unusually warm conditions across much of the country.
For households, farmers, travellers and outdoor workers, the practical lesson is to prepare for both sides of spring.
The next cold front may still bring a sharp temperature drop and damaging winds, while the following weather system could quickly bring sunshine and temperatures well above normal.
That spring rollercoaster is unlikely to disappear overnight. But the longer-term signal from the Bureau is clear: as Australia moves deeper into spring, warmer-than-average conditions are becoming an increasingly important part of the weather picture.