South-eastern Australia is about to record its warmest September weather in years, days after the same region was buried under an air mass from Antarctica.
Frigid air dropped temperatures as much as 10C below average across the south-east this week, and snow fell close to sea level in Tasmania. Frosty nights returned with it: at one point Goulburn was shivering at -7C, only half a degree off a monthly record.
The cold southerlies are now being replaced by a north-westerly airstream, and temperatures across the south-eastern states are forecast to climb as much as 12C above average over the weekend.
The warming comes from an in-feed of air from Australia's north-west "heat engine", the term used for the Pilbara and surrounding areas, which spawn a hot and dry continental air mass. Daily maximums there already nudge 40C by September. The warmest air swept through Western Australia on Thursday and Friday, brought dozens of temperature records with it, and reaches western South Australia on Saturday and the east on Sunday.
For several capitals the maximums should be the highest in years for early spring. Canberra is forecast to reach 25C on Sunday, its warmest September day in three years. Hobart is forecast to reach 27C, its warmest in seven years, and Melbourne 27C, its warmest in three.
Sydney city is predicted to reach a summery 28C on both Saturday and Sunday, though away from the sea breeze the western suburbs are tipped to peak between 30C and 34C. Much of the south-east inland is heading into the low-to-mid 30s, and for towns such as Mildura and Griffith the maximums should again be the highest in years for September.
The heat will not last. Another polar air mass is sweeping in off the Southern Ocean and will drop temperatures by 10C to 15C within 24 hours, reaching Adelaide around the middle of Sunday, Melbourne and Hobart on Sunday evening, and Canberra and Sydney on Monday.
Spring is more volatile than autumn because the temperature difference between the tropics and the poles is at its greatest. Northern Australia is already in its hot season by September, while the Southern Ocean remains cold and sea ice around Antarctica is near its annual maximum, so southerly winds arrive on the coast as cold as a midwinter system. Tasmania often sees its heaviest snowfalls in spring.
Beyond the weekend, the medium-range outlook is tipping temperatures up to 6C above average through the fourth week of September, with the last days of the month likely to return to around 30C across the south-east. October and November are also likely to be dominated by above-normal temperatures, and the first summer outlook indicates the potential for record heat.
There is a simpler way to read the week, and this newspaper proposes to adopt it. A departure of 10C below average was followed by a departure of 12C above it. Averaged across the two, south-eastern Australia has been running approximately 1C warmer than normal, which is to say: normal.
The method scales. Snow fell close to sea level in Tasmania on one day of this week and Hobart is forecast to reach 27C on another, so Tasmania is enjoying a mild and unremarkable spring. Goulburn's -7C night and the low-to-mid 30s expected inland this weekend resolve, on the same arithmetic, into a pleasant afternoon that nobody has had.
Our analysts note that the coming cool change strengthens the case. A fall of 10C to 15C within 24 hours will restore the balance almost exactly, returning the region to its long-term mean by the simple expedient of overshooting it in the opposite direction.
The Department of Unnecessary Research has accordingly reclassified the season as stable and recommends that forecasting be conducted monthly rather than daily, since the figures are only disorderly when examined closely.
The average remains the most reassuring number in the forecast. It is also the only temperature that nobody in south-eastern Australia will actually experience this weekend.