Spring see-saw to deliver hottest September weather in years

Following a polar blast this week, temperatures across south-east states will soar as much as 12 degrees Celsius above average this weekend.

The rapid warming is due to an in-feed of air from Australia's north-west "heat engine" which has already brought dozens of temperature records to Western Australia this week.

The first summer outlook has also now arrived, and in a repeat of the spring forecast, weather modelling is indicating the potential for record heat.

Warmest September weekend in years

Frigid air from Antarctica dropped temperatures as much as 10C below average this week over south-east Australia, and snow fell close to sea level in Tasmania.

Frosty nights also returned — at one point Goulburn was shivering at -7C, only half a degree off a monthly record.

But our infamously variable spring weather has again flipped as the cold southerlies are now being replaced by a north-westerly airstream.

The warmest air swept through WA on Thursday and Friday, it will reach western South Australia on Saturday, followed by the east on Sunday.

For several capitals, maximums should be the highest in years for early spring:

• Canberra, 25C on Sunday: The warmest September day in three years.
• Hobart, 27C on Sunday: The warmest September day in seven years.
• Melbourne, 27C on Sunday: The warmest September day in three years.

Sydney city is predicted to bathe in a summery 28C on Saturday and Sunday, but away from a sea breeze, western suburbs are tipped to peak from 30C to 34C.

Much of the south-east inland is also heading into the low-to-mid 30s this weekend, and again for some towns and cities, like Mildura and Griffith, maximums should be the highest in years for September.

However, just as quickly as the heat arrives, it will be flushed away by another polar air mass sweeping in off the Southern Ocean, dropping temperatures 10C to 15C in 24 hours.

The change should reach Adelaide around the middle of Sunday, Melbourne and Hobart on Sunday evening, followed by Canberra and Sydney on Monday.

Spring weather volatility explained

While both autumn and spring are transition seasons, the weather is far more changeable in spring thanks to a greater disparity between tropical and polar temperatures.

Northern Australia is already in the hot season by September as the sun migrates back to the equator following the austral winter.

The high angle of incoming sunlight then combines with cloudless skies to ignite Australia's "heat engine", the term used to describe the Pilbara and surrounding areas which spawn a hot and dry continental air mass.

Daily maximums within the "heat engine'" already nudge 40C by September, and when northerly winds develop over our interior, the large mass of hot air is then transported to southern states.

With a well-established source of heat, the right weather pattern can therefore deliver early spring temperatures above 30C as far south as Hobart.

However, while the tropics are already scorching by September, the Southern Ocean remains cold, and annual sea ice is reaching a maximum around Antarctica.

The result is when southerly winds arrive on the southern coastline, they are as comparatively cold as a midwinter system, and Tasmania in particular, often sees its heaviest snowfalls in spring.

Autumn though has more uniform temperatures between the poles and tropics, and therefore more stable temperatures in the mid-latitudes.

Northern Australia is often cloudy in autumn and the sun has already moved north of the equator by late March, while thanks to thermal lag and the high heat capacity of oceans, the Southern Ocean is still relatively warm.

First summer forecast revealed

Despite the current day-to-day fluctuations, the spring and summer temperature outlooks in general remain unanimous that Australia is facing record challenging heat.

Following the upcoming cool change, warmth will again build through the week, and the last days of the month are likely to see temperatures return to around 30C over south-east states.

October and November are also likely to be dominated by above-normal temperatures, potentially raising the season's average above the unprecedented hot springs from 2024 and 2020.

And while seasonal forecasts for precipitation carry uncertainty, the early outlooks for summer are also confident that Australia could breach records.

Below is a multi-agency forecast for temperatures in the Australian region from December to February.

The map averages the output from nine different models, including the Bureau Of Meteorology's, and the consensus shows temperatures are likely to land in the range of 1 to 2C above the baseline average from 1993 to 2016.

However, Australia's seasons are ranked against a cooler 1961 to 1990 baseline climate, meaning the forecast, when compared to previous summers, is actually 0.82C warmer than what has been displayed, and that puts the record hot 2018-19 summer under threat.