A quick glance at the national forecast this weekend and you could be forgiven for questioning what season we’re in.
While Perth bakes under sunny skies and mid-30 degrees Celsius heat, our south-east capitals will endure a spell of showery and wintry weather.
Polar air from the south will bring snow down to 600 metres elevation in Tasmania this weekend, and after a solid snowfall on Thursday a few flurries should also continue on the mainland Alps.
The disparity between the west and east will become even greater on Monday as a hot northerly sends southern Western Australia to near record-high October maximums, while another pulse of polar air increases snowfalls on the south-east highlands.
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Perth is forecast to reach 35C on Monday and Melbourne just 15C — with showers and small hail thrown in.
While the west is too hot, and south too cold, Sydney and Brisbane can expect a goldilocks kind of weekend … just right.
Both cities will welcome warm and sunny skies, light winds, and relatively low humidity, making it ideal weather for the beach, picnics, and weekend sport.
The colder air over south-east Australia will then move up the NSW coast on Monday before moving up to south-east Queensland on Tuesday, but not before Brisbane roasts in a Monday maximum of 34C.
Across the tropics during the coming days, while it may be officially the wet season, don’t expect clouds to provide shade from the sweltering heat.
Most towns away from the north-east coast can expect tops near 40C, which is warm even for northern Australia in the hot season.
Why is spring weather so changeable?
Spring’s variable weather across southern states is fairly exclusive — winter days are typically cold, summer days are mostly hot, and autumn brings extended periods of mild to warm weather.
So what is happening during spring that causes such volatile conditions?
Essentially the answer is that a battle is taking place between tropical air from the north and polar air from the south.
Let’s start with the pattern developing over the north.
By September 23, the equinox, the sun is already passing directly over the equator and before the true wet season arrives there is no cloud to prevent its high angle from generating a very hot and dry air mass over northern Australia.
This unimpaired exposure to long hours of direct radiation each day is why spring is the hottest time of year for much of Australia’s north, and when a northerly wind blows, the hot air is transported to southern states.
While tropical Australia is baking by early spring, Antarctica is still in darkness and sea ice is reaching a maximum.
When air escapes from the polar south and reaches Australia, it brings temperatures comparable to mid-winter.
The clash of hot and cold air not only leads to extreme weather but also strengthens the jet stream, which keeps weather systems moving rapidly.
It explains why we often see bushfires burning one day and snow falling the next, however this pattern is rarely repeated at the opposite equinox in March.
Although the sun’s angle is still high over tropical Australia in early autumn the still active monsoon brings a cooling influence from cloud and rain, while water temperatures in the Southern Ocean have only just passed their yearly maximum.
The result is a reduced contrast when the opposing air masses collide over southern Australia, making the weather far less volatile.
When is the next burst of heat for south-east states?
With one of the strongest positive IODs on record it wont take long for another round of warm temperatures to arrive over south-east states.
Modelling is currently suggesting the cold southerly winds over south-east states will swing to a northerly from Wednesday, starting in South Australia then moving east.
At this stage the spell of warmer weather looks short lived before the next front arrives from the Southern Ocean later this week and the spring see-saw swings on.
Some meteorologists have even theorised Katie Perry’s “Hot N Cold” number one hit was actually written due to her frustrations with spring weather rather than her lover.
