Australia Weather News

A cyclone spinning over waters to the west of Austraila is helping set up a large heatwave across the country this week. 

Forecasts from the Bureau of Meteorology's heatwave warning service show nearly half of Australia under varying levels of heatwave conditions between Tuesday and Saturday.

This includes 'severe' heatwave conditions through large portions of Western Australia and Queensland, including Perth and Brisbane, and some pockets of 'extreme' heatwave conditions in Queensland.

The Bureau's definition of a heatwave is based on three-day average temperatures (both day and night), taking into account what the average temperature of the area at that time of year is, as well what conditions have been like in the lead up.

Hot days meet warm nights

BoM senior forecaster Dean Narramore said both the day and night-time temperatures would be much warmer than normal over the next four days for regions under the warning area.

In Western Australia, maximum temperatures are forecast to be between 6 degrees Celsius and 12C higher than usual during the heatwave period.

This includes forecast tops between 39C and 41C in Perth from Tuesday and Friday, made to feel worse by high humidity on Tuesday — a feature not often associated with the western capital.

"There seems to be some moisture getting in from the ocean, particularly for those right on the beach, which can make it feel warmer than it actually is."

The run of warm weather comes after parts of Perth hit 45C on Monday, while the regional city of Geraldton — further north — equalled the record for its hottest-ever day with the mercury reaching 49.3C.

In Queensland, the worst of the heat is forecast to set in toward end of the working week, with maximum temperatures broadly forecast to reach between 6C and 12C higher than average during the day, according to Mr Narramore.

Brisbane is forecast to reach between 35-37C between Wednesday and Friday, though high humidity will likely make it feel more stifling there too.

In addition to the warm days, Mr Narramore said the overnight temperatures were also expected to be much higher than normal during the heatwave.

Night-time temperatures make up an important component of heatwave warnings, when it comes to the human impact. 

Studies have shown days with hot nights can significantly increase the mortality risk, compared with days that don't have hot nights. They also mean the next day's heat starts from a higher base.

"Perth is looking at four nights in a row above 25C, with some inland places in the high 20s, and Queensland's similar as well," Mr Narramore said.

"So that's pretty gross, no matter how you look at it," he said.

Victoria, New South Wales, Tasmania and South Australia are outside of the warning area, and are forecast to get milder summer weather for much of the week.

Cyclone helps drive hot winds 

Part of the reason for the prolonged and intense heat is related to the "very slow moving" cyclone, spinning over waters to Western Australia, according to Mr Narramore.

Cyclone Sean developed into a cyclone on Sunday, and remains at cyclone strength well off shore to the west of Carnarvon.

The system helped deliver Karratha nearly a year's worth of rain in one night on Sunday, breaking its record for daily rainfall with more than 274mm in the gauge. 

While the impacts of the cyclone are now mostly over, Mr Narramore said the system was now helping set up a "traffic jam" in the weather pattern, causing hot, desert winds to be fanned into the west for multiple days.

Typically, systems such as a cold fronts would move from west to east across the country, dragging hot air ahead of them but bringing cooler air behind them — meaning the heat only lasts a day or two.

But at the moment both the cyclone and a high pressure system located to the south of Australia are pushing against that flow. This has meant hot, easterly winds originating from the central desert are flowing into western and southern Australia day after day.

"When you have easterly winds with depth, it's like a big block or a car accident on a freeway for any weather system trying to come into that," Mr Narramore said.

"So you've got to go around the car crash, and that's what the weather is doing. All the weather coming from the west just goes around it, so that's why you're stuck with the same pattern for a few more days."

In Queensland, the heatwave conditions are being driven by a long period of west to north-westerly winds, bringing the heat from central and northern Australia into the state.

"Heatwaves anywhere in the country about where the winds are coming from," he said.

"So in Darwin you need it southerly, Melbourne you need it northerly, Perth you need it easterly and the east coast you need it westerly.

"[This week] we're seeing long duration period of kind of westerly winds through Queensland, so that's dragging the heat from the Northern Territory, South Australia and Birdsville into eastern and south-eastern Queensland and north east NSW."

Though it's not quite as direct as the impacts in Western Australia, Mr Narramore said the cyclone had a role to play in this weather pattern too.

This was because the 'traffic jam' in the west was causing a cold front to dip under Western Australia and move through south-eastern Australia, leading to a pattern of westerly winds across Queensland.

The late onset to the monsoon — a seasonal wind pattern that brings heavy rain to Australia's tropical north — has also added to the intensity of the heatwave on both sides of the country, according to Mr Narramore.