Perth will be by far the hottest Australian capital city on Christmas Day, with a maximum of 41°C expected as a broad area of heat bakes the west coast and nearby parts of Western Australia’s inland.
The mercury should also rise to the low-40s in WA’s fifth-largest city Geraldton (about 400km north of Perth) on Christmas Day for the third consecutive day.
The BoM is predicting low to severe heatwave conditions all the way from Broome in the Kimberley region to Albany in the South Coastal forecast district for the three days from this Tuesday, December 23.
Why so hot in the west?
A strong high pressure system centred over waters southwest of Australia is directing hot air from the continent’s interior over WA, as air circulates in an anti-clockwise direction. That’s part of the equation.
Image: Synoptic chart for Tuesday, December 23, 2025, showing the position of the high which is causing heatwave conditions in parts of Western Australia.
The other factor contributing to heatwave conditions is a heat trough currently positioned over WA's west coast.
Also locally known as a west coast trough, this is a low pressure zone that often forms during the warmest period of the year at the boundary between the flow of hot air from the interior and the cooler air over the Indian Ocean.
"The current trough's axis is oriented northeast to southwest, which means it's anchored over northwest WA and then pushes offshore," Weatherzone meteorologist Corine Brown explains.
"That means that sea breezes are prevented from developing along the coast, allowing heat to keep building throughout the day.
"A delayed sea breeze should kick in late on Christmas Day for coastal centres as the trough moves closer to the coast, but they'll have reached scorching temperatures well before then."
Image: The approximate position of the heat trough on Christmas Day is where hot winds from the east and cool winds from the west converge in this chart (the direction that wind the is travelling is indicated by the plain ends of the wind barbs).
A heat trough usually sticks around for at least a few days and can last for a week or more. This week’s event should dissipate after Christmas Day, with a return to more comfortable conditions in Perth and the entire South West Land Division from Boxing Day onwards for several days.
Perth should reach 27°C on Boxing Day after its 41-degree Christmas Day scorcher, with a run of three days with maximums of 29°C in Geraldton from Friday through to Sunday.
Image: Hourly graphs for Geraldton, WA, on the Weatherzone app.
Perth’s average December maximum is 29.5°C while Geraldton’s is 31.5°C.