Senior Weatherzone Meteorologist Jacob Cronje said temperatures were unlikely to exceed 25 degrees anywhere in Sydney on Christmas Day.
"We are looking at clearing showers later this morning into midday, but conditions will remain partly cloudy and windy from the south, therefore quite cool," he said.
"We're looking at temperatures ranging in the 23 to 25-degree mark anywhere in Sydney, possibly reaching about 26 in places like Penrith."
The inner city can expect a top of 23 degrees for Christmas Day, Parramatta is headed for a top of 24 degrees, and in Bondi it will be slightly cooler with top of 20.
While it's expected to stay cloudy, Mr Cronje said it doesn't look like Sydney will experience any afternoon showers.
"By midday we should have seen most showers abated and just the odd shower here and there," he said.
People out shopping in the Boxing Day sales can expect slightly warmer weather, with an isolated shower or two in the morning.
"Overnight we will see those winds shift and things will start to warm up as well tomorrow - not significantly, mind you, just by a degree or so - but we will have some showers in the morning here and there," Mr Cronje said.
The inner city is forecast to hit 24 degrees tomorrow, while Penrith and Parramatta will reach 26 and 25 degrees respectively on Boxing Day.
"We're looking at most likely in the morning very cloudy, becoming partly cloudy in the afternoon," he said.
The shift in wind direction from southerly and south-easterly to an easterly and north-easterly on Boxing Day will be good for the start of the Sydney to Hobart Yacht Race, Mr Cronje said. In Melbourne, the weather is also expected to be good for the Boxing Day Test.
"Even though it's cloudy and 22 today [in Melbourne], tomorrow will be 29 degrees, partly cloudy skies and we won't have any rain to speak of so it's going to be a pretty good day," he said.
Sydney's weather is going to heat up through the week, reaching the 30s by Friday, he said.
"By Wednesday temperatures will reach the mid to high 20s, and very likely by Friday we'll see temperatures in the low 30s in the city and the high 30s over the western parts of the Sydney basin."
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The story A cool start to Christmas, but the temperature will soon heat up first appeared on The Sydney Morning Herald.