Sydney health alert as mercury set to pass 40 degrees

Sydney's expected top temperature was trimmed by the Bureau of Meteorology to 35 degrees for the central business district - reversing an overnight revision higher.

Many western suburbs had already reported temperatures above 40 degrees by early afternoon, with Penrith setting its highest December mark with the mercury topping 43 degrees.

The bureau earlier issued a fire weather warning for the Sydney metro and Illawarra/Shoalhaven fire areas. Both districts have a total fire ban in place for the day.

"Very hot, dry and gusty northwesterly winds are expected ahead of a weak cold front that will bring a mild southerly change to southern and central coastal areas later in the evening," the bureau said.

The line tempering of the heat, though, should only extend to about Strathfield.

"East of that will see cooling in the afternoon once the sea breezes come through," Graeme Brittain, a meteorologist with Weatherzone, said
.
A cool changes of sorts - which is part of the prompt for the elevated fire risks - should reach Sydney at about midnight.

Much of Sydney can expect an uncomfortable night with a drop in temperatures towards the low-20s moderated by an increase in humidity, Mr Brittain said.

Record eclipsed

Sydney's CBD topped 30 degrees just after 10.30am and didn't get much hotter, reaching a maximumof 30.8 degrees, while Sydney Airport exceeded 35 degrees before 1pm.

Penrith reached 43.4 degrees just after 2.30pm, easily eclipsing its previous hottest December day of 42.3 degrees set in 2009, the bureau said.

Richmond, Camden and Bankstown were among the sites topping 41 degrees.

Most of NSW was in for a sizzler on Thursday. (See Bureau chart below.)

Health alert

NSW Health, meanwhile, has warned residents to try to stay cool and remain hydrated through the day.

"People can be unprepared for the first heat spike of summer, so we are reminding them to take safety measures against the effects of overheating and sun exposure," Ben Scalley, Director of Environmental Health, said in a statement.

"It's important people keep up their water intake, stay cool and avoid strenuous physical activity in the heat of the day," he said.

"Heat places a lot of strain on the body and can interfere with blood circulation and cause dehydration, heat exhaustion and heat stroke."

As reported by Fairfax Media, electric fans can provide considerable relief at as much as one-fiftieth the cost of air-conditioning.

Along with the heat, there is also a chance of thunderstorms in Sydney in the afternoon, as humidity starts to build.

The mild change is predicted to bring the maximum to 26 degrees on Friday - slightly above Sydney's long-term December average of 25.2 degrees.

Fire activity

Rural firefighters, meanwhile, are battling an out of control bushfire near Cessnock in the NSW Hunter region, AAP reported.

RFS issued a Watch and Act warning after 2pm on Thursday indicating the fire is moving in a southeast direction through Lovedale Road, Lovedale.

Crews on the ground are being supported by firefighting aircraft as the mercury hovers around 40 degrees.

More heat coming

Sunday may be the day to get done a lot of the remaining Christmas shopping because more heat is on the way.

"The early part of next week could see another burst of heat for Sydney," Mr Brittain said, adding the heat will be widespread across Australia's south east.

For Sydney's CBD, the mercury may not climb back into the 30s until next Tuesday, the bureau forecast.

Pentrith, though, has another hot week heat ahead, with 41 and 40 degrees predicted for Tuesday and Wednesday, respectively.

According to the bureau's heatwave forecast, that warmth with include a fair chunk of inland NSW facing "extreme" conditions. (See bureau chart below.)

Weatherzone is owned by Fairfax Media, publisher of this website.

With AAP