The first major heatwave of the summer started to lap Sydney's suburbs on Wednesday and is unlikely to fully recede for another week.
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Sydney will be spared the worst of the heat for most of the coming eight days as cooling sea breezes are expected to keep the lid on temperatures on the worst extremes.
But out west, the temperatures have already started to climb.
Temperatures rose past 40 degrees Celisus at Fowlers Gap, Smithville, Wilcannia and White Cliffs in the upper western forecast district on Wednesday.
While Broken Hill and Mildura did the same in the lower western.
And things aren’t expected to cool over the next few days with Bourke set for 43 today (Thursday), 40 on Friday, 42 on Saturday and Sunday and 43 on Monday.
In the western districts Ivanhoe is forecast to hit 40 degrees today, 38 on Friday, 39 across on Saturday, before climbing again on Sunday to 43, before hitting 44 on Monday.
In the Riverina Griffith is looking at 39 today, 37 on Friday before climbing to 41 on Sunday and a scorching 44 on Monday.
Young will sit in the high 30s, as will Yass.
Moree is headed for 38 today and Friday, before dropping a degree on Saturday, remaining at 37 through to Monday.
Dubbo is headed for 39 tomorrow, before dropping to 37 on Friday and 38 on Saturday and Sunday. It is forecast to climb to 39 on Monday.
Mudgee will also be sitting in the mid to high 30s for the weekend while Scone will climb to 41 today, and 38 on Friday.
Further north and Armidale will be cooler in the high 20s/ early 30s from today.
The NSW Rural Fire Service has also declared a total fire ban for the Sydney and Illawarra/Shoalhaven regions on Thursday.
Much of the continent will bake from Thursday through to possible next Wednesday, particularly north-western NSW and south-west Queensland. There the hot spell is likely to be the worst for December in eight years if not longer, Brett Dutschke, a senior meteorologist from Weatherzone, said.
"It's unusually intense and long lasting for this time of the year," Mr Dutschke said.
Parts of southern Australia are broiling on Wednesday, with Adelaide already reaching 39 degrees and Melbourne headed for 37 degrees.
Onshore winds, though, will keep Sydney - or at least places close to the harbour or the coast - to a more pleasant 29 degrees, the Bureau of Meteorology predicts.
(See BoM chart of Wednesday's extremes below.)
Hottest still to come
A couple of weak cool changes will provide some relief for those near the coast on Friday and Sunday, before the heat starts to build again for the NSW capital.
"The following three days at least [from Sunday] will be when we feel it - that's when the nights are going to stay warmer for longer compared to the next couple of nights," Mr Dutschke said.
There's little let-up in the heat for many parts of NSW - and many other inland regions - over the next week. Photo: John Veage
"It looks like there's a good chance that next Wednesday will be our hottest day [of the current spell] just ahead of a stronger, cooler change," he said.
Where possible people should take advantage of cooler evenings to open windows to help buildings lose some of their built-up heat.
Double block
A slow-moving high pressure in the Tasman Sea and a similarly stagnant low over north-western Western Australia are combining to push the heat over central Australia into the south and south-east.
The Sydney metropolitan region has a "very high" fire danger rating for Wednesday, and that will climb to "severe" on Thursday with the total fire ban coming to effect from midnight.
"It looks like the fire risk will be greatest [for Sydney] tomorrow, but it's not looking particularly bad," Mr Dutschke said, adding that next Wednesday "could rival that".
By Saturday, the three-day heatwave forecast has at least half the country in a low-intensity or even severe heatwave, the bureau predicts. (See chart below.)
If the town has more than seven such days, which is possible, it would be a 23-year high for this time of year.
Thursday has the potential to be Penrith's hottest December day in more than 20 years, most likely reaching 42-43 degrees, Mr Dutschke said.
It did, though, notch an even more sizzling 44.9 degrees in November 2014.
Weatherzone is owned by Fairfax Media, publisher of this website.