The first major heatwave of the summer started to lap Sydney's suburbs on Wednesday and is unlikely to fully recede for another week.
Sydney's central business district will be spared the worst of the heat for most of the coming seven days as cooling sea breezes are expected to keep the lid on worst extremes.
Western suburbs, though, sweltered from Wednesday with the mercury in Penrith nudging 40 degrees, with 42 degrees forecast there on Thursday. It may also touch 40 degrees again next Wednesday, the Bureau of Meteorology said.
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 during this period, 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 37 degrees.
Onshore winds, though, kept Sydney - or at least places close to the harbour or the coast - to a more pleasant 28 degrees, the bureau said.
(See BoM chart of Wednesday's extremes below.)
Weaker sea breezes on Thursday, though, will see a top in Sydney jump higher to 37 degrees.
Penrith is the hottest site across the city so far on Wednesday, with 39.8 degrees clocked at 3.30pm.
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.
Total Fire Bans will be in place for the Greater Sydney Region and the Illawarra/Shoalhaven tomorrow Thursday 14 December. Severe fire danger is forecast in these areas. #NSWRFS pic.twitter.com/c8D8GvcrpC??? NSW RFS (@NSWRFS) December 13, 2017
"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 should it top 42.3 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.
