Most of NSW will continue to bake in a prolonged heatwave well into next week, the second big warm event to get the year off to a hot start.
The roasting comes after the state posted its hottest year on record in 2017, a period which was also Australia's third-hottest year and equal-second warmest globally.
Sydney's west climbed above the 40-degree mark on Friday, with Penrith's 40.5 degrees the hottest for the city. Observatory Hill and other eastern sites were spared the worst, with sea breezes keeping the maximum to just over 28 degrees.
A cool change that moved across southern Victoria on Friday curtailed temperatures in that state but not before Melbourne - including the area where the Australian Open is being played - exceeded 40 degrees for a second day in a row.
That change, though, will stall over NSW and central Australia "due to no systems moving in to displace this hot air", said Grace Legge, a meteorologist with the Bureau of Meteorology.
The heat will continue through the weekend, with large areas of NSW expected to endure temperatures in the low- to mid-40s.
"Coastal regions of NSW will see some relief, with sea breezes expected to keep those temperatures down," Ms Legge said. "But just inland from the coast, we could see temperatures climb to the low 40s - including the western suburbs of Sydney, where over the next four days we're expecting to see temperatures above 40 degrees."
According to the bureau's heatwave service, virtually all of NSW can expect to have at least a "low-intensity" heatwave until at least Wednesday, with some regions reaching "severe" levels.
'Significant'
Blair Trewin, a senior climatologist with the bureau, said the heatwave was "significant but not in an overwhelming sense", and may be notable more for its duration than intensity.
By contrast, the intense heat event last February - that forced NSW to curtail electricity demand to stabilise the power grid - will likely feature in the full State of the Climate report being compiled by the World Meteorological Organisation when it is released in March, said Dr Trewin, speaking in his capacity as scientific co-ordinator of the global review.
Overnight, the WMO was one of several major agencies to release an initial assessment of worldwide weather conditions in 2017.
The agency declared global temperatures last year in effectively a dead-heat with 2015 as the second-hottest year since records began about 140 years ago, trailing only 2016.
"We can't split [2017 and 2015] in any meaningful sense," Dr Trewin said.
Notably, last year was the warmest on record without an El Nino event, which typically boosts temperatures as the Pacific takes up less heat than in other years.
(See chart below, showing the influence of El Nino, La Nina and neutral years.)
Last year and 2015 were about 1.1 degrees above pre-industrial levels, and 17 of the warmest 18 years have happened this century, the WMO said.
Other agencies, such as the US National Oceanic and Atmospheric Administration, NASA, the UK Met Office and Japan's Meteorological Agency, all ranked last year as the second- or third-warmest on record.
Stripping out the influences of El Nino or La Nina, the background warming trend from climate change has been a consistent 0.15-0.2 degrees per decade, Dr Trewin said.
While last year was notable for its average temperatures, extremes took their toll too.
"The warmth in 2017 was accompanied by extreme weather in many countries around the world," WMO secretary-general Petteri Taalas said.
"The USA had its most expensive year ever in terms of weather and climate disasters, whilst other countries saw their development slowed or reversed by tropical cyclones, floods and drought."
An example of the greater incidence of extreme weather days was Friday's reading of 46 degrees at Hopetoun Airport in Victoria's north-west.
It was the state's 28th day recorded at 45 degrees or warmer since 2001. Such a reading only happened once in the 17 years covering the 1984-2000 period, Dr Trewin said.
