The strength of Thursday's winds caught authorities on the hop, with the fire danger only rated "severe" rather than "extreme", as had been forecast for the previous Sunday's heat spike.
Bankstown, for instance, recorded 95 km/h winds, the strongest there for at least 10 years, said Ben McBurney, a meteorologist with Weatherzone.
Blustery conditions: A kitesurfer on Lake Illawarra. Photo: Dylan Robinson
"The bureau didn't expect wind speeds to go quite as high as they did," he said.
Humidity across the basin also fell as low as 10 per cent. "That's probably what led to these fires getting out of control," said McBurney.
Many areas saw their fire danger ratings rocket to "catastrophic", including for Richmond and Sydney Airport. Camden also had a "catastrophic" rating, the highest in 11 years.
Smoke over Sydney
People watch from Tamarama as a wave of smoke haze smothers the Eastern suburbs of Sydney, caused by bush fires across Sydney and NSW. Photo: Brianne Makin
While strong winds remained a concern, the arrival of cooler conditions late on Thursday gave firefighters in some areas a break. But relief may be temporary as a lack of rain and rising temperatures over the weekend will see high fire dangers return.
Friday's maximum will drop back to the long-run October average of 22 degrees and will seem almost cool after Sydneysiders have been baking in heat more akin to the height of summer than mid-spring.
Thursday saw the mercury climb to 33.6 degrees, the fourth day of 30-plus heat this month. Temperatures will mount again, with 24 tipped for Saturday, 28 on Sunday, and 31 for Monday – bringing the city just shy of the record six 30-plus days only reached in 1926 and 1968.
Live details of active fires - click on the icons for more information. Source: NSW Rural Fire Service.
Little or no rain is expected for the Sydney basin until Monday, while winds will again pick up ahead of the next cool change. Fire conditions will be high or very high by then, forecasters say.
Springwood, one area hit by Thursday's fires, has had only about 40 millimetres of rain over the past month compared to its long-term average of about 200 millimetres.
The welcome news, though, is that Monday's trough is likely to bring rain. More importantly, we may see at least a temporary breakdown of the regular cycle of warmer-than-average spells spiking to abnormally hot days.
“This is a real change,” said Peter Zmijewski, a senior forecaster at the Bureau of Meteorology. “We start to see an increase a bit in the moisture.”
Ben Domensino, a meteorologist at Weatherzone agreed, saying that weather patterns that had led to the build-up of a pool of heat over central Australia were starting to break up.
Heavy rain on Wednesday in parts of the Northern Territory was one signal that inland regions would start to see more cloud cover and a reduction in record-breaking early spring heat. Favourable westerly winds had been channelling some of that warmth to south-eastern Australia, putting Sydney on course for one of its hottest Octobers after setting new highs for September and the winter.
Mr Dominsino said next week's showers should bring 10-20 millimetres of rain to the Sydney basin and start a cooler cycle of weather. Even so, a wet early spring and then months of drying had left much of NSW facing elevated fire weather conditions.
"The lead-up to this year's [fire season] has been almost perfect for bad fire conditions," Mr Domensino said.
David Jones, head of climate analysis at the Bureau of Meteorology, said climate change will see fire conditions worsen for much of Australia over the longer run.
While weather patterns vary from year to year, southern Australia is already seeing springs and summer becoming hotter. As a result, there is a "trend towards more severe fire weather conditions across Australia.”
"We know from about March-April to around June, things have been drying out across southern Australia," he said.
"The fact there is less soil moisture increases the fire risk both towards the end of the fire season but also the subsequent fire season," Dr Jones said. "You have a lesser opportunity to really wet up the vegetation.”
“Global warming exacerbates a number of factors associated with fire," he said, adding that humidity is expected to decline over much of the continent particular in summer.