"There is every chance that we are going to see at the peak time during the day, some catastrophic ratings being reached," Mr Fitzsimmons said. Hot in the city: Sydney is likely to swelter on Thursday with temperatures predicted to reach 40 degrees. Credit:Steven Siewert "Because the extreme fire danger rating is at the upper end of the extreme scale, we could see, particularly with very strong wind gusts, a number of areas ... tipping into catastrophic," he said. Sydney is expected to see the mercury climb to 39 degrees, eclipsing the previous October record set on October 13, 2004, and topping the long-term average by 17 degrees. Temperatures are likely to remain warm well into the evening before a southerly buster blows in cooler conditions around midnight. "If they don't go to catastrophic it's only because the wind speeds are not as high as expected," said Ben Domensino, a senior meteorologist with Weatherzone. "The air mass is really hot."

"Most likely it will be about 4 o'clock," Dr Griffiths said. "Today we're expecting no sea breeze and it will keep warming until some time mid- to late-afternoon." North-westerly winds should be average 40-50km/h by the afternoon, with gusts reaching 70-75km/h. The southerly change, from about 10pm, should reach across the city by about midnight - making for a hot night ahead of the weather shift. Winds will strengthen particularly over the water, including Botany Bay and Sydney Harbour, reaching as much as knots (65km/h) and with gusts exceeding that, Dr Griffiths said. Winds over the city itself should gust to 70km/h with the change, potentially presenting fresh challenges for fire crews.

While the western and northern fringes of the city came under fire threat last month, the higher temperatures and prospect for winds gusting to as high as 70km/h have elevated the risk further. In fact, conditions are similar to last January when Sydney recorded it hottest day of 45.8 degrees. "We were very fortunate in January not to have those really strong winds," Mr Fitzsimmons said. "While temperatures are a little lower [on Thursday], we certainly have got a significant increase in the winds which is why the ratings are so high." A record mild winter followed by the hottest ever September have primed regions around Sydney for the worst fire risks since the 2001-02 and 2002-03 years, he said. Thousands of volunteers and paid firefighters are on standby, along with hundreds of fire trucks and dozens of aircraft, he said. Authorities have declared total fire bans in 15 of the state's 21 fire districts. Some 82 NSW parks are closed at least in part, with fires reported in five of them.

"All walking tracks and fire trails in Sydney national parks are closed today, with the exception of Lady Carrington Drive, Royal and Burrawang Walk along the foreshore in Kamay Botany Bay National Park," a spokeswoman for the Office of Environment and Heritage said. "West Head Road, North Head Scenic Drive and Henry Head Road are also closed within the national parks," she said. Fire authorities, which include crews from Fire and Rescue NSW and National Parks and Forestry, have already battled some 2500 fires this season, including about 1000 in September. While forest areas are of concern, grassland regions of the state will potentially be a higher threat, Mr Fitzsimmons said. That's partly because good rains in recent years have built up a fuel load that's now drying out. "You've really got to go back to the last big drought-breaking rains of the '70s and '80s before you are looking at conditions of some equivalency in those areas," he said.

North-westerlies are likely to whip up dust and pollen, adding to the day's discomfort. The arrival of the southerly buster will bring some relief but also stiffer gusts and little rain. "We think the strongest winds will come about half an hour to an hour after the change," Mr Domensino said. Western suburbs are unlikely to get any rain. The cool change, and the prospect of rain over the weekend, will only ease conditions temporarily. Sunday's forecast of 32 degrees and more days of unusual warmth should help nudge average temperatures for the city for the first half of October above the 1988 record of 26.2 degrees – placing yet another heat record at risk.

Loading "I'm confident it will be the hottest October on record, or close to it," Brett Dutschke, Weatherzone's senior meteorologist, said. Weatherzone is owned by Fairfax Media, publisher of this website.