Sydney is forecast to climb into the 30s for the first time this spring on Saturday before a strong cold front sweeps in, triggering a "drastic drop" in temperatures.
The Bureau of Meteorology is forecasting a top of 31 degrees including the first readings above 30 degrees this spring for Observatory Hill.
Days of 30 degrees this early in spring are fairly uncommon in Sydney but are becoming less so.
At Observatory Hill, only nine years have posted such readings by September 15 since records began in 1858. Of those, four have come since 2008, according to Blair Trewin, senior climatologist with the bureau.
The warmth won't linger, though, with the cold front likely to produce a "drastic drop" in temperatures when it rolls across Sydney from the south-west by late afternoon to early evening, Craig McIntosh, a Weatherzone meteorologist, said.
The mercury should drop rapidly to as low as 17 degrees in a couple of hours.
The Bureau of Meteorology has issued a severe weather warning for "vigorous" and damaging winds on Saturday for a region stretching from Wollongong south almost to the Victorian border.
The gusty conditions along with dry conditions prompted the NSW Rural Fire Service to raise the fire danger threat level for the Sydney Metropolitan region to "severe" for Saturday, triggering a total fire ban.
