Sydney's 35C September Morning and the 10-Degree Drop
Richmond in Sydney's outer west reached 35.4C on Monday 21 September 2026. Penrith reached 35C. Sydney Observatory Hill peaked at 32.7C at 10:40am, then fell by about 10C within a couple of hours as a south-southeasterly change moved through the city. The Guardian
The heat was concentrated in the morning. Temperatures in Sydney peaked above 35C before lunchtime on 21 September, according to Weatherzone data. The cooling was abrupt at suburb scale, with some Sydney suburbs recording a drop of 10 degrees within half an hour. Weatherzone
Monday followed an already hot weekend. On Sunday 20 September, Sydney's western suburbs recorded maximums of about 34C, while Sydney city recorded 28.9C. Forecast guidance issued on 19 September had put Sydney city at 28C for both Saturday 19 September and Sunday 20 September, part of what was expected to be the hottest September weather in years for south-east Australia. ABC News
The gap above normal was large for September. Temperatures across many parts of New South Wales were generally 6C to 12C above the September average over the weekend of 19-20 September. The September average maximum (the usual daytime high for the month) is 20.1C at Sydney Observatory Hill and 23.5C at Penrith. That places Monday's outer-west readings more than 11C above the monthly norm and Sunday's city maximum nearly 9C above it.
Heat and wind combined into elevated fire weather. High to extreme fire danger warnings were issued for Monday 21 September, particularly for the Greater Hunter, much of north-eastern New South Wales and northern inland areas. The warnings lined up with the pre-frontal phase of hot westerly advection (hot inland air carried east by westerly winds ahead of a front) before the coastal change, the weather pattern that typically produces the sharpest daytime temperature swings around Sydney.
Sydney city temperatures were forecast to be 18C to 19C on Tuesday 22 September, a fall of roughly 14C from Monday's Observatory Hill peak. Sydney temperatures were then forecast to be 22C to 24C on Wednesday 23 September, closer to late-September climatology (the typical pattern for this time of year) but still above the 20.1C average for Observatory Hill.
The broader context here is compression rather than just peak heat. A greater than 11C positive anomaly (a departure above normal) followed within hours by a 10C fall creates parallel management problems: heat exposure in outer-west populations during the morning, then fire-weather escalation to the north and east, then rapid adjustment of energy demand and transport conditions as the marine airmass (cool, moist air from the ocean) arrives. For forecast users, the salient variable was not only the maximum but the rate of change.
Looking at what this means for operational planning, shoulder-season events of this type test thresholds calibrated to summer. Fire danger ratings in September draw on fuel states (how dry grass, leaves and bush are) different from January, while public response to 35C in September differs from 35C in February. The sequence from 19 to 23 September, from forecast 28C weekend maxima to observed mid-30s then forecast sub-20C minima of the diurnal cycle (the daily rise and fall of temperature), shows how quickly New South Wales can shift when a continental hot airmass is displaced by a coastal southerly intrusion.
