A record spell of September heat is baking southern states in temperatures as much as 18 degrees Celsius above average, prompting a rare cool-season heatwave warning and an early end to the snow season.
The hot air mass responsible for the unseasonable warmth first descended on Western Australia 10 days ago, then expanded east last week engulfing most of central and southern Australia, reaching the NSW coast on Friday.
While a cool change is now gradually moving through South Australia and Victoria, for parts of NSW and southern Queensland the hottest weather is still ahead during the coming days.
A total fire ban is in place on Tuesday for the Greater Sydney region, as well as the far South Coast where 20 local schools are closed for the day due to the extreme heat.
Early spring records falling
About a dozen towns have already broken long-standing September records, including Alpine resorts last Thursday, Ceduna and Wudinna on Sunday, followed by Port Augusta, Moruya, Ulladulla and Forbes on Monday.
What’s unusual about this September’s hot weather is not just the absolute highest temperatures, but the longevity of the heat.
Sydney has daily weather data dating back to 1859 and the greatest number of 30C days in any previous September is three.
During a five-day stretch starting Saturday the city is on track to sweat through four days above 30C, setting a new record.
Western suburbs of Sydney are on track to record five to six consecutive days above 30C — also a record for early spring.
The heat has even reached an intensity matching summer hot spells, prompting the weather bureau to issue an out-of-season heatwave warning for NSW’s southern coastline, well before heatwave forecasts normally commence in November.
For Alpine regions, temperatures have climbed as high as 18 degrees in the past week, leading to a rapid snow melt and the early finish to the ski season in Thredbo, Mount Hotham and Mount Buller.
Hottest weather still to come
The underlying cause of the heatwave is an immobile high pressure system off the east coast.
Winds blow anticlockwise around a high, which is drawing a hot tropical air mass south across Australia.
While high pressure is a standard feature on a daily weather map, the system’s stationary position has allowed hot air to build in intensity through a prolonged period.
Tuesday will bring a continuation of the September scorching – a high in the Tasman Sea and hot north-westerly winds across south-east states, although a cooler air mass has now reached southern South Australia and south-west Victoria.
A cold front will spread the cooler air further north on Wednesday, however the front will first also strengthen the hot winds blowing through NSW.
The stronger winds will not only prevent a cooling sea-breeze from developing in Sydney, but also lift the fire danger to extreme.
With no sea breeze, Sydney is forecast to reach 34C on Wednesday, within a whisker of its all-time September record of 34.6C.
On Thursday, a cool southerly change will bring relief to most of NSW, dropping the temperature around 10C in 24 hours.
While relief arrives further south, the north-east of NSW and south-east Queensland will see their temperatures spike on Thursday, including Brisbane where 35C is predicted, the city’s warmest September day in six years.