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Sydney weather: City marks hottest month ever with every site setting records

Peter Hannam

Sydney has marked its hottest ever month, with extended warm spells setting records at all metropolitan sites for January mean temperatures.

Last month, Sydney's Observatory Hill notched its warmest January for maximum, mean and minimum temperatures in records going back to 1858, the Bureau of Meteorology said.

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The city's mean temperatures averaged 25.6 degrees in January - beating a high mark set in January 1991 by half a degree - and exceeding the long-run norm by 3.3 degrees.

It was also the highest temperature for any month in Sydney, and follows the hottest year set in 2016. The city's average maximum - at 29.6 degrees - shaded the record set in January 1896 by 0.1 degrees, and was also the hottest for any month, Agata Imielska, senior climatologist at the bureau, said.

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"We've seen back-to-back heatwave conditions," Ms Imielska said, adding that the lack of significant cool outbreaks, heavy rain and persistently warm offshore waters all played roles in the exceptional warmth.

With background conditions warming about a degree over the past century, climate change was also a factor.

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"It's about looking at the absence of cool conditions, which ... is another trend with global warming," she said.

Similar factors were at play nationally, with last month becoming the third hottest on record for minimum temperatures. Mean and maximum readings were also above average but less notable, Ms Imielska said.

Mild nights

While the city had its heat spikes - such as two days when 45.1 degrees were recorded at Badgerys Creek and Penrith Lakes, respectively - the month was also marked by its consistently mild nights.

During January, the mercury remained above 20 degrees for 25 nights, the most on record and almost triple the average of nine such nights. Meanwhile, 11 days topped 30 degrees at Observatory Hill, compared with the long-run norm of three such days.

(See chart below of the mean maximum anomalies for January, with NSW particularly warm.)

For the state as a whole, January was the third warmest on record for mean temperatures, running about 3.3 degrees above the norm.

Conditions were relatively dry, with rainfall down about half from a typical January for the city and 42 per cent for NSW overall.

The unusual heat is expected to linger well into February, with another heatwave heading towards Sydney for the coming weekend.

(See chart below showing parts of NSW face a severe heatwave in the three days from Saturday.)

The seasonal outlook suggests the relatively hot and dry conditions will extend at least until April, Ms Imielska said.

Since October, much of eastern Australia had shifted to sub-par rainfall readings after many regions had their wettest winter-spring periods on record.

Authorities are gearing up for a prolonged fire season as a result of the vegetation spurt during the cooler months followed by frequent hot spells that had dried out grasslands and forests.

As Weatherzone noted, parts of the North West Slopes and Plains of NSW were as much as 9 degrees above the January average for daytime temperatures for the entire month.

Walgett in the north-west exceeded 36 degrees each day during January and had 12 days above 40 degrees for the first time in 47 years, Weatherzone said.

"January's remarkable heat brought the town's run of days above 35 degrees to 36 in a row, eclipsing the old record of 29 days set back in 1896 and 1882," it said.

"Temperatures are forecast to reach 44-45 degrees each day until at least Tuesday, which would make the first seven days of this month the town's hottest week in 139 years of records."

Weatherzone is owned by Fairfax Media, publisher of this website.