Soggy end to summer masks another season of heat records for Sydney

With just a few days of the month to go, Sydney will notch up one of its hottest Februarys in 157 years of records. Added to the city's hottest ever month in January and its second-hottest December, Sydney will cap a summer without peer, temperature wise.

"At the moment, Sydney is running as the hottest summer for nights and days combined, and for nights and days alone," Brett Dutschke, senior meteorologist for Weatherzone, said.

Mean temperatures for Sydney's summer are so far running about 3 degrees above average at 24.9 degrees, well ahead of the previous hottest summer in 1990-91, he said.

Agata Imielska, senior climatologist at the Bureau of Meteorology, estimates the city's mean temperature summer record will be broken by about 0.1-0.2 degrees in a season where cool changes largely took a holiday.

"There was a lot of back-to-back heat," Ms Imielska said. "And in between, there really weren't any cool days."

Apart from setting a record for 11 summer days above 35 degrees - beating the previous high of nine set in 1894-95 - Sydney will also notch a number of records related to mild overnight temperatures, with about two-thirds of summer nights staying above 20 degrees.

Peak heat

The most remarkable summer heat peaked a fortnight ago on February 11 when the whole of NSW averaged 44 degrees.

That mark smashed the record for a month by 2 degrees - if the previous day's briefly held record of 42.4 degrees is excluded, the bureau said in a special climate report released on the event this week.

"The amount of heat and its extent was quite staggering," Ms Imielska said.

Before reaching the east, the heatwave broke South Australia's statewide maximum record for the month on February 8 with 43.92 degrees - about a third of a degree above the level that had stood since 1983.

The extreme heat in SA triggered some blackouts, stoking fears about reliability of the power grid in a state with a 40 per cent reliance on renewable energy.

However, NSW barely scraped through the event without widespread outages of its own despite draining all the electricity it could get from neighbouring states and its more abundant coal, gas and hydro fleet.

Dozens of sites set heat records for February or for any month, with some coastal regions smashing previous highs by several degrees, such as Port Macquarie.

Autumn outlook

The immediate influences for Sydney's record summer were unusually warm sea-surface temperatures that have kept overnight conditions much warmer than normal.

A high-pressure ridge also sat over much of NSW and southern Queensland for much of this year, creating relatively warm and stagnant air over the region that allowed heat to build up, Ms Imielska said.

The big heatwave was comparable with some of the biggest in Australian history for the area it covered, its longevity and extreme conditions, the bureau said. The January 1939 event with its huge bushfires was more intense, for instance, but confined largely to Victoria and NSW.

Those conditions are not going to break down quickly. The bureau this week released its seasonal outlook for autumn, revealing odds strongly favour a warmer and drier than typical autumn for much of the country.

"It will still be quite a warm March," Mr Dutschke said, with only heavy downpours likely to disturb the pattern. Without such a wet break, fire crews could be in for an active and extended season.

(See bureau chart below showing the chances of above-average March-May daytime temperatures.)

Climate view

Behind Sydney's record summer - which followed the city's hottest ever year in 2016 - is climate change.

According to Sarah Perkins-Kirkpatrick, a research fellow at the University of NSW, the background warming doubled the odds of a heatwave like this month's occurring.

"Currently we'll only see an event like that once ever 100, 120 years, but without climate change it was about once every 240 years," Dr Perkins-Kirkpatrick said. "And as climate change increases, we're going to see more and more of them - there's no doubt about that."

As the bureau's special report noted: "the 2017 warm event is the latest in a sequence of prolonged or intense warm spells that have affected Australia roughly every six weeks since the end of 2012 and, overall, the time between heat events is shortening."

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