How hot is summer actually going to be? A quick explainer.

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The summer heat hasn't waited for December, with heat records already tumbling across the Australian outback, setting an alarming new precedent for the season ahead.

The Bureau of Meteorology has shared a first glimpse at its long-range summer forecast — and weather experts have warned this could be just the beginning of unseasonable warmth.

So, let's dive into what the experts are predicting.

Listen: Why October makes us cranky, and how to fix it. Post continues below.

The Bureau's long-range forecast, which provides a snapshot from November to January 26, said Australians can expect a hot summer with above average rainfall.

They predict a 60 to 80 per cent chance that maximum temperatures across Australia will be above average over the next three months.

"Daytime temperatures for November to January are likely to be above average for much of Australia, while overnight temperatures are very likely to be above average almost nationwide," The Bureau said.

"November to January rainfall is likely to be above average for parts of northern Australia, particularly over far northern Queensland. Elsewhere, the forecast signal is mostly weak, meaning roughly equal chances of above or below average rainfall."

Watch: The Bureau delivers its long-range summer forecast. Post continues after video.

The other big question is whether we are about to enter a La Niña weather pattern, which is characterised by high rainfall and cooler-than-average sea surface temperatures.

"Looking ahead, sea surface temperatures for November to January are forecast to be warmer than average across much of the globe, including around Australia," the Bureau said.

"The El Niño–Southern Oscillation is neutral; however, La Niña development is possible over spring. To our west, a negative Indian Ocean Dipole event is underway."

Earlier this month, the US Climate Prediction Center (CPC) declared a La Niña event is officially underway in the tropical Pacific Ocean.

It's worth noting the CPC's definition of La Niña differs from the Bureau's.

New heat records set.

The Bureau's forecast was shared as October heat records started tumbling. Three states — New South Wales, South Australia and Queensland — clocked previously unrecorded outback heat levels for October, according to Weatherzone.

Sydney is also on track to break the average maximum temperature record for November.

Temperature records were broken across numerous locations in SA, QLD and NSW on Monday.

Based on current forecasts, which include highs of 33°C on Monday and 38°C on Wednesday this week, Sydney's average maximum temperature for October could come in at just above 27.1°C," Weatherzone said.

"This would be roughly 5°C above average for this time of year.

"If this happens, it will not only beat the previous October record of 26.2°C from 1988, but it will also trump the November record of 27.0°C from 2019. No spring month has ever been this hot in Sydney's meteorological history, which dates back to 1859."

Feature image: Getty.