Hottest summer weather in five years predicted for coming days as heatwave engulfs half of Australia.

Extract from ABC News

ABC meteorologist Tom Saunders 

• Topic:Weather Forecasts

1 hours ago

Maximum temperatures are predicted to exceed 40C across a broad swathe of Australia's interior and east during the next few days. (Supplied: Beth Bunn)

Link copied

An oppressive heatwave stretching across half the country from Darwin to Victoria will send temperatures soaring up to 15 degrees Celsius above average during the coming days.

Maximums are predicted to exceed 40C across a broad swathe of the interior and east — the warmest summer weather in years for much of south-east Australia, including highs on Monday of 41C in Melbourne and 38C in Canberra.

Across the eastern outback maximums will peak well above 45C, hot enough to challenge December records in a handful of western New South Wales towns.

The severe heat follows Australia's warmest spring on record and is likely to be followed by additional bursts of hot weather through the Christmas and New Year period.

A widespread heatwave will effect around half the country during the coming days.

Heatwave to engulf interior and east

This December is fast becoming one of the hottest on record as frequent heatwaves sweep across the nation.

While climate change and simmering seas surrounding Australia are having a background influence on the hot start to summer, the more immediate and direct cause of the current scorcher is the formation of a heat dome over tropical and central Australia this week.

Under a heat dome, air sinks towards the ground where it compresses and warms up while also preventing cooler surrounding air from mixing in, a process which can prolong hot weather from days to weeks.

Perth reached a maximum of 40.4C on Wednesday. (Supplied: Jeroen Gerritsen)

The western edge of the dome already sent Perth to a maximum of 40.4C on Wednesday, the city's hottest December day in three years.

The broad mass of 40C plus air is now shifting east and intensifying across central Australia and will peak over south-east states from Sunday to Tuesday as a north-westerly airstream carries the air mass well south.

So just how hot will each day become? 

Here is a day-by-day guide on which areas should exceed 40C.

Saturday: Northern SA, the inland NT and western Queensland.

Temperatures well above 40C are forecast for today across the central outback. (ABC News)

Sunday: Western Queensland, western NSW, most of the inland Northern Territory and South Australia..

Northerly winds will carry 40C heat to southern SA on Sunday, along with western parts of Queensland and NSW. (ABC News)

Monday: Most of Victoria, western NSW, western Queensland, eastern SA and the NT.

The heatwave will reach a peak on Monday with 40C temperatures reaching southern Victoria, and 45C plus maximums over the interior. (ABC News)

For many areas, including a handful of major cities, Monday's maximums are forecast to be the highest in years, including:

• Mildura 46C, warmest day since December 2019.
• Bendigo 43C, warmest day since January 2020.
• Melbourne 41C, warmest day since January 2020.
• Albury 43C, warmest day since January 2020.
• Wagga Wagga 42C, warmest day since February 2020.
• Canberra 38C, warmest day since February 2020 (if >38.0C).

For smaller towns over the western Murray Basin, highs could even break December records, including 47C at Ivanhoe and Wilcannia.

While eastern NSW also sweats through a heatwave from Saturday to Monday, the hottest weather for Sydney won't arrive until Tuesday when western suburbs potentially reach 41C.

Cool change preceded by fire threat

A cool and gusty south-west change will spread through southern states by Tuesday, and most of NSW by Wednesday, however, as is often the case, relief will arrive as a double-edged sword.

The clash of opposing cold and hot air masses along the change will lead to a strengthening of pre-change winds and when combined with low humidity this will lead to extreme fire danger on Monday from eastern SA to central Victoria.

Dangers may even approach catastrophic levels in western Victoria where winds will average 40 kilometres per hour, temperatures will reach around 45C and humidity will dip below 10 per cent — a combination which could cause fires to be fast moving and uncontrollable.

Extreme fire dangers are predicted on Monday across SA and Victoria which indicates dangerous conditions. (ABC News)

So when can our capitals expect relief? 

Here is a breakdown of the likely timing of the cool change:

• Adelaide, mid-afternoon Monday.
• Melbourne, Monday evening.
• Hobart, Monday evening.
• Canberra, Tuesday afternoon.
• Sydney, Tuesday night.

Elevated risk of scorching Christmas

While its too early for a precise forecast for Christmas Day, medium-to-long-range predictions of the general weather pattern weeks are more accurate than simply expecting the average weather or guessing.

The Bureau Of Meteorology's (BOM) method for outlooks weeks and months ahead is to run their model 99 times and use the variation in results to indicate the likely weather.

The coming fortnight from December 16, is likely to see extreme heat return to much of central and eastern Australia. (ABC News)

For the fortnight from December 16 to 29, the majority of those 99 runs show a strong swing favouring hot and dry weather across central and south-east Australia, and possibly further heatwaves.

This doesn't guarantee a hot and dry Christmas Day, but the odds are firmly tipped against widespread rain and unseasonably cool weather.

There is high chance rainfall will be below normal through Christmas across eastern Australia.  (ABC News)

One hot day doesn't make a heatwave?

The term heatwave is often thrown around flippantly when discussing hot weather, but the word in meteorology has a strict definition since heatwaves have claimed more lives than any other natural hazard in Australia and can affect infrastructure such as public transport and electricity supply.

The BOM classifies a heatwave as a period when temperatures are unusually hot over three days, however the threshold for what's considered 'hot' varies by location and time of year.

In simple terms the declaration of a heatwave is determined by comparing temperatures over a three-day period with:

• What would be considered hot at that location.
• The observed temperatures at that location over the past 30 days.

Heatwaves have claimed more lives than any other natural hazard in Australia. (Supplied: Tim Richie)

The calculation therefore factors in the ability to adapt, for example, a 35C day will feel much hotter to a resident of Hobart in November than a resident of Brisbane in February as the latter will be acclimatised by both a sub-tropical climate and the recent passing of peak summer.

However, it's not only daily maximums evaluated, minimums are also critical since warm nights prevent recovery from each day's heat while also allowing the next day to warm up quickly.

Like most weather threats, heatwaves are sub-classified into categories to help communicate the level of risk.

Low-intensity heatwaves are frequent during summer. Most people can cope during these heatwaves.

Severe heatwaves are less frequent. They are likely to be more challenging for vulnerable people.

Extreme heatwaves are rare. They can be dangerous even for healthy people who exercise or work outdoors.

The current heatwave is classified as either low-intensity or severe in most areas, and extreme across the central NT.