Early February is still the height of summer but a polar air mass currently on a journey north across the Southern Ocean is about to drop temperatures to their lowest February levels in nearly two decades.
By Friday snow will be falling across the Alps as maximums in Melbourne, Adelaide, Canberra, and Hobart all fall below 20 degrees Celsius.
While residents across the south-east will be diving into their winter wardrobe, further north to Sydney and northern NSW the unseasonable chill will be welcomed with open doors and windows, as the cool Antarctic air flushes out the stifling summer heat and humidity.
A rare wintry day in February
Temperatures are already dropping below average across southern states as the first of two fronts brings cool Southern Ocean air, but a more vigorous front with air directly from Antarctica will reach South Australia and Victoria on Thursday before sweeping through southern NSW overnight.
What will follow on Friday is a mid-summer day reminiscent of winter as maximum temperatures plunge as much as 15C below average.
It will be especially cold along the ranges, bringing snow down to an elevation of between 1,300 and 1,500 metres, equivalent to the snowline often seen during winter.
This event could bring the heaviest February snow since 20cm fell in 2005.
Away from the Alps the Great Dividing Range will shiver through maximums in the low to mid-teens, while even towns and cities near sea-level will struggle to reach 20C, the coldest February weather in 18 years.
Friday's forecast and the last time south-east cities have experienced a February day this cold:
- Adelaide 19C — 6 years (17.4C in 2017)
- Albury 19C — 18 years (12.1C in 2005)
- Ballarat 14C — 18 years (10.0C in 2005)
- Bendigo 17C — 18 years (11.5C in 2005)
- Canberra 18C — 5 years (17.1C in 2018)
- Melbourne 18C — 3 years (15.7C in 2020)
- Wagga Wagga 20C — 18 years (13.4C in 2005)
- Perisher 6C — 14 years (4.5C in 2009)
- Mt Hotham 2C — 18 years (-0.4C in 2005)
For the NSW coast the most notable impact will be a rapid drop in humidity.
Sydney has been sweating in afternoon humidity over 70 per cent during the past week but will enjoy levels closer to 30 per cent by Saturday.
How a mid-summer day can bring snow
Meteorologists track and forecast air masses as they develop, transform, and move around the globe.
One method of tracking a parcel of air is called a reverse trajectory which involves starting with the landing position of an air mass, your location of interest, and working backwards to investigate where the air originated.
Selecting the centre of south-east Australia on Friday morning reveals the origin of the wind four days earlier was right along the coastline of Antarctica.
This explains why even in February when Southern Ocean temperatures are near a peak the polar air will remain sufficiently cold on its journey north to produce snow at southern Australia's sub-tropical latitudes.
How long will the unseasonable cool weather last?
Temperatures will remain well below average through the weekend in a south-westerly airstream but will recover to near average on Monday as winds tend more easterly — a direction far more common during the summer months.
