A rapidly deepening low-pressure system in the Tasman Sea will track towards the coast this weekend, the precursor to a bout of severe winter weather across eastern Australia.
The system will bring multiple threats to south-east states from Sunday, including gale-force winds, heavy rain, huge surf and possibly Alpine blizzards.
The intense low will also sling a cold polar air mass a long way north, bringing icy temperatures from Tasmania to central Queensland, and the first snow in two years to parts of the New South Wales ranges.
Deep low to generate fierce winds and rain
The Tasman low initially formed on Friday and will further intensify off the coast this weekend.
The system's centre will track south on Saturday around 600 kilometres off the NSW coast, then begin a clockwise loop from Sunday, bringing the storm within striking distance of the east coast.
Modelling predicts the low will approach Tasmania on Sunday, then make a right-hand turn from Monday to Wednesday, shifting up the coast to Victoria and NSW.
The precise wind threat is dependent on the low's exact position, with a track nearer to the coast resulting in severe gales across Tasmania, eastern Victoria and south-east NSW, along with possible damaging gusts extending up the NSW ranges.
The ferocity of winds could down trees and lead to power outages, while bringing a significant wind chill — feeling as much as 10 degrees colder than the observed air temperature.
Along with strong winds, the low will additionally generate widespread showers, intensifying to rain near the centre of the system.
As with wind, the intensity of rain also depends on the low's path – with a close swipe of the coast bringing up to 150 millimetres to Gippsland Victoria and eastern Tasmania (including Hobart).
The heavy falls could trigger areas of flash and river flooding in both states, although major flooding is unlikely.
For the rest of south-east Australia, the system will bring widespread showers, including a string of cold, wet days in Melbourne and Adelaide, along with the odd shower in Canberra and Sydney.
The remaining threat is huge surf, whipped by offshore winds well above gale force.
Waves should peak at about 5 to 6 metres during the peak of the event, a height unusual for the normally protected coasts of eastern Tasmania and Gippsland.
Polar air and snow track well north
Since winds spin clockwise around a low, a very cold southerly airstream from the sub-Antarctic will engulf eastern states through the next five days.
The cold snap will be noteworthy for both its extent and longevity, commencing in Tasmania on Saturday and then penetrating well north into central Queensland by the middle of the week.
With the blast of polar air coinciding with the middle of winter, temperatures should be comfortably cold enough for snowfalls across the Alps, however a few non-alpine areas are also likely to see snow this week.
Flakes should commence falling on Sunday across highland parts of Tasmania and Victoria, falling to an elevation of about 800 to 900 metres.
A few flakes will then shift to the NSW central ranges on Monday and northern ranges on Tuesday, bringing the first snowfall in two years, after 2023's warm winter failed to deliver a single snow day.
So how widespread could the snowfalls be?
Current modelling indicates the snow level should lower to an elevation of about 1,000m on the NSW ranges, which could bring a dusting to the regional cities of Armidale, Orange and Lithgow, along with dozens of elevated towns right up to perhaps Glen Innes, including Jindabyne, Cooma and Walcha.
The heaviest snow should accumulate in the very highest towns, including Guyra and Oberon where accumulations could reach a few centimetres.
However, how much snow actually falls in any one location will greatly depend on moisture, or in other words, whether or not the clouds are thick enough to produce precipitation.
Dismal ski season to receive much-needed top-up
While the south-east collectively bunkers down in the storm, for Australia's ski resorts the system could not arrive soon enough.
Despite cold temperatures, this winter has produced only a handful of light to moderate snowfalls, mostly on only the higher slopes.
Snowy Hydro's latest measurement of the snow depth at Deep Creek, which sits at 1,620m, was 0cm — nearly 50cm below average, and the first time in 23 years no snow was observed in the second week of July.
Thankfully, the upcoming system will bring much-needed fresh snowfalls to finally build a base, however the quantity of snow will largely depend on the position of the Tasman low.
Despite the uncertainty, the majority of models indicate an average of at least 30cm for most resorts.
Antarctic air behind cold spell but well off records
The cold weather ahead will result from a prolonged 90-degree swing in the prevailing winds to Australia's south.
On an average day, westerly winds blow between Australia and Antarctica, which prevents the coldest polar air from reaching our shores.
However, the airflow will shift to a southerly this weekend and by Tuesday stretch across an expanse of around 3,000km, arriving from deep in the Southern Ocean.
This abnormally long fetch of polar air will bring unusually cold air well north and cause temperatures to plummet as much as 8 degrees below average.
The duration of the icy winds is also noteworthy, likely to last around five days, as the southerlies are squeezed north between the deep Tasman low and a strong high-pressure system near Western Australia – a set-up that will drive numerous pulses of polar air across the eastern states
While snow outside of the Alps is rare in Australia due to our low latitude, this event is unlikely to match some of the more newsworthy events of the past decade, including snow in Hobart and Launceston in 2020, Canberra in 2019, and a heavy snowfall in Stanthorpe in 2015.
Going back decades, snow settled in parts of Melbourne in 1986, a short and sharp event where melting snowflakes were even reported from Sydney's northern suburbs.
In terms of northern extent, an invasion of polar air brought snow to the ranges west of Mackay in 1965.
ABC
