Australia's typical settled winter weather will head off the rails during the next week.
Firstly, polar air from Antarctica is surging towards south-east Australia and will bring gusty winds, showers and low-level snow this weekend.
While midwinter cold outbreaks are typical, the subsequent weather through the coming week is nothing short of bizarre.
A strong high-pressure system is forecast to stall near Tasmania by Monday, a very unusual position for July, which will reverse the direction of prevailing winter winds and result in another extended spell of wet days along the east coast, prolonged icy weather across the interior and south, and possibly culminate in a major inland rain event towards next weekend.
The blocking high is looking so dominant it may even challenge Australia's all-time air pressure record.
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Antarctic winds and snow close to Hobart, Melbourne
During the past 48 hours, a cold air mass originating over Antarctic waters has surged north and is now making landfall on Australia's southern coastline.
The intrusion of polar air into the mid-latitudes has generated a powerful cold front on its leading edge, whose passage over South Australia, Victoria and Tasmania will bring showers, gusty winds and alpine snow through Saturday.
The front is the strongest so far this winter and should bring a welcome 5–20mm of rain to drought-affected parts of southern SA and south-west Victoria, while dumping up to 20cm of snow on ski resorts.
Strong winds will also accompany the system, and warnings are current for damaging gusts along pockets of the ranges.
The rainband and front will then sweep north through western and southern NSW late on Saturday, before reaching northern NSW and southern Queensland on Sunday.
After milder weather during the past few days, the front's arrival will cause temperatures to drop, including maximums of 12C in Adelaide and Melbourne on Saturday, then on Sunday just 9C in Hobart and Canberra, and 15C in Sydney.
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The post-frontal Antarctic winds will also lower the snow level — falling on Sunday to around 1,000 metres in Victoria and 700 metres in Tasmania, although the dryness of the polar air will limit snowfalls to only a dusting in most non-alpine regions.
Around Hobart, snow will blanket Mount Wellington and the odd flake may also settle on the higher hills on the city's outskirts.
The closest snow to Melbourne this weekend will fall around Mount Donna Buang, just north of Warburton.
Stalling high and week of rain for NSW, QLD coast
The front will clear off the east coast by Monday. However, behind it, a strong high-pressure system is predicted to stall near Tasmania, leading to another week where our weather grinds to a halt.
The high's southern position is abnormal for July, when high-pressure systems normally migrate to central Australia, leading to a reversal of the wind direction from a westerly to a south-easterly across southern states.
The prolonged spell of south-easterly winds will produce a string of wet days for the east coast as showers are driven in off the Coral and Tasman seas — a near-exact replica of the pattern in May when a blocking high brought 13 consecutive days of rain to Sydney.
Modelling is indicating the heaviest falls should impact the NSW north coast, where up to around 100mm is possible by next weekend, although, spread over a week, the rain is unlikely to trigger flooding.
Sydney could potentially see close to 100mm and Brisbane 50mm, nearly a month's worth of rain, continuing the 2024 trend where our east coast capitals' rainfall has far exceeded the southern states.
Sydney's rainfall in 2024 is the stand-out, already surpassing the annual average and exceeding the combined total from Melbourne, Adelaide, Canberra, Perth and Hobart.
Initially, this week's rain will mostly impact the east coast, however, modelling is also hinting of a major inland rain event towards next weekend as the easterly winds ultimately spread moisture into the interior.
The event could produce a season's worth of rain in just a few days across the outback, although it's too far ahead for a precise forecast.
Frosty week to bring Melbourne's coldest night in two years
Away from the east coast, the blocking high will lead to a string of chilly days and icy nights as the cold post-frontal air becomes trapped.
Adelaide is predicted by the Bureau of Meteorology to have both minimums and maximums below average until at least Thursday, while Melbourne shivers down to a frosty 1C on Wednesday morning, the city's coldest temperature in two years.
Subzero temperatures are even possible as far north as Alice Springs, where temperatures should drop from a high of 30C on Saturday to a minimum of -1C on Wednesday morning.
For alpine areas, the week will provide ample opportunity for snowmaking, however further natural falls (after this weekend) are unlikely into the foreseeable future.
High could break all-time pressure record
Blocking highs are common through autumn and early winter but become rare by July, when stronger jet streams keep weather patterns mobile.
This particular high is also abnormal due to its strength — at its peak, a central pressure predicted to exceed 1040 hectopascals.
The current all-time air pressure record in Australia is 1044hPa, observed at Launceston Airport on June 7, 1967, a figure which could be challenged if the high's centre drifts directly over Tasmania.
So how will the high pressure be felt on the ground?
Apart from lower-than-normal sea levels, the majority of humans can not detect changes in atmospheric pressure unless they ascend high into the atmosphere, however, many studies have linked changing pressure to the severity of some medical conditions like arthritis and headaches.
