Pre-Christmas cold spell delivers snow and near-record-low summer temperatures – ABC News

Pre-Christmas cold spell delivers snow and near-record-low summer temperatures
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It's beginning to look a lot like Christmas in south-east Australia, but far from the typical sun-drenched version.
The unseasonable spell of wintry conditions will last right through the week as a classic 'blocking' weather pattern prevents warm air from returning to our southern shore.
The warm weekend weather came to a rapid end on Monday as a cold air mass from the Southern Ocean moved north over south-east Australia.
Melbourne Airport reached 29 degrees Celsius on Sunday but spent most of Monday below 15C and at 2pm Tuesday was shivering on just 12C.
That's colder than the average maximum during the middle of winter.
NSW recorded minimum temperatures on Tuesday morning 12 degrees below average, while Griffith's low of 4.2C was the town's equal-coldest summer temperature on record.
A high of only 1.6C at Falls Creek at 4pm on Tuesday was well short of its December average of 15C.
Short bursts of cold weather are far from unprecedented in early summer but normally last no longer than a day or two.
The current spell of chilly weather is unusual in its longevity due to a blocking pattern where a high-pressure system stalls and prevents the usual eastward progression of pressure systems.
A blocking high causes the weather to essentially become jammed, leading to a prolonged spell of invariable conditions.
This week's cell will stall south of the Great Australian Bight. Winds spin anticlockwise around a high in the Southern Hemisphere so a conveyor belt of cold southerlies will feed bursts of modified polar air across south-east states.
Melbourne, Hobart and Canberra will spend the working week with maximums in the teens while Adelaide and Sydney are stuck in the low 20s. 
Although southerly winds in summer don't normally bring snow, the origin of this week's is just off the coast of Antarctica.
As the waters surrounding Antarctica take months to warm up after the winter freeze, they can still support conditions to produce Australian snow in December.
Snow fell on Monday above an elevation of about 1,500m and returned late Tuesday to even lower levels as the next pulse of polar air moved in.
Snow will continue across the Alps and Tasmanian Highlands through Wednesday and Thursday and may even accumulate to a depth sufficient for a summer ski.
The big question is how long will the blocking pattern last?
The high pressure system will dawdle towards Tasmania this weekend but won't move east of Australia until next week, ensuring below-average temperatures until at least December 20.
It's still too far out to make a solid Christmas Day forecast, however indications from modelling suggest the days leading up to Christmas will see a return of warmer northerly winds for south-east states.
The Bureau of Meteorology won't issue a forecast for the 25th until late Sunday, but most of the models the Bureau uses will have Christmas Day data available as early as Friday.
However, as it's still 10 days away, the accuracy is substantially reduced.
The Southern Ocean acts like a large heater in winter, warming up polar air masses before they reach our shore.
And in summer that same ocean prevents air from warming up.
The last white Christmas for mainland Australia was in 2006 and potentially the only snowy December 25th in recent decades.
Temperatures that year were more than 12C below average and snow even fell briefly on Mount Dandenong.
This year's December snow is unfortunately 10 days too early for an Australian white Christmas.
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