This week's near-record snowfalls in the east, and the days and days of upcoming rain forecast for Perth, share one crucial factor with the Lismore floods, last year's 49-degree Celsius heatwave in Canada, and many of Australia's most destructive extreme weather events.
Right now, much of Australia's weather is hardly changing, day after day after day.
The south east of the country is being raked by cold, wet winds and the south west is expecting rain every day for at least the next week.
This is because the weather systems driving the wind and rain have stopped moving west to east like they normally do.
As meteorologists would say, Australia's weather systems have become "blocked".
"It's like a traffic jam. You've got weather systems that can't move on," Kimberley Reid, a climatologist from the ARC Centre of Excellence for Climate Extremes, said.
Blocking high pressure systems act like weather amplifiers, allowing heat or rain or snow to build up and up and up, with at times disastrous consequences.
Atmospheric river held in place
In February this year, a blocking high pressure system settled between Australia and New Zealand.
Looking on was Kimberley Reid.
Ms Reid is an expert in atmospheric rivers, a phenomenon where huge volumes of rain can be channelled from the tropics over Australia.
She watched as an atmospheric river formed off the Queensland coast, funnelling tropical moisture from New Caledonia right over the Queensland-New South Wales border region.
The blocking high prevented the weather from moving out to sea, as it otherwise might have done.
"You had air that's come off the Pacific Ocean flowing over the east coast for days. And as we saw that led to multi-day rainfall that just didn't stop and, of course, devastating floods," she said.
Blocked weather systems aren't always disastrous. This week's cold, wet weather in the east has led to one of the best starts to the ski season in decades.
And south-west Western Australia could do with some rain.
But blocking high pressure systems are behind some of Australia's deadliest extreme weather events.
Blocking highs drive heatwaves
In January 2019, a blocking high set in off Australia's east coast. Instead of rain, the high pressure system brought a record-breaking heatwave.
"There were winds coming off the interior of the continent that were hot and dry, sitting there for days at a time," Ms Reid said.
The blocking high, on top of global warming, led to heat records around the country being smashed.
January 2019 was the hottest month on record nationally by a margin of 0.99C, and was also the hottest month on record for every state and territory except South Australia and Western Australia.
New South Wales broke its previous record by more than 2C.
And blocked weather doesn't just impact Australia.
The Omega Block
Last year, a high-pressure system stalled over Canada in a distinctive pattern called an "Omega Block".
Again, heat built up and up, day after day.
On June 29, the temperature in Lytton, British Columbia, hit 49.6C, the highest temperature ever recorded in Canada.
Shortly after, the town was destroyed by fire.
Blocking events and climate change
In the wake of the disaster, some climatologists, including prominent scientist Professor Michael Mann, Director of the Earth System Science Centre at Pennsylvania State University, argued that climate change was making blocking events like the Canada heatwave more frequent.
He proposed that a warming Arctic was making the northern hemisphere jet stream wavier and more meandering, leading to more blocking events.
Writing in the New York Times, Professor Mann wrote the heatwave was "an example of a phenomenon known as wave resonance, which scientists (including one of us) have shown is increasingly favoured by the considerable warming of the Arctic".
Kimberley Reid explained the theory.
"The hypothesis, and I emphasise that this is still a debated hypothesis, is that as the Arctic warms up, the temperature difference between the poles and the mid latitude region decreases, which means that the jet stream is no longer really tight and confined to the polar region, but it spreads further south and becomes wavy," she said.
"In fact, some models are even suggesting a decrease in blocking. So for these reasons, it is a very hot area of research right now.
"Because of these recent extreme events that we've seen, especially the flooding in February, March of this year, there is currently a lot of interest in the climate community on blocking highs".
