Communities in southeast Queensland and northern New South Wales have been bracing for the impact of Cyclone Alfred for several days as the slow moving storm system sits off the Australian east coast. 1News weather presenter Daniel Corbett explains what makes this storm different.
In many ways, most people will not have seen a tropical cyclone like Tropical Cyclone Alfred.
That might not be because of top wind gusts or rainfall amounts — but more for the combination of factors.
The big difference of course is the major right turn the cyclone took late last week.
In most cases, tropical cyclones move southeast out of the tropics and gradually dissipate as they reach the cooler waters of higher latitudes and engage in the upper level westerlies.
The sharp right turn that Alfred took is not solely unique but our warming planet could be playing a part in this too. It is known that the planetary warming has caused the jet stream to become more erratic and wobbly in recent years.
The cause of the westward drift of Cyclone Alfred was mainly down to the position of the jet stream — the meandering river of wind in the higher atmosphere.
There are several others but the one we’ll talk about is the sub-tropical jet stream.
Late last week, the system that moved up across New Zealand and the Tasman Sea was steered north by a bend in the upper jet stream winds. An upper trough.
Residual upper energy
At the top end of it some residual upper energy lingered across the north of Australia. This clockwise swirling band of wind in the higher atmosphere happen to drift into northeast Australia and the coral sea just west of Cyclone Alfred.
By doing so, this influenced the steering winds around Alfred. In the days leading up to this, the steering winds around the cyclone had become very slack — a no-flow area — so it almost ground to a halt.
The upper trough to the west had enough pull that it started to shift Alfred west. There was a bit of back and forth over the days that followed but, eventually, a second upper trough from the southwest rolled into the west of the slow-moving cyclone and began to grab it properly to shift it west.
One of the biggest risks in tropical cyclones is torrential rain and flooding. The risks for this in Alfred will be extremely high and life threatening due to the slow progression to make landfall.
There are no strong steering winds to bring the storm onshore so the rain continues to fall over many days — increasing the flood risk.
That’s one of the factors that make this such an unusual storm, and such a threat to people in the affected areas.
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