Scientists explain Tropical Cyclone Alfred’s midnight burnouts

Why does Tropical Cyclone Alfred keep doing burnouts in the middle of the night? Scientists reveal why.

Tropical Cyclone Alfred

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Tropical Cyclone Alfred is in the middle of a tug of war between two rival steering currents in the atmosphere, causing the “erratic” category-2 storm to perform slow midnight doughnuts.

On Thursday and Friday mornings, residents in the path of the cyclone woke up to discover Alfred’s progress had stalled because it had moved in a circle overnight, instead of heading straight for the east coast of Australia.

Central Queensland University adjunct professor Steve Turton, an environmental geographer, has been studying tropical cyclones for 30 years and has lived through five of natural disasters when he lived in north Queensland. He now lives on the Sunshine Coast, and is again in the firing line, joking that he’s the magnet for the first cyclone to hit southeast Queensland since 1974.

Professor Turton said it was not unusual for cyclones to “wobble along their track” because they were affected by atmospheric winds steering the storms in different directions.

“At the moment, just north of the cyclone, we have what’s called an area of relatively higher pressure, and that’s wanting to push Alfred out to sea, wanting to push it east, but it’s quite a weak steering current,” he said.

“And then (to the south) over the Tasman Sea, there’s another high pressure one but a much stronger one and that’s wanting to push Alfred in a westerly track, towards the coast.”

“So it’s a tug of war between the two. The net result is it’s shifting to the west, but for this reason it’s moving more slowly.”

“That fluctuates … so the last two nights that steering current to the north could have taken over and made it do a loop, and the other one then became dominant again.”

Every hour, the Bureau of Meteorology has been releasing a fresh tracking map for Tropical Cyclone Alfred’s predicted path. On Friday, on the hour, Alfred’s projected landfall moved an hour later, and it continued to move at about 7km/hr.

Professor Turton said cyclones also wobbled north and south along their track, which caused uncertainty about where exactly it would cross the coast.

For those who have not experienced a cyclone before, Professor Turton said people sheltering in their homes could expect trees, branches and debris flying around and hitting their homes, similar to what was experienced during a supercell thunderstorm.

Tropical Cyclone Alfred is predicted to cause destructive winds of up to 155km/hr on the outer Moreton Bay islands and in exposed places on the coast, while Brisbane itself could be hit with damaging gusts of 130km/hr.

Professor Turton said even that gentler wind was powerful enough to easily lift trampolines high into the air and fling them around.

“The rain will be torrential and horizontal and it’ll sound pretty noisy, not just because of the wind, and the vegetation bashing the side of your house.”

He warned people not to leave their homes for a stickybeak after the cyclone crossed overhead when the conditions first calmed, because they most likely would be in the eye of the storm. Professor Turton told people to stay inside until the back side of the cyclone had passed.

“The eye is about 40 or 50km in diameter … and if the cyclone’s moving at 7km/hr still … it could take several hours of that area of basically no wind at all to pass over,” Professor Turton said.

“There’s an area in the middle where the wind will be dropped way down, the sky will brighten up, and people might wander out and do silly things.”

Weatherwatch meteorologist Anthony Cornelius said cyclones were often moved around the ocean as if they were “big sailboats”.

“They get pushed around by these upper-level systems … and the winds are somewhat opposing, so that’s why the system has slowed down so much, and has been tracking slowly,” Mr Cornelius said.

He said the slower Alfred went, the more rain the cyclone would deliver.

“I actually think the highest concern for rainfall is going to be after Alfred crosses the coastline – and for some people, that’s going to be the most severe part.”

Falls of up to 400mm a day are being predicted across widespread parts of the region in the cyclone’s path.

Bureau of Meteorology senior meteorologist Jonathan How said Alfred was now “making a move” free from the impact of the Tasman Sea high-pressure system.

“That does mean the tropical cyclone can now make an unobstructed way towards the coast,” Mr How told the ABC, but warned it might slow further.

“The next obstacle will be when it starts to interact with the land, also that more shallow water around Moreton Bay,” he said. “That’s when we will start to see it really slow down, particularly into (Saturday) morning.”