Millions of people in south-east Queensland and northern New South Wales awoke on Thursday expecting a day of damaging winds and rainfall as Tropical Cyclone Alfred bore down.
But overnight the system slowed down, with forecasts that it might not make landfall until early Saturday. So what’s been happening to Tropical Cyclone Alfred?
What has changed?
On Wednesday morning the various models the Bureau of Meteorology’s cyclone team analyses were suggesting that Alfred would probably make landfall early on Friday morning.
But by Wednesday evening the eye of the storm was slightly further away from the Queensland coast, after performing a full loop during the day instead of continuing to move towards the coast.
“Alfred basically stalled and almost did a loop-de-loop back on itself,” said a BoM meteorologist, Christie Johnson.
Why has Alfred slowed down?
The movement of a cyclone is down to the “steering flow” – the average wind speed between about 2km and 10km in the atmosphere.
On Tuesday Tropical Cyclone Alfred ran into a high-pressure ridge that is sitting over the Tasman Sea which pushed it off its south-east trajectory and turned it towards the Queensland coast.
Prof Liz Ritchie-Tyo, a cyclone expert at Monash University, said: “When it took that sharp turn, it came under the influence of that ridge and they tend to have much lighter winds associated with them.”
But why didn’t the models pick up that the cyclone would slow down?
Ritchie-Tyo said the cyclone had taken its turn to the coast slightly further north than many of the models expected, and this meant the steering flow had been slightly weaker.
“This is like the butterfly effect, where a small error in a model means the prediction farther out is error-prone. But now the model runs are picking this up pretty well.”
How fast is the cyclone moving and when might it reach the Queensland coast?
On Thursday morning Alfred was moving at about 7km/h – relatively slow for a cyclone, according to Ritchie-Tyo.
“There’s no suggestion it’s going to pick up much speed,” she said. “So that means the winds and rain will stay over the same area for longer.”
Johnson said Alfred was expected to move slowly and there was a suggestion it could stall as it approached the coast.
“Cyclones can get steered by other influences and can be a bit like a leaf in a storm, with eddies pushing this way and that,” she said.
Another area of low pressure over the north of the country could interact with the storm, causing it to slow and stall again off the coast. There was even a suggestion that for a short time it could speed up overnight Thursday into Friday, before slowing again.
On Thursday the Queensland premier, David Crisafulli, said the most likely scenario was a coastal crossing Saturday morning – pretty much 24 hours later than communities had been preparing for.
When might people start to feel the impacts?
Coastal communities on the Gold Coast and northern New South Wales were already feeling the effects and these would continue to intensify, according to Johnson.
Much of the south-east Queensland coast, from Gympie to the NSW border, was under a severe weather warning on Friday for intense rainfall and damaging winds.
What intensity will Alfred be when it makes landfall?
Johnson said on Wednesday that some models were suggesting Alfred could intensify into a category-three system before it made landfall, with even more destructive winds, but the chances of that had fallen away.
“The chances of it intensifying has reduced and it’s now expected to be a category one at landfall but for much of the approach it could still be a high-end category two,” she said.
On Friday morning, the cyclone was forecast to weaken slightly to a category one storm in the final hours before landfall, expected around 10am on Saturday in an area between Double Island Point and just south of Brisbane.
Cyclone categories only reflect wind speeds and don’t account for the amount of rainfall or flooding.
Johnson said while the landfall was expected to be in the vicinity of Brisbane city, with one US model having it slightly north, it was important people did not focus on that aspect.
Destructive winds, extreme rainfall, flooding and coastal impacts were likely to occur over a wide area, from the Sunshine Coast to northern NSW.
Read more of Guardian Australia’s Tropical Cyclone Alfred coverage:
