As residents in south-east Queensland and northern New South Wales wait for Tropical Cyclone Alfred’s next move, the Bureau of Meteorology’s regular “track map” will be a source of information for many.
But behind the snaking track lines and coloured blobs on the map are more than a hundred scenarios from different computer models that are trying to work out how the system will react to what’s happening in the atmosphere and ocean.
So how do cyclones move and how does all this complexity distill down into a map that can warn millions of people of what’s to come?
How does the Bureau of Meteorology create the track maps for Cyclone Alfred?
BoM senior meteorologist Dean Narramore has been part of a team working over the past 11 days to produce updated maps of Cyclone Alfred.
Every six hours, a new map is issued after the bureau’s cyclone team has looked at the output of models from Europe, the US, Japan and Australia’s model – the Australian Community Climate and Earth-System Simulator.
“When you look at all the model runs there can be 100 to 150 different tracks. It looks a bit like spaghetti,” Narramore says.
Early into Alfred’s life, Narramore says the modelled tracks were “all over the place” but a few days ago – even though the models are slightly different – they began to align.
“When all the lines start to come together into a consistent pattern, that gives us more confidence. So then it’s still like spaghetti, but now you’re holding it in your hand.”
The bureau will start issuing a track map every three hours once a tropical cyclone warning is issued – a warning triggered when gale-force winds above 63 km/h are expected within 24 hours.
That could be as early as Tuesday afternoon.
What forces are trying to steer Cyclone Alfred?
So far, winds in the upper part of the atmosphere have been pushing Tropical Cyclone Alfred south-east from where it formed in the Coral Sea at the end of February.
Dr Hamish Ramsay, a cyclone expert at the CSIRO, says the direction a cyclone moves in is dictated by something called the “steering flow” – the prevailing winds between two and 10 kilometres up in the atmosphere.
Ramsay says it is not unusual for cyclones to track so far south – however it is unusual for them to make landfall so far down.
That’s because a cyclone level with the south-east Queensland coast isinfluenced by mid-latitude weather systems that are usually blowing towards the east, taking most systems away from Australia.
On Tuesday, Alfred is expected to move into an area where the “steering flow” is going west – thanks to an area of high pressure over the Tasman Sea.
Where is Cyclone Alfred expected to hit?
The bureau’s track maps also include a large area in grey outside the main track that represents the area where the cyclone could go.
Narramore said once Alfred moves towards the coast later on Tuesday, that uncertainty is likely to narrow – giving the public and authorities a clearer idea of where the worst impact could be.
Staying up to date with the maps is the best way to stay informed, but on Monday he said the models were showing a likely landfall late on Thursday or early Friday between Hervey Bay and the Gold Coast.
But this could change.
What intensity will Cyclone Alfred be when it hits?
The track maps also include the expected intensity of the cyclone, which relates to the wind speeds they produce.
A category two cyclone – the expected rating for cyclone Alfred when it is forecast to make landfall – suggests average winds between 89 and 117km/h with gusts up to 164km/h.
But Ramsay says the category of a cyclone does not necessarily reflect how damaging it could be.
Alfred was forecast to move over an area of unusually warm ocean surface, giving more energy to the system in terms of the rain it could produce.
If the cyclone slows down as it approaches the coast, this would also see higher rainfall.
“The highest mortality from tropical cyclones is associated with the rainfall,” he said. “Wind speed in this instance might not be the most damaging.”
What should people be looking for?
The bureau’s track maps shows the likely route for the cyclone and the areas that will be hit by destructive winds.
In the southern hemisphere, areas to the south of a cyclone’s centre experience the worst of the weather.
Narramore says a very large area is likely to experience gale force winds hundreds of kilometres away from where the cyclone makes landfall, including the Sunshine Coast, Brisbane, Gold Coast and northern NSW.
The timing of the landfall could also be critical to the coastal damage from waves and inundation.
If the cyclone strikes at high tide, this could add a further metre to the expected tide, with the damaging waves adding even further height.
“A few hours can make a big difference,” he said.
