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Cyclone Debbie: Biggest threat may come from 'storm tide' as tempest builds

Peter Hannam

The biggest threat to residents from tropical cyclone Debbie - and similar tempests - typically comes from the huge storm surge that can inundate low-lying coastal regions rather than the winds, researchers say.

Cyclone Debbie remains on track to cross the coast near Bowen - midway between Townsville and Mackay - on Tuesday morning, with a "very destructive core" bringing winds gusts potentially reaching 260 km/h, the Bureau of Meteorology said. That would make it category 4 or severe cyclone.

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The "very destructive wind core" of Cyclone Debbie is 100km wide and will hit the north Queensland coast before 8am on Tuesday. Nine News

"Residents between Cape Upstart and St Lawrence are specifically warned of the dangerous storm tide as the cyclone crosses the coast on Tuesday morning," the bureau said.

Cyclone Yasi, which struck north Queensland in 2011, powered a storm surge that reached 7.5 metres between Cardwell and Tully Heads, akin to a tsunami, said David King, the director of the Centre for Disaster Studies at James Cook University in Townsville.

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Storm surges "bring in flooding", Professor King said. "The water moves in at a great speed."

Waters under the eye of the cyclone swell because of the deep low pressure. Strong winds also whip up waves, with the impact intensified if the storm's arrival coincides with a high tide.

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On current forecasts, the storm is predicted to make landfall at 10 am local time, or close to the 9.45 am high tide timing at Bowen, according to the bureau.

"More people have historically died from storm surges than the wind damage," Jonathan Nott, a specialist in extreme weather also at James Cook University (JCU), said, noting the Great Bhola Cyclone killed half a million people in Bangladesh in 1970.

"[Yasi] literally swept houses...off their foundations and they were totally gone," Professor Nott said. "Others were destroyed."

"We could be looking at 3-4 metres with Debbie as it intensifies," he said.

(See bureau chart below of the current forecast track of what is expected to be a category four cyclone.)

Jeff Kepert, head of severe weather research at the bureau, said it's long been understood that flooding poses a greater risk for people living in a cyclone's path.

"You hide from the wind but you flee from the water," Dr Kepert said.

Shifts in the forecast track of just 30 kilometres can add a metre in storm-surge height if the storm hits a bay rather than a headland.

While the science of cyclones has advanced rapidly in recent decades, more improvements will be introduced by the end of 2017, Dr Kepert said. These include advanced modelling, which will allow forecasters to give better "worst-case" predictions for coastal inundation, and more precise cyclone strength estimates.

"Intensity forecasts have scarcely improved at all," compared with other recent advances, he said.

Rain, reef

Heavy rainfall, perhaps as much as 400 millimetres, could also lead to inundation particularly if it combines with swollen rivers and high tides.

"Localised flash flooding is likely, and the public is urged to stay tuned for warnings," the bureau said..

"Townsville had a very poor wet season," so the catchment is really dry ahead of Cyclone Debbie, Professor King said.

Both professors said cyclones such as Debbie were important for bringing rainfall to inland parts of Queensland. They also help ease the threat of bleaching of coral reefs by lowering water temperatures through increased cloud cover and ocean mixing.

Much of the corals of the central Great Barrier Reef appear to be bleaching this year, adding to the damage from last year's massive event.

Climate signal

Research in recent years indicates that climate change is reducing the overall number of cyclones forming off Queensland, but the number of intense ones is increasing.

Tropical cyclones leave an historical mark on the landscape in part because of that storm surge. Unlike the extra-tropical east coast low that hit Sydney last year and caused large-scale beach erosion at places such as Collaroy, tropical tempests tend to dump large amounts of sand inland, Professor Nott said.

JCU researchers are going to be in the field collecting data on the wind speed and strength of the storm surge but also the chemistry of the rain dumped on the region.

A clutter of boats and yachts are piled together at Port Hinchinbrook after Cyclone Yasi.

A clutter of boats and yachts are piled together at Port Hinchinbrook after Cyclone Yasi. Photo: Rick Rycroft

Interestingly, rain and water vapour from cyclones has a distinct isotopic signature, with most of the oxygen-18 isotopes rained out because of the intense convection of the event and how high the molecules rise within the storm.

Using the chemical record within limestone and other sources, Professor Nott and other scientists have been able to show that cyclones are now the fewest in at least 800 years for the Cairns region and 1500 years for Western Australia. (See Nature paper here.)

"We've already been able to show that climate change is already affecting the behaviour of tropical cyclones in Australia," Professor Nott said.

"One of major impacts is a very marked increase in the intensity and a marked decrease in the frequency [of cyclones]," he said. "This really started occurring quite dramatically in 1970."

Some other regions, such as the north-west Pacific, are experiencing more and stronger cyclones.

Models predict the number of El Nino events will double this century with climate change, a shift that implies the trend towards fewer but more powerful cyclones in our zone will continue.

"It doesn't sound very good for coral reefs and it doesn't sound very good for rain-penetrating systems getting into the interior of Queensland," Professor Nott said.

Record heat

While meteorologists are focused on Cyclone Debbie, a slew of heat records are being set across inland Australia

On Sunday, three states set late-season records for maximum temperatures and more may be set before the heatwave ebbs.

Ceduna reached 44.6 degrees, the hottest for this late in the year for South Australia.

In NSW, Tibooburra closed 41.6 degrees, while Birdsville in Queensland touched 43 degrees.

Wilcannia on Monday broke the record again, with 44 degrees as of 3.50 pm. Birdsville also beat Sunday's mark with 43.7 degrees reached so far.

"We've had very hot conditions in central Australia for about a week now," Blair Trewin, senior bureau climatologist, said, noting that Alice Springs had days above 38 degrees since last Monday and about three weeks above 35 degees.

Birdsville topped 40 degrees on Monday for a seventh day in a row, matching the record for March, with another two days forecast.

Dr Trewin said there was unlikely to be a direct connection between the record warmth and cyclone Debbie, although regions to the north of the storm will likely see unusually warm weather as westerlies drag in inland heat.

Cairns, for instance, had 33.6 degrees on Sunday, or three degrees above the March average.