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Once Cyclone Kirrily crosses the coast, a tropical low could pose a serious flood risk to large areas of Queensland


Queensland’s newest cyclone remains in its adolescence, still not yet fully formed and swirling off the coast as a tropical low. 

It is expected to bring significant rainfall totals to large parts of Queensland, from Cairns to Brisbane.

The Bureau of Meteorology (BOM) expects the system to develop into Tropical Cyclone Kirrily sometime on Wednesday.

For now, it has slowed its march towards the mainland, stalling slightly in the northern Coral Sea.

It’s currently forecast to hit the coast late on Thursday night and into Friday morning.

In the days following, it is expected to revert to a tropical low, dumping heavy rain.

With so many catchments across the state already saturated, there is a risk of serious flash flooding.

What is a tropical low?

Dr Annie Lau from the University of Queensland’s School of Environment said the term tropical low — or tropical depression in the US — refers to a low-pressure system similar to a tropical cyclone, but of lower intensity.

“A tropical depression has to get low pressure to form — a lot of humidity in the atmosphere and also high temperatures,” Dr Lau said.

“We call it a moderate strength, low-pressure system. It’s often associated with monsoon troughs and is something that we see a lot in the tropics.

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The weather bureau’s senior meteorologist Miriam Bradbury says the system is projected to become a category two cyclone rather than a three cyclone at landfall as previously expected.

“Not every depression will develop into a cyclone. The atmospheric conditions have to be really favourable.”

BOM senior forecaster Felim Hanniffy said a tropical low is upgraded when its winds reach a certain speed.

“Once a system has wind speeds of more than gale force — that’s winds in excess of 65 kilometres an hour — and is more than halfway around the system’s low-pressure centre, then it becomes a named system.”

He said tropical lows needed very warm sea temperatures to form, along with lots of moisture in the air.

“Sea temperatures above 26.5 degrees are usually considered the threshold to sustain these systems, as they derive all their energy from that interaction.”

How does speed affect the area of impact?

Dr Lau said the risk of heavy falls increases the longer a tropical low remains stalled over the Coral Sea.

“The more time the cyclone stays offshore to gather moisture, the more rainfall it will bring to the inland and the coastal area.

“The other thing is for a slower-moving cyclone, which has a slower forward speed, it doesn’t push that much water to the coast, so the storm surge risk would be slightly lower.”



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