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 parts of Queensland.

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 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.

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."

Mr Hanniffy said slower cyclones often affect inland areas more than coastal areas.

"Once they come inland they slow more, which means they can sit over an area for an extended period of time and dump vast amounts of rainfall," he said.

A a tropical low could move inland for "several days well after the initial landfall", Mr Hanniffy said.

"And that just spreads heavy rainfall risk across a very wide area, and obviously increases the risk of some widespread flooding as well," he said.

"Localised daily falls of 200 to 300 millimetres, if not more for a couple of days, wouldn't be out of the question."

What will happen to areas already saturated?

The tropical low from ex-Tropical Cyclone Jasper that remained stalled over Cairns and surrounds brought record-breaking rainfall totals to Far North Queensland, causing severe flooding.

The Daintree River rose to 14.85 metres, smashing the previous record of 12.6m, while the Barron River reached 4.4m, breaking the 1977 record of 3.8m.

South-east Queensland was hit by devastating storms through December and into the New Year period, with many weather stations recording 300 millimetres or more already this month.

Mr Hanniffy said the ground in those areas would not absorb heavy falls.

"We've seen persistent shower and thunderstorm activity and a lot of catchments have moistened," he said.

"When you get a slow-moving tropical cyclone, there's a significant risk of widespread flooding."

How is this system similar to Cyclone Debbie?

Tropical Cyclone Debbie battered the Queensland coast in March 2017, making landfall in the Whitsunday Islands and Airlie Beach, devastating the region.

As the system lost power, it became a tropical low and made its way down the coast, bringing damaging winds and torrential rainfall to central and south-east Queensland.

Falls of almost 1 metre were recorded west of Mackay in two days, while the Gold Coast Hinterland received more than 600mm of rain in one day.

Mr Hanniffy said the current tropical low bears some similarities to Debbie.

"Cyclone Debbie made landfall as category four, it was probably closer to the coast and moved inland and then moved south along the east coast," he said.

"This system probably looks like going further inland and is likely to be a weaker system even by the time it makes the coast, but we are looking at several days with this system lingering inland."