Explained: Could three years of La Nina mean flooding hell AGAIN?

There is now a 70 per cent chance of a third La Nina forming by early spring, with experts saying they cannot rule out a repeat of the 1974 floods.

There is now a 70 per cent chance of a third La Nina forming by early Spring as experts warn a repeat of the 1974 floods is “unlikely” but not impossible.

On Tuesday, the Bureau of Meteorology’s El Nino–Southern Oscillation (ENSO outlook) was raised from La Nina watch to La Nina alert following a “renewed cooling” in the Pacific Ocean.

It comes as climate models indicate further cooling is likely, with four of seven models suggesting La Nina could return by early spring.

But Head of Brisbane Weather, David Taylor said it would be “very premature” to confirm a third La Nina.

Mr Taylor said although he is “sceptical” of a third La Nina, if it does develop it will be “very weak”.

“We are still predicting above average rainfall and flooding but if a La Nina develops it will increase those chances significantly,” he said.

“It is still touch and go but we could see a similar outlook to the February floods.”

Irrespective of whether a third La Nina forms, Mr Taylor said there are “ still a number of variables” which influence rainfall and subsequent flooding including the Southern Annular Mode, a climate driver and “key player”.

Above-median rainfall is very likely from September to November for much of the eastern two-thirds of the mainland (chances are greater than 80 per cent)

While back-to-back La Nina events are not uncommon, and have occurred in approximately half of all past events since 1900, three in a row is less common and has only occurred three times since 1900: 1954-57, 1973-76, and 1998 to 2001.

But Mr Taylor said previous La Ninas which occurred third times in a row were “stronger”.

“The La Nina that affected us earlier in the year actually weakened to a non La Nina event in winter – previous La Ninas have continued for three years without this break,” he said.

Mr Taylor said a repeat of the 1974 floods is “very unlikely” despite a negative Indian Ocean Dipole (IOD) index value.

A negative IOD increases the chances of above average winter to spring rainfall.

All five international climate models surveyed by the Bureau anticipate a negative IOD is likely to persist through November, with a “rapid decline” in December. 

BOM climatologist Hugh McDowell said La Nina Alert is just a “change in the confidence of La Nina developing for this current Spring season”.

There is now a 70 per cent chance of a third La Nina forming by early Spring

Mr McDowell said La Nina conditions often led to more frequent tropical cyclones and above-average rainfall across Queensland. 

“There are subtle differences between La Nina’s – we don’t necessarily subscribe particular impacts to the strength of La Ninas,” he said.

“But we could potentially see the impact of the ’74 floods with the increased chance of rainfall ... whether it will be as bad, I can’t tell you.”

“Whether that leads to particularly bad flooding events or not is something that will be in the shorter range forecasts rather than the climate outlook.”

“But the forecast across the whole of Queensland is for a high chance of above average rainfall.”

Originally published as Explained: Could three years of La Nina mean flooding hell AGAIN?