RAIN, HAIL, OR SHINE: What BOM predicts for autumn

The Bureau of Meteorology has released its climate outlook for autumn.

Rain, hail, or shine, what the Bureau of Meteorology has predicted for autumn.

A wetter than average summer and a wetter than average autumn forecast could combine to increase the risk of flooding on the Northern Rivers, the Bureau of Meteorology said.

Senior climatologist Andrew Watkins said last year, Australia experienced its coolest and wettest summer in at least five years, and the third wettest December since national records began in 1900.

In 2020, Lismore received 1705mm of rain - the town's median yearly rainfall was 1212.9mm.

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A third of that yearly total fell in the last month of the year, with Lismore getting 565.6mm of rain in December - the wettest December since records began in 1884.

Lismore also received more rainfall than usual in January, 122.8mm, and February, 244.4mm. The February total was almost double the median for the month.

La Nina, which brings increased chance of rain, was still impacting the Northern Rivers, but was past its peak, Mr Watkins said, and would likely return to neutral during autumn.

BOM said the Northern Rivers has more than a 70 per cent chance of exceeding its average rainfall for autumn.

Coastal areas have a 75 per cent chance of receiving 400-600mm of rain in autumn.

The median autumn rainfall for most of the Northern Rivers was between 100-200mm, with the exception of the area surrounding Ballina, which had a median of 300mm.

Mr Watkins said good rainfall in summer meant soil moisture levels were very much higher than average, increasing the run-off from predicted rain and the likelihood of flooding.

"The risk of widespread flooding was highest for parts of eastern and northern Australia, where soils and catchments are already wet and we're likely to see above average rainfall," he said.

The Northern Rivers had a 50 per cent chance of being warmer in autumn than average, but that was likely to only experience a rise in maximum temperatures by 1C.

The average maximum autumn temperatures for the Northern Rivers ranged between 21-27C.

Warmer minimum temperatures, of about 1-2C higher, were also likely across most of the Northern Rivers in autumn.