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

BOM OUTLOOK: Wet soils increase risk of future flooding

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.

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.

In 2020 a number of rainfall records were broken across the Clarence Valley, with much of the rainfall battering down in the later stages of the year.

Wooli Beach saw a new record in total rainfall in 2020, with 2201.6mm, eclipsing the previous record of 2004.8mm set in 2011. Grafton Research Station also had its wettest year since 1959, with 1600.2mm recorded.

>>> RELATED: FIRST FLOODS: Grafton's wettest December on record

Yamba's Pilot Station saw 1746.6mm of rainfall in 2020, which was around 20 per cent higher than the average annual rainfall total for the station. Grafton Airport AWS saw a total of 1527.8mm last year, a 33 per cent increase on the station's rainfall average.

Meanwhile Grafton experienced its third driest November since 1917 with just 7mm of rain recorded, immediately followed by its wettest December in 164 years of data, recording 513mm.

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The first two months of 2021 continued the damp trend in the Clarence, with Yamba copping 144.4mm in January and 242.4mm in February. Grafton Research station saw 101.8mm and 178.2mm to start the year, while the observation station at Grafton Regional Airport recorded 75.2mm and 244mm in January and February.

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.

The BOM said the Northern Rivers has more than a 70 per cent chance this year of exceeding its average rainfall for autumn. Coastal areas have a 75 per cent chance of receiving 400-600mm of rain in autumn.

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.

>>>> RELATED: New record in Clarence Valley's 2020 rainfall stats

Yamba Pilot Station and Grafton Airport AWS mean minimum and maximum temperatures were above the average last year.

The mean maximum temperature for Yamba was 24.4C, 1C above the average. The mean minimum temperature was 16.5C in Yamba, again 1C above average.

At Grafton Airport AWS the mean maximum temperature for 2020 was 26.2C, 0.3C above average while the mean minimum temperature was 13.3C, 0.5C above average.

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Motorists are urged to drive safely to the conditions.