Rainfall tipped for dip

The Department of Agriculture and Food’s latest Statistical Seasonal Forecast (SSF) indicates below average rainfall for the next three months across the grainbelt, although more positive falls are forecast for a select area in the central and southern regions.

The general forecast across the south-west of WA is for a 60 to 80 per cent chance of below average rainfall for May to July, based on a predictive skill of 50 to 75 per cent.

However, the forecast is more positive for an area including Tammin, Kellerberrin and Bruce Rock and east of Albany, where there is a 60-70 per cent chance of exceeding median rainfall.

The Bureau of Meteorology’s (BoM) current seasonal outlook is slightly more favourable, with a 40-60 per cent chance of above median rainfall for the grainbelt.

The BoM outlook for May rainfall is consistent with the SSF outlook of below median outlook, with the bureau indicating a 30-45 per cent chance of above median rainfall forecast for the South West.

The SSF’s longer-term outlook for May to October predicts below average to average rainfall for the majority of the grainbelt.

Better odds of a neutral outlook with a decile range of 4-7 is forecast for shires between Greenough and Dandaragan, as well as a line from Dalwallinu south to Narembeen and an area around Jerramungup and Ravensthorpe.

Meanwhile, Westonia, Merredin, Nungarin, Trayning and Kellerberrin shires have even better odds with up to 70 per cent chance of above average rainfall.

The rainfall during April across the grainbelt and the South West was average to above average.

Department research officer Meredith Guthrie said the forecast for the next three months was influenced by the current higher than normal sea level pressure over the south-west of the State.

“While current warmer than normal sea surface temperatures in the Indian Ocean and a rapidly decaying El Nino in the Pacific would ordinarily result in more favourable rainfall, the mean sea level pressure over WA is higher than normal, which is causing high pressure systems to block any large rain systems,” she said.

Dr Guthrie said the declining El Nino was likely to be replaced by a La Nina system.

“Climate models indicate a likely return to neutral conditions by mid-year,” she said.

“However, there is a 50 per cent chance – double the normal odds – of a La Nina forming in 2016. La Nina’s generally bring above average rainfall to eastern Australia but can also result in above average rainfall for the South West.”

The current Soil Water Map, from May 2, indicates very good levels of plant available water, except for an area in the far north east.

Department research officer David Ferris said the early break of season and good soil moisture levels across most regions had helped crop emergence but warned pests were now a looming threat.

“Early-sown canola is being damaged by vegetable weevils and mites in some areas and rust is likely to be an issue early in the season in higher rainfall zones,” he said.

Dr Ferris encouraged growers to tailor the SSF online tool to their individual location to assist decisions about ongoing fertiliser applications.

“The variable SSF forecast indicates that growers still need to plan top up fertiliser inputs carefully and apply nitrogen according to how the season unfolds, as follow-up rainfall might not be adequate to produce above average yield,” he said.

The department’s SSF forecast is updated at the start of every month and can be viewed at agric.wa.gov.au by searching for ssf.