Dr David Jones, Head of Climate Analysis at the Bureau's National Climate Centre, said that if recent trends in Pacific climate patterns were to be maintained, an El Niño event would be established by mid-winter.

"This view is supported by several computer forecasts of El Niño, which have firmed in their predictions of an event in 2009," Dr Jones said.

The odds of an El Niño are now thought to be above 50 per cent, which is more than double the normal risk of an El Niño in any year.

According to the Bureau, the surface of the equatorial Pacific currently stands at around 0.5°C warmer than normal, while the Southern Oscillation Index (SOI) dropped from +9 in April to -5 in May.

During a typical El Niño event, central equatorial Pacific Ocean temperatures are at least 0.8°C warmer than normal for around four to six months during which time the SOI persists at values –7 or lower.

The Pacific easterly Trade Winds were also suppressed in May.

The Bureau says that historically the main impacts from El Niño events usually occur in the second half of the year.

During this period eastern, northern and parts of southern Australia face an increased risk of below-average rainfall and above-average daytime temperatures.

El Niño events are frequently associated with drought, as in the recent 2002 and 2006 El Niño events when rainfall deficiencies were widespread and severe.

However, the Bureau says it is possible for timely falls of rain to limit the worst effects to relatively small areas, as happened during the intense 1997 El Niño event.

"While there is an underlying tendency for El Niño events to be drier than normal, the detailed impacts of each El Niño are different," the Bureau said.

"However, it's still possible that the recent trends may stall without El Niño thresholds being reached. Such an outcome, however, may still bias the outlook towards drier than average conditions, especially if the SOI remains negative."