Climate models project the Pacific will soon be ''on the cusp'' of an El Nino event, with warmer ocean surface temperatures already contributing to record warm and increasingly dry conditions across south-eastern Australia, the weather bureau said.
Surface temperatures in the tropical Pacific are running at 0.5 to 1.5 degrees above normal, and the normal trade winds have dropped or reversed to become westerlies, said David Jones, head of climate monitoring at the bureau. Sub-surface temperatures are 2 to 5 degrees warmer than usual.
''A lot of the precursory signatures of an El Nino event are already out there in the Pacific,'' Dr Jones said, adding that there was a 70 per cent or higher chance such an event would be declared.
El Ninos happen about every three to eight years and tend to result in higher global temperatures. For much of Australia and countries to the north, they typically bring droughts and more active bushfire seasons as rainfall shifts eastward.
Some US scientists say the early signs point to a ''super El Nino'' like that of 1997, which propelled global temperatures to the warmest on record in 1998 - before 2005 and 2010 exceeded that mark. Recent weak El Ninos have had more severe effects on Australia than the 1997 event, with 1994, 2002 and 2006 events leading to worse droughts, Dr Jones said.
Australia is already seeing El Nino-like conditions. May has been ''almost rainless through the bulk of Queensland and most of NSW'', he said, with Victoria and Tasmania also mostly dry.
''Both Melbourne and Sydney have the potential to have their warmest May on record,'' Dr Jones said - particularly Sydney.
Melbourne had its 11th consecutive day of 20 degrees or warmer weather on Wednesday, beating the previous record of seven for this late in autumn, with Thursday likely to be the last in the series ahead of cooler conditions from Friday..
Sydney posted its 12th consecutive day of at least 22 degrees on Wednesday, compared with a previous record of six this late in the season. The weather bureau has forecast another six such days, with most reaching 25 to 26 degrees, compared with a May average of 19.4.
As Fairfax Media reported on Monday, the warm spell has left alpine areas little snow less than three weeks before the season's official opening.
''The odds are certainly favouring a below-average ski season,'' Dr Jones said.