WA Seasonal Rainfall Outlook: probabilities for October to December 2004, issued 16th September 2004

Above average falls more likely in parts of WA

There is a moderate shift in the odds towards above average rainfall for the December quarter in parts of northwest and central Western Australia, the Bureau of Meteorology announced today. For the rest of the State, the chances of accumulating at least average rain over the last three months of 2004 are mostly close to 50%.

probability of exceeding median rainfall - click on the map for a larger version of the map

In an area extending south from Port Hedland across parts of the Pilbara, Gascoyne, Goldfields and Interior districts, the chances of a wetter than average October to December period are between 60 and 65%. So in years with ocean patterns like the current, about six October to December periods out of ten are expected to be wetter than average in these area, with about four out of ten being drier. It should be noted though, that the December quarter is a seasonally dry time of year in the Fortescue and West Gascoyne with heavy rain being uncommon.

However, when looking at the growing season (April-November) as a whole over southern and western WA, widespread below average falls up to the end of August mean that many places are unlikely to reach their total growing seasonal average by the end of November. The chances of receiving the required rain are below 30% in the Central Wheat Belt, Great Southern and Southern Coastal districts, dropping to below 10% in parts of the Central West and Lower West. For more information on the recent dry conditions, see the Drought Statement.

The current pattern of outlook probabilities is mostly due to recent temperature patterns in the Indian Ocean.

Outlook confidence is related to the influence of Pacific and Indian Ocean temperatures on seasonal rainfall. During the December quarter, history shows this influence to be moderately consistent across much of WA (see background information).

The chance of a late-developing El Niño event increased over the past month, with several but not all indicators reaching their El Ni˜o thresholds. The mounting evidence includes warming of the central Pacific, reduced Trade Winds in the same area and continued negative values of the Southern Oscillation Index (SOI) which came in at –8 for August. The approximate SOI for the 30 days ending 13th September was –3.

For routine updates on the latest data relating to El Niño, together with details on what the phenomenon is and how it has affected Australia in the past, see the El Niño Wrap-Up.


Click on the map above for a larger version of the map. Use the reload/refresh button to ensure the latest forecast map is displayed.

More information on this outlook is available from 9.00am to 5.00pm (WST) Monday to Friday by contacting the Climate and Consultancy section in the Bureau's Perth Office: (08) 9263 2222.


Corresponding temperature outlook

August 2004 rainfall in historical perspective

June to August 2004 rainfall in historical perspective


Background Information