Generally neutral outlook for summer rainfall

The chances of accumulating at least average summer (Dec-Feb) rain are close to 50% for most of the country, the Bureau of Meteorology announced today. The one exception is in an area of northeast Queensland where there is a moderate shift in the odds towards below average rainfall for summer.

Within about 100 to 200 km of Townsville in northern Queensland, the chances of above median rainfall for the December to February period are between 35 and 40%. So in years with ocean patterns like the current, about six summers out of ten are expected to be drier than average in this region, with about four out of ten being wetter.

The current pattern of outlook probabilities is mainly due to above average temperatures in the Pacific Ocean.

Outlook confidence is related to the influence of Pacific and Indian Ocean temperatures on seasonal rainfall. During summer, history shows this influence to be moderately consistent across much of WA and scattered areas in eastern Australia, but generally weakly or very weakly consistent through the centre of the country (see background information).

Progress towards a late-developing El Niño event has slowed over the past two months, with consistent cloud and wind patterns failing to materialize. The central Pacific remains warmer than average and at levels consistent with El Niño, but the Southern Oscillation Index (SOI) was only weakly negative, coming in at –4 for October. The approximate SOI for the 30 days ending 16th November was –5.

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.

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Regional commentary is available from the Climate and Consultancy Sections in the Bureau's Regional Offices:

Queensland -	(07) 3239 8669 or (07) 3239 8666
New South Wales -	(02) 9296 1522
Victoria -	(03) 9669 4949
Tasmania -	(03) 6221 2043
South Australia -	(08) 8366 2664
Western Australia -	(08) 9263 2222
The Northern Territory -	(08) 8920 3813

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 17^th DECEMBER 2004.

Corresponding temperature outlook

October 2004 rainfall in historical perspective

August to October 2004 rainfall in historical perspective

• The Bureau's seasonal outlooks are general statements about the probability or risk of wetter or drier than average weather over a three-month period. The outlooks are based on the statistics of chance (the odds) taken from Australian rainfall/temperatures and sea surface temperature records for the tropical Pacific and Indian Oceans. They are not, however, categorical predictions about future rainfall, and they are not about rainfall within individual months of the three-month outlook period. The temperature outlooks are for the average maximum and minimum temperatures for the entire three-month outlook period. Information about whether individual days or weeks may be unusually hot or cold, is unavailable.

• This outlook is a summary. More detail is available from the contact people or from SILO (www.bom.gov.au/silo/products/SClimate.shtml).

• Probability outlooks should not be used as if they were categorical forecasts. More on probabilities is contained in the booklet The Seasonal Climate Outlook - What it is and how to use it, available from the National Climate Centre. These outlooks should be used as a tool in risk management and decision making. The benefits accrue from long-term use, say over 10 years. At any given time, the probabilities may seem inaccurate, but taken over several years, the advantages of taking account of the risks should outweigh the disadvantages. For more information on the use of probabilities, farmers could contact their local departments of agriculture or primary industry.

• Model Consistency and Outlook Confidence: Strong consistency means that tests of the model on historical data show a high correlation between the most likely outlook category (above/below median) and the verifying observation (above/below median). In this situation relatively high confidence can be placed in the outlook probabilities. Low consistency means the historical relationship, and therefore outlook confidence, is weak. In the places and seasons where the outlooks are most skilful, the category of the eventual outcome (above or below median) is consistent with the category favoured in the outlook about 75% of the time. In the least skilful areas, the outlooks perform no better than random chance or guessing. The rainfall outlooks perform best in eastern and northern Australia between July and January, but are less useful in autumn and in the west of the continent. The skill at predicting seasonal maximum temperature peaks in early winter and drops off marginally during the second half of the year. The lowest point in skill occurs in early autumn. The skill at predicting seasonal minimum temperature peaks in late autumn and again in mid-spring. There are also two distinct periods when the skill is lowest - namely late summer and mid-winter. However, it must always be remembered that the outlooks are statements of chance or risk. For example, if you were told there was a 50:50 chance of a horse winning a race but it ran second, the original assessment of a 50:50 chance could still have been correct.

• The Southern Oscillation Index (SOI) is calculated using the barometric pressure difference between Tahiti and Darwin. The SOI is one indicator of the stage of El Niño or La Niña events in the tropical Pacific Ocean. It is best considered in conjunction with sea-surface temperatures, which form the basis of the outlooks. A moderate to strongly negative SOI (persistently below –10) is usually characteristic of El Niño, which is often associated with below average rainfall over eastern Australia, and a weaker than normal monsoon in the north. A moderate to strongly positive SOI (persistently above +10) is usually characteristic of La Niña, which is often associated with above average rainfall over parts of tropical and eastern Australia, and an earlier than normal start to the northern monsoon season. The Australian impacts of 23 El Niño events since 1900 are summarized on the Bureau's web site (www.bom.gov.au/climate/enso/).