Below average rainfall possible in southwest Australia in autumn

The Western Australia outlook for total autumn rainfall shows increased chances of being drier than normal over the Southwest Land Division during autumn.

The pattern of seasonal rainfall odds across Australia is mainly a result of warm conditions in the Indian Ocean in January, the Pacific Ocean had little contribution to this forecast.

The chance of exceeding median rainfall is about 40% over most of the Southwest Land Division, in other words, the chance of being drier than normal is about 60% in these areas. This means that for every ten years with ocean patterns like the current, about six years are expected to be drier than average in this region.

For the remainder of the state, the chance of exceeding median rainfall during autumn is about 50% to 60%, meaning that above average falls are about as equally likely as below average falls.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During autumn, history shows the effect to be moderately consistent across much of the state, whilst the effect is only weakly or very weakly consistent in some pockets in the north Kimberley, west Pilbara, Southwest and eastern Interior (see background information).

Some warming of the equatorial Pacific Ocean occurred in January, following cooling in December, so that sea surface temperatures (SST) in the central Pacific have now eased back to neutral values. While sub-surface waters are cool in the eastern Pacific, those in the west of the basin are warmer than normal and there are indications that these will impact on the surface in coming months. Furthermore, most current model outlooks suggest further warming in the Pacific. The most likely scenario is for conditions to remain neutral. The SOI remains positive at approximately +15 for the 30 days ending 17 February. For routine updates and comprehensive discussion on any developments please see the ENSO 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 detailed forecast maps, including the probabilities of seasonal rainfall exceeding given totals, can be found here.

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 24^th March 2009

Corresponding temperature outlook

January 2009 rainfall in historical perspective

November 2008 to January 2009 rainfall in historical perspective

Background Information

• 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/).