Risk of further below average rainfall in southwest WA

There is a moderate shift in the odds towards below average September quarter (July-September) rainfall across a significant part of southwest Western Australia, the Bureau of Meteorology announced today. The autumn-early winter rainy season has been very dry in southwest WA, so a persistence of below average falls would have serious consequences.

The pattern of seasonal rainfall odds across Australia is largely a result of above average temperatures in the Pacific Ocean. Conditions in the Indian Ocean are near normal and have had little influence on this outlook.

For the July to September period, the chances of above median rainfall are between 35 and 40% over parts of southwest WA (see map). This means that below average falls have a 60 to 65% chance of occurring in these regions. So in years with ocean patterns like the current, about six September quarters out of 10 are expected to be drier than average in these areas of southwest WA, with about four out of ten being wetter.

In contrast, a small part of the Gascoyne in WA has an increased risk of above average seasonal falls. The chance that the seasonal total will exceed the median in this area is 60 to 65%. For much of WA though, the chances are close to 50% for accumulating at least the long-term median rain during the coming three months.

However, it should be noted that July to September is the heart of the dry season across northern Australia and heavy rain is uncommon during this period.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During July to September, history shows this effect to be moderately consistent in parts of central and eastern WA, but only weakly consistent across most of the State, including the southwest (see background information).

After moderately positive values of +14 and +15 in March and April respectively, the Southern Oscillation Index (SOI) dropped substantially in May to a value of −10. The approximate SOI for the 30 days ending 24th June is −9.

It is likely that neutral ENSO conditions will remain in place for the remainder of 2006. For routine updates and comprehensive discussion on the latest data relating to ENSO, together with details on what the phenomenon is and how it has affected Australia in the past, 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.

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 25^th JULY 2006

Corresponding temperature outlook

May 2006 rainfall in historical perspective

March to May 2006 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/).