WA rainfall odds mostly neutral for late winter/mid-spring

The Bureau's latest seasonal rainfall outlook shows near 50:50 chances across most of the country with no big swing towards wetter or drier conditions for the late winter to mid-spring period.

For the August to October period in Western Australia, the chances of above median rainfall are between 40 and 60%, apart from a small region near Meekatharra where they're below 40% (see map). So with climate patterns like the current, about 5 seasons out of 10 are expected to be wetter than average across the vast majority of the State, with about 5 out of 10 being drier.

It should also be noted that the August to October period is in the heart of the dry season across north and northwest WA and heavy rain at that time of year is uncommon.

The Bureau's August to October rainfall outlooks have low reliability over most of Western Australia (see background information).

The Outlook probabilities are based on recent Indian and Pacific Ocean temperatures. The tropical Indian Ocean is warmer than average, whilst the Pacific is showing a mixed pattern of warmer and cooler than average. For more detail see the El Niño Wrap-Up. The excess warmth in the Indian Ocean has had the greater influence on the overall pattern of probabilities.

June's value of the Southern Oscillation Index (SOI) was –12, five points below the May value of –7. The approximate SOI for the 30 days ending 13th July was –3.

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 19^th AUGUST 2003.

June 2003 rainfall in historical perspective

April to June 2003 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 Reliability: Strong reliability 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). Low reliability means the historical relationship 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/).