Contrasting June-quarter rainfall odds across SE Australia

For southeastern Australia, the outlook for total June quarter rainfall (April-June), shows a moderate shift in the odds favouring wetter than normal conditions over eastern and northern NSW. In contrast, there is a moderate to strong shift in the odds favouring drier than normal conditions over much of South Australia, Victoria and northern Tasmania.

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

For the April to June period, the chance of exceeding median rainfall is between 60 and 70% in a region covering much of eastern and northern NSW (see map). This means that for every ten years with ocean patterns like the current, about six or seven years are expected to be wetter than average over this area, while about three or four years are expected to be drier.

Conversely, the chance of above median rainfall is below 40% in a band extending from northwest SA across southwest NSW and Victoria (except East Gippsland) to northern Tasmania (see map). The probabilities drop below 30% in the central pastoral districts of South Australia. This means that below median falls, i.e. drier than normal conditions, have a 60 to 75% chance of occurring across this broad region. In other words, for every ten years with ocean patterns like the current, about six to seven years are expected to be drier than normal in these areas.

New: Under the WATL part of the Bureau's website, there is an expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall exceeding given totals (e.g. 200 mm).

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During April to June, history shows this effect to be moderately consistent in a band covering most of South Australia, the far southwest of NSW and over most of Victoria. There is also a moderate consistency over most of the northeast corner of NSW and the eastern half of Tasmania. However, confidence is only weakly to very weakly consistent in a band stretching from northeast SA to southeast NSW and part of western Tasmania (see background information). In these areas where outlook confidence is not high caution should be used when interpreting these outlooks.

Pacific climate patterns were bordering on La Niña for several months, with above average wet season falls in the Australian tropics a direct result. The equatorial Pacific Ocean is warming and the consensus from computer models is for near average temperatures in the middle of the year. The SOI remains positive at approximately +3 for the 30 days ending 21 March. 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.

More information on this outlook is available Monday to Friday from 9.00am to 5.00pm local time by contacting the Bureau's Climate Services sections in Queensland, NSW, SA, Victoria and Tasmania at the following numbers:

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

Corresponding temperature outlook

February 2009 rainfall in historical perspective

December 2008 to February 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/).