Southeastern Aust Seasonal Rainfall Outlook: probabilities for Spring 2009, issued 26th August 2009

Drier spring favoured over much of SE Australia

For southeastern Australia, the outlook for total rainfall for spring (September and November) shows a moderate to strong shift in the odds favouring drier conditions over most of SA, Victoria, Tasmania and southwestern NSW. Only the northern half of NSW and northeastern part of SA show no strong shift favouring either wetter or drier conditions.

The pattern of seasonal rainfall odds across SE Australia is a result of recent warm conditions in the Pacific Oceans (El Niño).

probability of exceeding median rainfall - click on the map for a larger version of the map

During spring (September to November), the chance of exceeding is below 40% in an area encompassing much of SA, southern NSW west of the Great Divide, most of Victoria, and northern and eastern Tasmania (see map). In central and eastern SA, and the far northwest of Victoria, chances drop to below 30%. This means that for every ten years with ocean patterns like the current, about six or seven years are expected to be drier than average during spring across this large part of the southeast, while about three or four years are expected to be wetter.

New: An expanded set of seasonal rainfall outlook maps and tables, including the probabilities of seasonal rainfall exceeding given totals (e.g. 200 mm), is available on the "Water and the Land" (WATL) part of the Bureau's website.

Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian rainfall. During spring (September to November), history shows confidence to be moderately consistent over southern and northeastern SA, most of NSW and Victoria and northern Tasmania. Over the remaining areas, especially western SA and southern Tasmania, the effect is only weakly to very weakly consistent. In these areas where outlook confidence is not high (especially central SA), caution should be used when interpreting these outlooks (see background information).

An El Niño event looks to be developing across the Pacific: the latest outputs from computer models indicate it will reach peak intensity late in the year. El Niño events are usually (but not always) associated with below normal rainfall in the second half of the year across large parts of southern and inland eastern Australia. The SOI is approximately −7 for the 30 days ending 23 August. 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:

Sydney -(02) 9296 1555
Adelaide -(08) 8366 2664
Melbourne -(03) 9669 4949
Hobart -(03) 6221 2043
 

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 22nd September 2009

Corresponding temperature outlook

July 2009 rainfall in historical perspective

May to July 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 (Seasonal Climate Outlook Products).

  • 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 25 El Niño events since 1900 are summarized on the Bureau's web site (El Niño - Detailed Australian Analysis).