Mixed odds for summer rainfall across southeastern Australia.

Southeastern Aust Seasonal Rainfall Outlook: probabilities for Summer 2010/2011, issued 23rd November 2010

Mixed odds for summer rainfall across southeastern Australia.

The December to February outlook for rainfall has a wetter than average summer predicted for northeastern New South Wales. Drier than average conditions are more likely across South Australian agricultural areas and southern pastoral areas, as well as western Victoria, and the northern half of Tasmania.

The pattern of seasonal rainfall odds across Australia is a result of warm conditions in the Indian Ocean, as well as cool conditions in the equatorial Pacific Ocean associated with the current La Niña.

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

The chances of exceeding the median rainfall for December to February are over 60% for the northeastern half of New South Wales, and exceed 75% in the far northeast of the state. This means that for every ten years with ocean temperature patterns like the current, about six to eight years would be expected to be wetter than average over these areas, while about two to four years would be expected to be drier.

In northern Tasmania, western Victoria and South Australian agricultural areas and southern pastoral areas, the outlook favours drier conditions with odds of around 35 to 40%. This means for every ten years with ocean patterns like the current, about six to seven years would be expected to be drier than average over this summer period while about three to four would be wetter. In remaining areas, which include central and eastern Victoria, southern Tasmania and central and northern pastoral areas in South Australia, the outlook is neutral with odds around 50%. This means that the chance of a wetter than average December quarter are about as likely as the chance of below average conditions in these areas.

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 December to February, history shows this effect to be strongly consistent over much of the eastern half of New South Wales and parts of Victoria, though much of this state sees only a moderately consistent influence for this time of the year. For all of Tasmania and almost all of South Australia there is only a weak consistency between summer rainfall and observed ocean temperature patterns (see background information).

A moderate to strong La Niña event remains in place across the tropical Pacific Ocean. Computer models surveyed by the Bureau suggest the current La Niña event will persist into at least the first quarter of 2011. For routine updates and comprehensive discussion on any developments regarding El Niño and La Niña, 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 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 17th December 2010

Corresponding temperature outlook

October 2010 rainfall in historical perspective

August to October 2010 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.

  • 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 past El Niño events since 1900 are summarized on the Bureau's web site (El Niño - Detailed Australian Analysis), and past La Niña events (La Niña - Detailed Australian Analysis)

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