Warm days and nights for SE Australia

For southeast Australia, the outlook for temperatures over spring (September to November) shows warmer days and nights are favoured in most areas.

The pattern of seasonal temperature odds across southeast Australia is a result of recent warm conditions in both the Pacific and Indian Oceans. The chances of exceeding the median maximum temperature during spring (September to November) are between 60% and 70% for mainland southeast Australia, and between 55 and 60% over Tasmania. This means that for every ten years with ocean patterns like the current, about six or seven years would be expected to have warmer than average daytime temperatures over spring, while about three or four would be cooler. Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During spring (September to November), history shows this effect on maximum temperatures to be moderate to strongly consistent over most of SA, northern half of NSW and small parts of the north coast of Tasmania (see background information). However, confidence is lower where the effect is weakly to very weakly consistent in remaining areas - namely parts of southern NSW, all of Victoria and most of Tasmania. In areas where outlook confidence is not high, caution should be used when interpreting these outlooks. The outlook for spring mean minimum temperatures shows 60 to 70% chances of exceeding the median minimum temperature for much of SA, inland NSW and most of Victoria. Higher odds of 70 to 80% are forecast for northwestern SA. History shows the oceans' effect on minimum temperatures during spring to be moderate to strongly consistent over most of SA, and parts of inland NSW. However, over much of eastern and southern NSW, Victoria and Tasmania, the effect is generally only weakly consistent, so this outlook needs to be used with caution in these areas.

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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 22^nd September 2009

Corresponding rainfall outlook

Maximum temperature departures from average for May to July 2009 - base period 1961-1990

Minimum temperature departures from average for May to July 2009 - base period 1961-1990

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).