Warmer than average summer favoured in the south

The outlook for daytime temperatures averaged over summer (December to February) shows a moderate to strong shift in the odds favouring warmer than normal conditions over southern regions of southeastern Australia. However, the outlook for overnight temperatures shows no significant shifts in the odds favouring either warmer or cooler than normal conditions.

The chances of increased daytime temperatures over summer (December to February) are around 60 to 70% for Tasmania, Victoria, southern NSW and the agricultural areas of South Australia (see map). This means for every ten years with ocean patterns like the current, about six or seven would be expected to be warmer than average in terms of daytime temperatures averaged over December to February, while three or four would be expected to be cooler. Across the rest of the southeastern region, the odds are between 50 and 60%, which means roughly equal chances of warmer or cooler than normal conditions. The pattern of seasonal maximum temperature odds across southeastern Australia reflects a moderate to strong contribution from the Indian Ocean, where sea surface temperatures in the central and southeastern Indian Ocean and especially off the west coast of WA have recently been warmer than average. The equatorial Pacific is generally still in a neutral state and not contributing significantly to the temperature outlook for southeastern Australia. Outlook confidence is related to how consistently the Pacific and Indian Oceans affect Australian temperatures. During summer, history shows this effect on maximum temperatures to be moderately consistent over much of southeastern Australia. However, over southern NSW, central Victoria and much of central and northern SA confidence is only weakly consistent, so this outlook needs to be used with caution in these areas (see background information). For most of southeastern Australia the outlook for overnight minimum temperatures shows no significant shifts in the odds favouring either warmer or cooler than normal conditions. In other words, the chances of minimums being warmer than normal in most districts are about the same as the chances of being cooler (see map). The only exception is the far northeast of NSW where there is a moderate shift in the odds favouring warmer than normal minimum temperatures. In this region the odds favouring above normal minimum temperatures are around 60%. History shows the oceans' effect on minimum temperatures during summer to be moderately consistent over northern NSW, northern SA and most of Tasmania. However, over the remainder of the southeastern region, 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 17^th December 2008

Corresponding rainfall outlook

Maximum temperature departures from average for August to October 2008 - base period 1961-1990

Minimum temperature departures from average for August to October 2008 - 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 (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/).